Fragmentation mechanisms for methane induced by 55 eV, 75 eV, and 100 eV electron impact B. Wei, Y. Zhang, X. Wang, D. Lu, G. C. Lu, B. H. Zhang, Y. J. Tang, R. Hutton, and Y. Zou Citation: The Journal of Chemical Physics 140, 124303 (2014); doi: 10.1063/1.4868651 View online: http://dx.doi.org/10.1063/1.4868651 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/140/12?ver=pdfcov Published by the AIP Publishing Articles you may be interested in An (e, 2e + ion) study of low-energy electron-impact ionization and fragmentation of tetrahydrofuran with high mass and energy resolutions J. Chem. Phys. 141, 134314 (2014); 10.1063/1.4896614 Studies of the fragmentation of the monocation and dication of methanol J. Chem. Phys. 131, 224305 (2009); 10.1063/1.3266940 Electron impact ionization of CHF 2 Cl : Unusual ordering of ionization energies for parent and fragment ions J. Chem. Phys. 119, 11704 (2003); 10.1063/1.1622665 Formation of anion fragments from gas-phase glycine by low energy (0–15 eV) electron impact J. Chem. Phys. 116, 10164 (2002); 10.1063/1.1479348 Absolute partial cross sections for electron-impact ionization of CH4 from threshold to 1000 eV J. Chem. Phys. 106, 4430 (1997); 10.1063/1.473468
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THE JOURNAL OF CHEMICAL PHYSICS 140, 124303 (2014)
Fragmentation mechanisms for methane induced by 55 eV, 75 eV, and 100 eV electron impact B. Wei,1,2 Y. Zhang,1,2 X. Wang,1,2,a) D. Lu,1,2 G. C. Lu,1,2 B. H. Zhang,3 Y. J. Tang,3 R. Hutton,1,2 and Y. Zou1,2 1
Applied Ion Beam Physics Laboratory, Fudan University, Key Laboratory of the Ministry of Education, Shanghai 200433, China 2 Institute of Modern Physics, Department of Nuclear Science and Technology, Fudan University, Shanghai 200433, China 3 Research Center of Laser Fusion, China Academy of Engineering Physics, P.O. Box 919-986, Mianyang 621900, China
(Received 5 December 2013; accepted 5 March 2014; published online 24 March 2014) The fragmentation of CH4 2+ dications following 55 eV, 75 eV, and 100 eV electron impact double ionization of methane was studied using a cold target recoil-ion momentum spectroscopy. From the measured momentum of each recoil ion, the momentum of the neutral particles has been deduced and the kinetic energy release distribution for the different fragmentation channels has been obtained. The doubly charged molecular ions break up into three or more fragments in one or twostep processes, resulting in different signatures in the data. We observed the fragmentation of CH4 2+ dications through different mechanisms according to the momentum of the neutral particles. For example, our result shows that there are three reaction channels to form CH2 + , H+ , and H, one synchronous concerted reaction channel and two two-step reaction channels. For even more complicated fragmentation processes of CH4 2+ dications, the fragmentation mechanism can still be identified in the present measurements. The slopes of the peak in the ion-ion coincidence spectra were also estimated here, as they are also related to the fragmentation mechanism. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4868651] I. INTRODUCTION
Cold target recoil-ion momentum spectroscopy (COLTRIMS) has been developed for more than 20 years, this rapidly developing technique provides an efficient and precise experimental tool to study various of atomic and molecular reactions.1–4 The advantage of the COLTRIMS is that it allows measuring the time of flight (TOF), and the impact position on the detector of the all fragments produced in the collision, from which the collision process can be reconstructed. Concretely, after the collision process, the charged particles are extracted by an electronic field, through a field free drift tube and finally detected with a time and position sensitive detector. The TOF act as a mass spectrometer which can be used to determine the mass and charge state of the recoil-ion, and three-dimensional momentum vectors can be deduced for both electrons and recoil ions. Today, COLTRIMSs are widely equipped to study different molecular fragmentation processes. Methane as a critically important molecule in planetary atmospheres5, 6 and in Tokamak plasmas7 constitutes an interesting case to study the ionization, excitation, and fragmentation of this molecule both from the experimental and theoretical points of view. A lot of effort has been directed to studying the ionization and fragmentation of CH4 induced by electrons,8–13 ions,14–16 synchrotron radiation,17–22 and lasers.23–25 The first measurement of the CH4 ionization cross a) E-mail: [email protected]
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section by electrons was performed by Hughes and his colleagues in 1924,26 since those early days many other experimental works have been undertaken. Also since then, the absolute partial cross section of methane impacted by electrons from threshold to hundreds of eV has been measured.27–29 More recently, with time-of-flight (TOF) mass spectrometry and two-dimensional ion-ion coincidence techniques, Ward et al. studied the dissociation of singly charged and multiply charged methane,8 and the relative cross sections for ions from different reaction channels were measured. A comprehensive review on the studies of electron-impact ionization of hydrocarbons can be found in the paper recently published by Reiter and Janev.30 Due to the tetrahedral structure of the methane ground state, collision induced fragmentation is a very active area of research in recent years. With electron-ion or ion-ion coincidence techniques, different fragmentation pathways of the methane ion (or molecule) can be identified. Fragment ions from different dissociation processes have distinct kinetic energy distributions. However, it is difficult to detect the neutral atom/molecule. An indirect way is to detect all the charged ions and then deduce the momentum of the neutral fragment according to momentum conservation. By measuring the kinetic energy release (KER), the electronic structure and the dissociation mechanism of methane molecules has been analyzed.8–10, 18, 19, 21, 22 Earlier studies of the KER of hydrocarbons found that the fragment ions have complex energy distributions, which suggests that the product ions are produced in more than one dissociation channels. For example,
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Flammini et al.9 have studied the fragmentation mechanism of methane induced by 4 keV electrons using Auger electronion-ion coincidence spectroscopies. They found that the fragmentation is a stepwise process and occurs mainly via the formation and dissociation of the CH3 + intermediate. Using recoil ion momentum spectroscopy, Singh et al.10 measured the KER of methane impacted by 10 keV electrons and they also analyzed the fragmentation process of methane dications. Those studies focused on the fragmentation following coreelectron ionization of methane. For valence-electron ionization, the fragmentation patterns are quite different.22 Ward et al.8 have studied the fragmentation of the lower energy electronic states of CH4 2+ populated by electron double ionization of CH4 at 55 eV, but they were more interested in the partial cross section study of different fragments and the dependence of the fragmentation on the excitation energy. In most cases of the previous studies, TOF spectrometers were used, thus only one-dimensional information was obtained and therefore the fragmentation patterns of CH4 2+ dications was mainly deduced from the slope of the peak in the ion-ion coincidence spectrum. From the ion-ion coincidence spectrum and the KER distribution, a lot of information can be drawn, e.g., the slope of the peak in the ion-ion coincidence spectrum can provide information about the fragmentation mechanism, i.e., whether it is a one-step or twostep process.31, 32 However, for more complicated fragmentation processes, it is difficult to deduce the fragmentation mechanism just from the slope and KER. For example, the metastable H2 +∗ related reaction channel of CH4 2+∗ → CH2 + + H2 +∗ → CH2 + + H+ + H was ruled out in Ref. 8. In this work, we use a COLTRIMS to study the fragmentation process of methane impacted by 55eV, 75 eV, and 100 eV electrons, in which valence-electron ionization dominates, and all three-dimensional momentum information of the recoil ions was obtained. KER was studied for the Coulomb explosion process of CH4 2+ dications. From the ion-ion coincidence spectrum the slope of the peak for different fragmentation channel has been estimated. Furthermore, since the momentum of all the recoil ion has been measured, the momentum of the neutral fragments can be estimated, this gives a direct indication of the fragmentation mechanism. Finally, the fragmentation mechanism was analyzed in more detail using Newton diagrams and Dalitz plots, and direct evidence of the different fragmentation mechanism of CH4 2+ dications, i.e., in one-step or two-step manner, was shown, e.g., the metastable H2 +∗ related channel mentioned above was observed for the first time. II. EXPERIMENT SETUP
The present study of methane fragmentation following impact by low energy electrons (55 eV, 75 eV, and 100 eV) has been performed on a COLTRIMS at Fudan University. The experimental setup has been described in Ref. 33. We will just describe it briefly here. A pulsed electron beam produced by a thermo cathode electron gun with energy ranging from a few eV up to 2 keV34 was directed to intersect with a cold methane gas jet target. To pulse the electron beam, a grid electrode is placed between the cathode and the anode of the gun.
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A floated pulsed voltage (+50 V, 1 ns, and 10 kHz) is applied on the grid which has a bias voltage of −25 V with respect to the cathode. To avoid double hit events, the electron beam was kept at about 104 electrons per pulse during the measurement. The cold target was provided by a supersonic gas jet, in which high pressure gas (2 bar) flowed through a 10 μm nozzle into the first stage of the vacuum chamber at room temperature. A 0.1 mm skimmer at a variable distance of about 10 mm downstream of the nozzle was used to extract a geometrically welldefined gas beam. A multistage pumping structure was used to make sure that the vacuum in the collision chamber was better than 3 × 10−10 torr. A pair of coils was used for shielding the background magnetic field, and the residual magnetic field is then lower than 10% of the Earths magnetic field. The recoil-ions produced in the collision region were extracted by a pulsed electric field E (= 25 V/cm), and then passed through a field-free drift region, finally they were detected by a delay-line anode position sensitive detector (PSD) with a diameter of 75 mm. The advantage of the pulsed extraction field is that the field has no influence on the incident electron beam, which is especially important for low energy electron beams. A time-focusing geometry is adopted for the TOF, i.e., the length d (= 20 cm) of drift region is twice the length a (= 10 cm) of the extraction field. This way the influence from the finite reaction length along the TOF direction can be suppressed.35 According to the measured TOF data and the position information on the detector, the three dimension momentum vectors (px , py , pz ) of the recoil-ions can be reconstructed.3 In this work, the recoil-ion momentum resolution for CH4 + is about 2 a.u. A high performance Multi-hit Time to Digital Convertor (TDC) with 52 μs full scale range and 5 ns double hit resolution36 was adopted here and the measurement was recorded event by event. During the experiment, the recoil-ion signals served as starts and the delayed pulse signal of the electron beam was used as a common stop of a measurement event. III. RESULTS
A typical TOF spectrum of methane after impact by 100 eV electrons is shown in Fig. 1. The data were accumulated for more than 100 h. The observed ions are mainly singly charged fragments such as CHn + (n = 0–3), H+ and H2 + . Since the natural abundance of13 C is about 1.1%, Fig. 1 also contains a small peak of13 CH4 + . The narrow peak superimposed on the wide H2 + distribution results from dissociation of CH4 + into H2 + and a neutral fragment (for example, CH2 ) which leads to low translational energy ions. The broad part of the H2 + peak is formed by the Coulomb explosion process in which CH4 2+ fragments into H2 + and another ion (for example CH2 + , CH+ , etc).37 The peaks of the fragment ions CHn + (n = 0–3) are broader because these ions carry higher kinetic energy. It has to be clarified that the voltage applied in the TOF is not sufficient to collect all the ions in our experiment, for example, the H+ ions from the Coulomb explosion process with an initial perpendicular energy larger than 1.2 eV to the TOF direction cannot be collected. However, the finite angular acceptance of the TOF does not affect the momentum measurement.
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FIG. 1. A typical TOF spectrum of methane impacted by 100 eV electrons.
Taking advantage of ion-ion coincidence, the Coulomb explosion process can be separated and we can generate an ion-ion coincidence spectrum. Fig. 2 shows a 2D ion-ion spectrum of methane impacted by 100 eV electrons. The time of flight of the first ion detected T1 is plotted on the horizontal axis, while that of the second ion detected T2 is plotted on the vertical axis. Due to the limited angular acceptance of the TOF, the line structure on the coincidence map reduced to two enlarged spots. Namely, the first detected ion with an initial velocity towards (backwards) the detector and the second ion with an initial velocity backwards (towards) the detector gave the contribution to the left (right) spot. Eight coincidence channels have been identified and marked in the figure. As we used here a cold target and the thermal velocity is dramatically reduced, consequently the shape of the peaks for CH3 + -H+ and CH2 + -H2 + are quite narrow. In the previous studies,8–10 the fragmentation mechanism was mainly analyzed from the slopes. For comparison, same work has also been done in the present measurement. The slope of the ion-ion coincidence map was extracted for each coincidence channel using the method of least squares,38 and shown in Table I along with results from previous experiment. The slopes of CH3 + -H+ and CH2 + -H2 + are −1.0. This is the consequence of momentum conservation, as both cations were initially at rest, they must have equal momentum with
FIG. 2. Ion-ion coincidence map recorded from dissociative ionization of CH4 2+ produced in collisions between methane molecule and 100 eV electrons.
opposite direction as the amplitude of the Coulomb force on each other is the same. However, when the CH4 2+ dication breaks up into three or more fragments, the reaction process is more complicated and the slope of the ion-ion coincidence map depends on both the “happen time” of the fragmentation and the reaction pathway. The fragmentation mechanism can be categorized as one-step process or synchronous concerted if the bonds are cleaved instantaneously, otherwise it is termed as two-step process,9 which includes the asynchronous concerted process and sequential fragmentation according to the nomenclature of Ref. 39. In Sec. IV, we will try to distinguish one-step process and two-step process according to the momentum of the fragments. The reaction related to the ion-ion pairs of CH2 + -H+ , CH+ -H+ , and C+ -H+ will be discussed in detail in Sec. IV. The slopes for ion-ion pair of CH+ -H2 + and C+ -H2 + are also listed in Table I. However, due to the poor statistics, the fragmentation processes related to those ion pairs will not be discussed further. The benefit of COLTRIMS is that it allows us to measure the TOF and corresponding position information on the position sensitive detector (PSD), therefore, the three-dimensional momentum vectors of the recoil-ions can be reconstructed, and then the kinetic energy for each recoil-ion can be
TABLE I. Experimental slopes extracted from the ion-ion coincidence map. The Coulomb explosion of CH4 induced by the incident electron with the energy of 55 eV, 75 eV, and 100 eV in the present measurement. For comparison, the earlier experimental data were plotted. The 4 keV data are corresponding to the Auger electron energy of 250 eV in Ref. 9. Reaction channel
55 eVa
75 eV
100 eV
4 keV9
10 keV10
CH3 + + H+ CH2 + + H+ + H CH+ + H+ + 2H C+ + H+ + 3H CH2 + + H2 + CH+ + H2 + + H C+ + H2 + + 2H
−1.00 ± 0.01 −0.95 ± 0.02 −0.87 ± 0.05 −0.80 ± 0.10 −1.00 ± 0.01 −0.85 ± 0.10 −0.69 ± 0.11
−1.00 ± 0.01 −0.95 ± 0.02 −0.85 ± 0.03 −0.68 ± 0.05 −1.00 ± 0.01 −0.84 ± 0.10 −0.75 ± 0.10
−1.00 ± 0.01 −0.95 ± 0.01 −0.81 ± 0.02 −0.61 ± 0.03 −1.00 ± 0.01 −0.78 ± 0.08 −0.74 ± 0.10
−1.03 ± 0.17 −1.11 ± 0.18 −0.93 ± 0.16 −0.80 ± 0.16 −1.01 ± 0.17
−1.00 ± 0.02 −0.94 ± 0.04 −0.90 ± 0.04 −0.68 ± 0.06 −1.00 ± 0.02 −0.97 ± 0.08 −0.80 ± 0.08
a
Ward et al.8 also measured the slopes of the ion-ion coincidence map in their experiment of CH4 impacted by 55 eV electron, which is −0.97 ± 0.06 for CH2 + -H+ and 1.01 ± 0.06 for CH+ -H+ .
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TABLE II. Kinetic energy release in different fragmentation channels of the doubly charged methane induced by 55 eV, 75 eV, and 100 eV electrons. Previous measurements of the KER for the fragmentation processes of CH4 2+ dication induced by electron, proton, and photon were summarized and shown in this table for comparison. Unit in eV. Electron impact Present Reaction channel +
H+
CH3 + CH2 + + H2 + CH2 + + H+ + H CH+ + H+ + 2H C+ + H+ + 3H
Proton impact
Photon impact
Ref. 8
Ref. 9
Ref. 10
Ref. 14
Ref. 18 38.5 eV
55 eV
75 eV
100 eV
55 eV
4 keV
10 keV
2 keV
3.0 ± 0.2 3.4 ± 0.2 3.2 ± 0.2 3.6 ± 0.2 3.6 ± 0.2
3. 0 ± 0.2 3.5 ± 0.2 3.3 ± 0.2 4.0 ± 0.2 4.3 ± 0.2
3.1 ± 0.1 3.5 ± 0.1 3.3 ± 0.1 4.2 ± 0.1 4.8 ± 0.1
6.4 ± 0.6 6.8 ± 0.6 7.4 ± 0.4 7.6 ± 0.5
4.34 ± 0.89 5.14 ± 0.71 4.41 ± 0.90 4.33 ± 0.89 5.8 ± 1.27
3.0 ± 0.4 3.5 ± 0.4 5.0 ± 0.8 4.7 ± 0.9 6.5 ± 1.0
6 ± 0.4 6 ± 0.4 5.1 ± 0.4 5.1 ± 0.4
calculated. In the simplest case, if the doubly charged CH4 2+ ion dissociates into two charged fragments, the KER is just the sum of the kinetic energy of those two fragments. If there are neutral atoms/molecules produced in the reaction channel, those neutral atoms/molecules will be considered as a whole molecule in this work. With the principle of momentum conservation, the momentum vectors of the neutral fragment can be estimated. Then the KER for each fragmentation events can be deduced and thus the KER distributions for the Coulomb explosion processes of CH4 2+ were obtained. By fitting the distribution, we use the peak value to represent the KER for various reaction channels shown in Table II. For comparison, the previous experiment results are also summarized in Table II. It should be noted that the values given by Flammini et al.9 are the upper bounds of the kinetic energy distribution, thus are all larger than our measurement. Measuring the KER for the dissociation process would enable us to estimate the excitation state of the precursor CH4 2+ dication. The threshold energy for different dissociation channels of CH4 2+ dications has been theoretically calculated.40 Summing the threshold energy and the KER, the lower energy limit of CH4 2+ dication could be estimated.14 We have summarized the lower limit energy for each ion-ion pair in Table III. The KERs are corresponding to CH4 2+ dication produced in the collision of methane with 55 eV in the table. The possible excited states for CH4 2+ given by Flammini et al.9 were also listed.
5.3 5.3 5 5
be measured directly, and the KER distributions for CH3 + H+ and CH2 + -H2 + are shown in Figs. 2(a) and 3(b), respectively. Increasing the incident electron energy from 55 eV to 100 eV does not change the KER distributions for either reaction channels in the present measurement. It could be explained that 55 eV of the incident electron energy is enough to excite methane which dissociates following those two reaction channels. Higher energy states of precursor CH4 2+ might undergo further fragmentation9 as listed above. The measured KER for the reaction channel CH3 + -H+ induced by 55 eV electrons is 3.0 ± 0.2 eV in this work, which is in good agreement with the data given by Flammini et al.9 and Singh et al.10 The lower limit on the energy of CH4 2+ ion for formation of CH3 + -H+ is 30.8 ± 0.2 eV in this work, which is close to the energy of the excited state of 1 E.9 This indicates that the ion pair of CH3 + -H+ is mainly produced from this state. The measured KER for reaction channel of CH2 + -H2 + in the collision of methane with 55 eV electrons is 3.4 ± 0.2 eV, which is also in good agreement with the data measured by Singh et al.10 According to the theoretical threshold energy for this reaction channel,18, 40 the lower limit on the energy of the CH4 2+ ion is about 33.9 ± 0.2 eV in our measurement, which is close to the energy of the excited state of 3 T1 .9 B. CH2 + + H+
There are four possible reaction channels to produce the ion pair of CH2 + + H+ for CH4 2+ dications, i.e.,
IV. DISCUSSION A. CH3 + + H+ and CH2 + + H2 + +
+
+
CH2 4+∗ → CH2 + + H+ + H,
+
The ion pairs of CH3 -H and CH2 -H2 were produced in the two-body Coulomb explosion processes of CH4 2+ . The momentum of the two charged fragments can
(1)
CH4 2+∗ → CH3 +∗ + H+ → CH2 + + H+ + H,
(2)
TABLE III. The lower limits energies for different fragments produced in the collision of methane with 55 eV electrons. The threshold energy given by Refs.18 and 40, the ionic state of CH4 2+ and Min. energy given by Ref. 9. “Min. energy” is the energy of the minimum of the potential energy surface. Fragments Threshold energy (eV) KER in this work (eV) Lower limits energy (eV) Ionic state Min. energy (eV)
CH3 + + H+
CH2 + + H2 +
CH2 + + H+ + H
CH+ + H+ + H2
C+ + H+ + 3H
27.8 3.0 ± 0.2 30.8 ± 0.2 1E 31.1
30.5 3.4 ± 0.2 33.9 ± 0.2 3T 1 33.8
33.3 3.2 ± 0.2 36.5 ± 0.2 1T 2 35.9
33.4 3.6 ± 0.2 37.0 ± 0.2 1A 1 38.5
37.5 3.6 ± 0.2 41.1 ± 0.2
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FIG. 3. KER distributions for the two-body Coulomb explosion channels: (a) CH3 + -H+ and (b) CH2 + -H2 + produced in the dissociation of CH4 2+ in collision of CH4 with 55 eV, 75 eV, and 100 eV electrons, respectively.
CH4 2+∗ → CH2 + + H2 +∗ → CH2 + + H+ + H, CH4 2+∗ → CH3 2+∗ + H → CH2 + + H+ + H.
(3) (4)
Since there are no distinct counts of CH3 2+ ions in our TOF spectra and also in previous measurement by Ward et al.,8 the reaction channel (4) could be disregarded. Channel (1) is a synchronous concerted dissociation and channels (2) and (3) are two-step fragmentation. In the Coulomb explosion process of multi-charged molecules, the momentum as well as the kinetic energy of the fragments results mainly from Coulomb repulsion. So the final momentum of the neutral H depends on the fragmentation mechanism. For sequential fragmentation, the neutral H will share the momentum with the CH2 + ion in channel (2) and with H+ in channel (3). As stated before, the momentum of the neutral fragments can also be calculated according to momentum conservation. Fig. 4(a) shows the KER distributions for the formation of CH2 + , H+ , and H induced by electron impact with the incident energy of 55 eV, 75 eV, and 100 eV. It can be found in Fig. 4(a) that there is not much difference in the KER distributions induced by different energy of the incident electrons. However, as the incident electron energy increases, the KER slowly increased. So we can still conclude that the incident energy of 55 eV is enough to excite methane to dissociate into CH2 + , H+ , and H. Theoretical threshold energy for formation of CH2 + -H+ -H is 33.3 eV,18, 40 and the lowest KER in the present measurement in the collision of methane with 55 eV electrons is 3.2 ± 0.2 eV. Then the lower limit on the energy of CH4 2+ ions is 36.5 ± 0.2 eV, which is slightly higher than the energy of 35.9 eV in the excited state of 1 T2 .
FIG. 4. Comparison for the fragmentation of CH4 2+ diaction produced in the collisions of methane impacted by 55 eV, 75 eV, and 100 eV electrons, respectively. (a) KER distribution corresponding to the ion pair of CH2 + -H+ , (b) the momentum distribution of neutral H, positive means the momentum of H has the same direction with H+ ions and negative means the momentum of H has the same direction with CH2 + ions.
FIG. 5. (a) Newton diagram for the Coulomb explosion process of CH4 2+ → CH2 + + H+ + H, which induced by 100 eV electron impact. The momentum vector of H+ ion defines the x axis, while the relative momentum vectors of CH2 + ions and neutral H are mapped in the upper and lower half, respectively. (b) Dalitz plot with measured data, the three particles are represented by three edges, the distance of each point from the three edges represents the relative squared momenta, more detail can be found in text.
The slope of the CH2 + -H+ ion pair is constant with different incident electron energy and the value is −0.95 ± 0.02 in the present measurement (in Table I), and Fig. 4(b) shows that there is no difference for the momentum distribution of neutral H with the change of the incident electron energy. We could conclude that the CH2 + -H+ ion pair is produced by the same fragmentation mechanisms for the 55 eV, 75 eV, and 100 eV incident energy of electron. Flammini et al.9 have calculated that the slope for the pure sequential reaction is −0.93, larger than our result. Thus the synchronous concerted fragmentation mechanism has to be taken into account in our case. To identify the fragmentation mechanism of CH4 2+ ions into three bodies, Newton diagram and Dalitz plot41 were used in our analysis. Figs. 5(a) and 5(b) show the Newton and Dalitz plots for the CH4 2+ diaction dissociating into CH2 + , H+ , and H induced by 100 eV electrons. The momentum vector of H+ ion defines the x axis, shown in Fig. 5(a) as an arrow. The relative momentum vectors of CH2 + ions and neutral H are mapped in the upper and lower half, respectively. The momentum of neutral H ranges from −20 a.u. to + 20 a.u. In the fourth quadrant, there are some neutral H atoms with a momentum in the same direction as H+ . While in the third quadrant, there are some neutral H atoms with a momentum in the same direction as CH2 + . It indicates that neutral H shared momentum with H+ and CH2 + , respectively. It should be noted that the arrow in Fig. 5(a) only gives the direction of the momentum, but the amplitude is not constrained. Dalitz plots have been widely used and described in earlier works.42, 43 Briefly, the three-body momentum balance is displayed in an equilateral triangle. Each edge represents one of the final-state fragments. The distances of a given data point from the threeedges are equal to the relative squared momenta πi = pi2 / pj2 , where pj is the momentum of the jth particle. For example, data points in the center of the triangle are at equal distance from all three edges. This region represents equal momentum of all three particles. Combining the Newton diagram and Dalitz plot, we could obtain more information about the fragmentation mechanism. The relationship between the momenta of the three fragments is shown in Fig. 5(b) . In the bottom of the equilateral triangle (in Fig. 5(b)), the momentum of H is very small and
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the momenta of CH2 + and H+ are almost equal to each other, these events correspond to the synchronous concerted fragmentation process (1), as in this fragmentation channel, the momentum was carried by the two charged fragments, and the neutral fragment did not take part in the Coulomb interaction. In fact, we have selected a small region close to the H edge, rebuilt the ion-ion coincidence map, the slope we get is then very close to −1, which is a signature of synchronous concerted fragmentation. On the right part of the equilateral triangle, CH2 + ions shared the momentum with neutral H and must be produced in the sequential fragmentation channel of (2). Namely, in the first fragmentation step, the excited CH3 +∗ ions have the same momentum as H+ but with opposite direction, and then the momentum is shared by CH2 + and neutral H in the second step. While on the left part, H+ ions share the momentum with neutral H and should be produced in the sequential fragmentation channel of (3). On the other hand, stable H2 + peak has been observed in the TOF spectrum with the flight time about 2.9 μs (shown in Fig. 1), it could be an indirect evidence for the formation of CH2 + -H+ ion pair through the reaction channel (3). To be noted, there is no clear boundary on either Newton diagram or Dalitz plot to separate the fragmentation pathway of (1), (2), and (3). C. CH+ + H+
There are many reaction channels to produce the ion pair of CH+ -H+ for the CH4 2+ dication. Since we did not observe the peaks of CH3 2+ , CH2 2+ , and H3 + in the TOF spectrum, here we list the only four possible reaction channels in our measurement CH4 2+∗ → CH+ + H+ + H2 (or 2H) ,
(5)
CH4 2+∗ → CH3 +∗ + H+ → CH+ + H+ + H2 (or 2H) , (6) CH4 2+∗ → CH+ + H2 +∗ + H → CH+ + H+ + 2H,
(7)
CH4 2+∗ → CH2 +∗ + H2 +∗ → CH+ + H+ + 2H.
(8)
Fig. 5(a) shows the KER distributions for CH4 2+ diaction dissociating into CH+ , H+ , and 2H (or H2 ) induced by electron impact with the incident energy of 55 eV, 75 eV, and 100 eV. As the energy of incident electrons increasing, the distributions of KER shift to higher energy as well as the KERs (in Table II). This indicates that the excitation energy of CH4 2+ dication increased. Theoretical threshold energies for formation of CH+ -H+ -H2 and CH+ -H+ -H-H are 33.4 eV and 37.9 eV, respectively.18, 40 The measured KER for formation of CH+ -H+ in the collision of methane with 55 eV electrons is 3.6 ± 0.2 eV, the lower limit on the energy of CH4 2+ ion is 37.0 ± 0.2 eV. The slope corresponding to the CH+ -H+ ion-ion pair increased slowly from −0.87 to −0.81 as the incident energy was raised from 55 eV to 100 eV along with the KER (in Fig. 6(a)), this can be interpreted as the higher excitation energy of the CH4 2+ diaction induced by the higher energy of the incident electrons gives rise to a more dramatic fragmentation in the present measurement. However, the profiles of
FIG. 6. Same as Fig. 4, (a) KER distribution corresponding to the ion pair of CH+ -H+ and (b) the momentum distribution of neutral particle 2H (or H2 ).
the momentum distribution for the neutral particles, which were produced in the dissociation of CH4 2+ induced by the different incident energy of electrons, have almost the same features as seen in Fig. 6(b). So the fragmentation mechanism could be analogous and we will discuss here only the case of 100 eV incident electrons. In previous measurement, Flammini et al.9 has confirmed that the reaction channel (6) has to be taken into account in their collision experiments of methane with 4 keV electrons. In fact, the slope strongly depends on the fragmentation mechanism. And in our measurement, the slopes are larger than −1, which indicates the two-step reaction mechanism can contribute significantly in the formation of the CH+ -H+ ion pair. The Newton diagram (shown in Fig. 7(a)) and Dalitz plot (shown in Fig. 7(b)) for the Coulomb explosion process of CH4 2+ → CH+ + H+ + 2H has been plotted here. One can find that the momentum of the neutral particle varied from −30 a.u. to + 20 a.u. and the direction could be any out of 4π in Fig. 7(a). Fig. 7(b) gives the relationship of the fragments momenta. Apparently, the points in the right part close to the CH+ edge correspond to the neutral particles momentum obtained from the CH2 +∗ ion during a delayed dissociation process of (6), and those points in the left part close to the H+ edge correspond to neutral particles momentum obtained from the H2 +∗ ion during a delay dissociation process of (7). The reaction channel (5) gives the contribution to those points in the bottom of the equilateral triangle. However, there are points in the center of the triangle, which means that both CH+ and H+ shared their momentum with neutral particles. We attribute those points to the reaction channel (8). Namely, both CH+ and H+ were slowed down by the neutral H. Again,
FIG. 7. Same as Fig. 5, (a) Newton diagram and (b) Dalitz plot for the Coulomb explosion process of CH4 2+ → CH+ + H+ + 2H (or H2 ), which induced by 100 eV electron impact.
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FIG. 8. Same as Fig. 4, (a) KER distribution corresponding to the ion pair of C+ -H+ and (b) the momentum distribution of neutral particle 3H.
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FIG. 9. Same as Fig. 5, (a) Newton diagram and (b) Dalitz plot for the Coulomb explosion process of CH4 2+ → C+ + H+ + 3H, which induced by 100 eV electron impact.
there is no boundary on both the Newton and Dalitz plot to separate the different fragmentation pathways. D. C+ + H+
Again, there are many reaction channels which can lead to the production of the ions pair of C+ -H+ , and we are not going to list them here. The threshold energy for the formation of the ions pair C+ -H+ is 37.5 eV and 42.0 eV corresponding to the fragments of C+ -H+ -H-H2 and C+ -H+ 3H, respectively.18, 40 In the present experiment, the measured KER is 3.6 ± 0.2 eV in the dissociation of CH4 2+ dication induced by 55 eV electron impact. Accordingly, the lower limit on the energy of the CH4 2+ ion is 41.1 ± 0.2 eV for the formation of C+ -H+ -H-H2 and 45.6 ± 0.2 eV for the formation of C+ -H+ -3H. Which means the incident energy of 55 eV is enough to excite methane to produce ions pair of C+ -H+ . However, with increasing the incident energy from 55 eV to 100 eV, the KER increased, see Table II. It could be found that, in the KER distribution shown in Fig. 8(a), the higher energy part was absent for the incident energy of 55 eV. The reason is that the CH4 2+ dication obtained more excitation energy in the collision with 100 eV electrons comparing to 55 eV electrons, and then more fragmentation channels were involved with a higher energy release. It is the same situation for the momentum distribution of 3H shown in Fig. 8(b). However, the missed counts mainly appeared in the left part for the incident energy of 55 eV, which is corresponding to the momentum of 3H in the same direction with the momentum of C+ ion. The neutral fragments with the momentum in the same direction as C+ ion are only produced in the sequential dissociation process of CHn +∗ (n = 1, 2, 3) ions. So with increasing the incident energy of electrons from 55 eV to 100 eV, there are more excited CHn +∗ (n = 1, 2, 3) ions produced in the first fragmentation step and then these broke up to form C+ ions in the second fragmentation step. This can be confirmed by the slopes of C+ -H+ ion-ion pair in Table I, which varied from −0.80 to 0.61 with the incident energy increasing from 55 eV to 100 eV. To obtain more information about the fragmentation mechanism, the Newton diagram and Dalitz plot for the Coulomb explosion process of CH4 2+ → C+ + H+ + 3H induced by 100 eV electron impact is presented in Fig. 9. The momentum of neutral fragments ranges from −40 a.u. to 20 a.u. in Fig. 9(a). And the reaction channels could be recognized in Fig. 9(b). The points in the bottom of the triangle
should be produced in a synchronous concerted fragmentation process, and those points closed to the edge of H+ ion in the left part could be produced in a sequential fragmentation of H2 +∗ ion. Since there are more sequential reaction channel to produce C+ ion in the two-step dissociation process of CHn +∗ (n = 1, 2, 3) ions, the distribution of the experiment points in Fig. 9(b) is shifted to the edge of C+ . One also can find some points in the middle of the triangle, which could be produced in the sequential fragmentation of both H2 +∗ and CHn +∗ (n = 1, 2) ions. In fact, by comparing the momentum of neutral fragment shown in Figs. 5(a), 7(a), and 9(a), one could find that the maximum momentum of neutral fragments was always smaller than 20 a.u. with the same direction as the momentum of H+ ion. This could be explained in a simple way. Assuming the neutral H atoms are produced in the sequential fragmentation process, in which the excited H2 +∗ ion broke up into H+ and H. The 20 a.u. momentum of neutral H fragments is just half of the maximum momentum of H2 + , corresponding to the second step happened far away from the Coulomb explosion center. However, the maximum momentum of neutral fragments with the same direction as CHn + (n = 0, 1, 2) increased from 20 a.u. in Fig. 5(a) to 40 a.u. in Fig. 9(a). This cannot be interpreted that the momentum was averagely shared by fragments during the sequential dissociation, because the mass of C+ is much bigger than 3H. To explain the slope of C+ -H+ -3H, Flammini et al.9 have assumed that the fragments CH3 +∗ keeping a pyramidal form after the first fragmentation. We think it could be the case in the present measurement. The three neutral H were launched in the same time with an opposite momentum direction of H+ , and the C+ as the top of the pyramid was decelerated in an asynchronous concerted fragmentation, in which the momentum was mainly carried by the neutral fragments. Nevertheless, to understand this, we need more theoretical analysis in the future.
V. CONCLUSIONS
The fragmentation of CH4 2+ dications induced by 55 eV, 75 eV, and 100 eV electrons was studied using a COLTRIMS. With the measured momentum of each recoil ion, the momentum of neutral particle has been deduced and the kinetic energy release distribution for the different fragmentation channel has been plotted. The KERs for the reactions
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related to the ions pair of CH3 + -H+ , CH2 + -H2 + , and CH2 + H+ -H are constant for different incident energy of electrons in the present measurement energy region. This can be interpreted that the incident energy of 55 eV is enough to excite the CH4 molecule proceeding the fragmentation corresponding to those fragments. With increasing incident energy, the excitation energy of CH4 2+ increases and more reaction channels will be opened. This could be confirmed by comparing the KER for those reaction channels related to the fragments of CH+ -H+ -2H and C+ -H+ -3H in Table II. The fragmentation mechanism for the doubly charged molecular ion breaking up into three or more fragments was studied here. The slopes of the peak in the ion-ion coincidence spectrum have also been estimated. Using the benefits of COLTRIMS, the momentum of the neutral fragment was deduced using the principle of momentum conservation. Based on the momentum of the neutral particles, the fragmentation of CH4 2+ dications in either synchronous concerted or a two-step manner has been distinguished experimentally. Namely, the neutral fragments produced in the two-step fragmentation shared the momentum with the charged fragments. However, in the synchronous concerted fragmentation, the neutral fragments did not take part in the Coulomb interaction, and thus there is no momentum sharing between the neutral fragments and the charged fragments. For the formation of CH2 + , H+ , and H from CH4 2+ , we have clarified the existence of one synchronous concerted reaction channel and two two-step reaction channels. With the help of the Dalitz plot, for the sequential reaction channel, the neutral H produced in the delayed dissociation of CH3 +∗ and H2 +∗ , could be distinguished in our measurement. The same fragmentation pattern has been identified for the dissociation of CH4 2+ dications related to the fragments of CH+ -H+ and C+ -H+ in the present incident energy region. Furthermore, sequential fragmentation processes have been observed in which both of the two charged fragments are in excited states with a delayed dissociation.
ACKNOWLEDGMENTS
This work was supported by the National Magnetic Confinement Fusion Program under Grant No. 2009GB106001, IAEA (CRP; No. 15735) and the National Science Foundation of China under Contract No. 10904019. This work was also supported by Shanghai Leading Academic Discipline Project (Project No. B107). 1 R.
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THE JOURNAL OF CHEMICAL PHYSICS 140, 124303 (2014)
Fragmentation mechanisms for methane induced by 55 eV, 75 eV, and 100 eV electron impact B. Wei,1,2 Y. Zhang,1,2 X. Wang,1,2,a) D. Lu,1,2 G. C. Lu,1,2 B. H. Zhang,3 Y. J. Tang,3 R. Hutton,1,2 and Y. Zou1,2 1
Applied Ion Beam Physics Laboratory, Fudan University, Key Laboratory of the Ministry of Education, Shanghai 200433, China 2 Institute of Modern Physics, Department of Nuclear Science and Technology, Fudan University, Shanghai 200433, China 3 Research Center of Laser Fusion, China Academy of Engineering Physics, P.O. Box 919-986, Mianyang 621900, China
(Received 5 December 2013; accepted 5 March 2014; published online 24 March 2014) The fragmentation of CH4 2+ dications following 55 eV, 75 eV, and 100 eV electron impact double ionization of methane was studied using a cold target recoil-ion momentum spectroscopy. From the measured momentum of each recoil ion, the momentum of the neutral particles has been deduced and the kinetic energy release distribution for the different fragmentation channels has been obtained. The doubly charged molecular ions break up into three or more fragments in one or twostep processes, resulting in different signatures in the data. We observed the fragmentation of CH4 2+ dications through different mechanisms according to the momentum of the neutral particles. For example, our result shows that there are three reaction channels to form CH2 + , H+ , and H, one synchronous concerted reaction channel and two two-step reaction channels. For even more complicated fragmentation processes of CH4 2+ dications, the fragmentation mechanism can still be identified in the present measurements. The slopes of the peak in the ion-ion coincidence spectra were also estimated here, as they are also related to the fragmentation mechanism. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4868651] I. INTRODUCTION
Cold target recoil-ion momentum spectroscopy (COLTRIMS) has been developed for more than 20 years, this rapidly developing technique provides an efficient and precise experimental tool to study various of atomic and molecular reactions.1–4 The advantage of the COLTRIMS is that it allows measuring the time of flight (TOF), and the impact position on the detector of the all fragments produced in the collision, from which the collision process can be reconstructed. Concretely, after the collision process, the charged particles are extracted by an electronic field, through a field free drift tube and finally detected with a time and position sensitive detector. The TOF act as a mass spectrometer which can be used to determine the mass and charge state of the recoil-ion, and three-dimensional momentum vectors can be deduced for both electrons and recoil ions. Today, COLTRIMSs are widely equipped to study different molecular fragmentation processes. Methane as a critically important molecule in planetary atmospheres5, 6 and in Tokamak plasmas7 constitutes an interesting case to study the ionization, excitation, and fragmentation of this molecule both from the experimental and theoretical points of view. A lot of effort has been directed to studying the ionization and fragmentation of CH4 induced by electrons,8–13 ions,14–16 synchrotron radiation,17–22 and lasers.23–25 The first measurement of the CH4 ionization cross a) E-mail: [email protected]
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section by electrons was performed by Hughes and his colleagues in 1924,26 since those early days many other experimental works have been undertaken. Also since then, the absolute partial cross section of methane impacted by electrons from threshold to hundreds of eV has been measured.27–29 More recently, with time-of-flight (TOF) mass spectrometry and two-dimensional ion-ion coincidence techniques, Ward et al. studied the dissociation of singly charged and multiply charged methane,8 and the relative cross sections for ions from different reaction channels were measured. A comprehensive review on the studies of electron-impact ionization of hydrocarbons can be found in the paper recently published by Reiter and Janev.30 Due to the tetrahedral structure of the methane ground state, collision induced fragmentation is a very active area of research in recent years. With electron-ion or ion-ion coincidence techniques, different fragmentation pathways of the methane ion (or molecule) can be identified. Fragment ions from different dissociation processes have distinct kinetic energy distributions. However, it is difficult to detect the neutral atom/molecule. An indirect way is to detect all the charged ions and then deduce the momentum of the neutral fragment according to momentum conservation. By measuring the kinetic energy release (KER), the electronic structure and the dissociation mechanism of methane molecules has been analyzed.8–10, 18, 19, 21, 22 Earlier studies of the KER of hydrocarbons found that the fragment ions have complex energy distributions, which suggests that the product ions are produced in more than one dissociation channels. For example,
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Flammini et al.9 have studied the fragmentation mechanism of methane induced by 4 keV electrons using Auger electronion-ion coincidence spectroscopies. They found that the fragmentation is a stepwise process and occurs mainly via the formation and dissociation of the CH3 + intermediate. Using recoil ion momentum spectroscopy, Singh et al.10 measured the KER of methane impacted by 10 keV electrons and they also analyzed the fragmentation process of methane dications. Those studies focused on the fragmentation following coreelectron ionization of methane. For valence-electron ionization, the fragmentation patterns are quite different.22 Ward et al.8 have studied the fragmentation of the lower energy electronic states of CH4 2+ populated by electron double ionization of CH4 at 55 eV, but they were more interested in the partial cross section study of different fragments and the dependence of the fragmentation on the excitation energy. In most cases of the previous studies, TOF spectrometers were used, thus only one-dimensional information was obtained and therefore the fragmentation patterns of CH4 2+ dications was mainly deduced from the slope of the peak in the ion-ion coincidence spectrum. From the ion-ion coincidence spectrum and the KER distribution, a lot of information can be drawn, e.g., the slope of the peak in the ion-ion coincidence spectrum can provide information about the fragmentation mechanism, i.e., whether it is a one-step or twostep process.31, 32 However, for more complicated fragmentation processes, it is difficult to deduce the fragmentation mechanism just from the slope and KER. For example, the metastable H2 +∗ related reaction channel of CH4 2+∗ → CH2 + + H2 +∗ → CH2 + + H+ + H was ruled out in Ref. 8. In this work, we use a COLTRIMS to study the fragmentation process of methane impacted by 55eV, 75 eV, and 100 eV electrons, in which valence-electron ionization dominates, and all three-dimensional momentum information of the recoil ions was obtained. KER was studied for the Coulomb explosion process of CH4 2+ dications. From the ion-ion coincidence spectrum the slope of the peak for different fragmentation channel has been estimated. Furthermore, since the momentum of all the recoil ion has been measured, the momentum of the neutral fragments can be estimated, this gives a direct indication of the fragmentation mechanism. Finally, the fragmentation mechanism was analyzed in more detail using Newton diagrams and Dalitz plots, and direct evidence of the different fragmentation mechanism of CH4 2+ dications, i.e., in one-step or two-step manner, was shown, e.g., the metastable H2 +∗ related channel mentioned above was observed for the first time. II. EXPERIMENT SETUP
The present study of methane fragmentation following impact by low energy electrons (55 eV, 75 eV, and 100 eV) has been performed on a COLTRIMS at Fudan University. The experimental setup has been described in Ref. 33. We will just describe it briefly here. A pulsed electron beam produced by a thermo cathode electron gun with energy ranging from a few eV up to 2 keV34 was directed to intersect with a cold methane gas jet target. To pulse the electron beam, a grid electrode is placed between the cathode and the anode of the gun.
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A floated pulsed voltage (+50 V, 1 ns, and 10 kHz) is applied on the grid which has a bias voltage of −25 V with respect to the cathode. To avoid double hit events, the electron beam was kept at about 104 electrons per pulse during the measurement. The cold target was provided by a supersonic gas jet, in which high pressure gas (2 bar) flowed through a 10 μm nozzle into the first stage of the vacuum chamber at room temperature. A 0.1 mm skimmer at a variable distance of about 10 mm downstream of the nozzle was used to extract a geometrically welldefined gas beam. A multistage pumping structure was used to make sure that the vacuum in the collision chamber was better than 3 × 10−10 torr. A pair of coils was used for shielding the background magnetic field, and the residual magnetic field is then lower than 10% of the Earths magnetic field. The recoil-ions produced in the collision region were extracted by a pulsed electric field E (= 25 V/cm), and then passed through a field-free drift region, finally they were detected by a delay-line anode position sensitive detector (PSD) with a diameter of 75 mm. The advantage of the pulsed extraction field is that the field has no influence on the incident electron beam, which is especially important for low energy electron beams. A time-focusing geometry is adopted for the TOF, i.e., the length d (= 20 cm) of drift region is twice the length a (= 10 cm) of the extraction field. This way the influence from the finite reaction length along the TOF direction can be suppressed.35 According to the measured TOF data and the position information on the detector, the three dimension momentum vectors (px , py , pz ) of the recoil-ions can be reconstructed.3 In this work, the recoil-ion momentum resolution for CH4 + is about 2 a.u. A high performance Multi-hit Time to Digital Convertor (TDC) with 52 μs full scale range and 5 ns double hit resolution36 was adopted here and the measurement was recorded event by event. During the experiment, the recoil-ion signals served as starts and the delayed pulse signal of the electron beam was used as a common stop of a measurement event. III. RESULTS
A typical TOF spectrum of methane after impact by 100 eV electrons is shown in Fig. 1. The data were accumulated for more than 100 h. The observed ions are mainly singly charged fragments such as CHn + (n = 0–3), H+ and H2 + . Since the natural abundance of13 C is about 1.1%, Fig. 1 also contains a small peak of13 CH4 + . The narrow peak superimposed on the wide H2 + distribution results from dissociation of CH4 + into H2 + and a neutral fragment (for example, CH2 ) which leads to low translational energy ions. The broad part of the H2 + peak is formed by the Coulomb explosion process in which CH4 2+ fragments into H2 + and another ion (for example CH2 + , CH+ , etc).37 The peaks of the fragment ions CHn + (n = 0–3) are broader because these ions carry higher kinetic energy. It has to be clarified that the voltage applied in the TOF is not sufficient to collect all the ions in our experiment, for example, the H+ ions from the Coulomb explosion process with an initial perpendicular energy larger than 1.2 eV to the TOF direction cannot be collected. However, the finite angular acceptance of the TOF does not affect the momentum measurement.
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FIG. 1. A typical TOF spectrum of methane impacted by 100 eV electrons.
Taking advantage of ion-ion coincidence, the Coulomb explosion process can be separated and we can generate an ion-ion coincidence spectrum. Fig. 2 shows a 2D ion-ion spectrum of methane impacted by 100 eV electrons. The time of flight of the first ion detected T1 is plotted on the horizontal axis, while that of the second ion detected T2 is plotted on the vertical axis. Due to the limited angular acceptance of the TOF, the line structure on the coincidence map reduced to two enlarged spots. Namely, the first detected ion with an initial velocity towards (backwards) the detector and the second ion with an initial velocity backwards (towards) the detector gave the contribution to the left (right) spot. Eight coincidence channels have been identified and marked in the figure. As we used here a cold target and the thermal velocity is dramatically reduced, consequently the shape of the peaks for CH3 + -H+ and CH2 + -H2 + are quite narrow. In the previous studies,8–10 the fragmentation mechanism was mainly analyzed from the slopes. For comparison, same work has also been done in the present measurement. The slope of the ion-ion coincidence map was extracted for each coincidence channel using the method of least squares,38 and shown in Table I along with results from previous experiment. The slopes of CH3 + -H+ and CH2 + -H2 + are −1.0. This is the consequence of momentum conservation, as both cations were initially at rest, they must have equal momentum with
FIG. 2. Ion-ion coincidence map recorded from dissociative ionization of CH4 2+ produced in collisions between methane molecule and 100 eV electrons.
opposite direction as the amplitude of the Coulomb force on each other is the same. However, when the CH4 2+ dication breaks up into three or more fragments, the reaction process is more complicated and the slope of the ion-ion coincidence map depends on both the “happen time” of the fragmentation and the reaction pathway. The fragmentation mechanism can be categorized as one-step process or synchronous concerted if the bonds are cleaved instantaneously, otherwise it is termed as two-step process,9 which includes the asynchronous concerted process and sequential fragmentation according to the nomenclature of Ref. 39. In Sec. IV, we will try to distinguish one-step process and two-step process according to the momentum of the fragments. The reaction related to the ion-ion pairs of CH2 + -H+ , CH+ -H+ , and C+ -H+ will be discussed in detail in Sec. IV. The slopes for ion-ion pair of CH+ -H2 + and C+ -H2 + are also listed in Table I. However, due to the poor statistics, the fragmentation processes related to those ion pairs will not be discussed further. The benefit of COLTRIMS is that it allows us to measure the TOF and corresponding position information on the position sensitive detector (PSD), therefore, the three-dimensional momentum vectors of the recoil-ions can be reconstructed, and then the kinetic energy for each recoil-ion can be
TABLE I. Experimental slopes extracted from the ion-ion coincidence map. The Coulomb explosion of CH4 induced by the incident electron with the energy of 55 eV, 75 eV, and 100 eV in the present measurement. For comparison, the earlier experimental data were plotted. The 4 keV data are corresponding to the Auger electron energy of 250 eV in Ref. 9. Reaction channel
55 eVa
75 eV
100 eV
4 keV9
10 keV10
CH3 + + H+ CH2 + + H+ + H CH+ + H+ + 2H C+ + H+ + 3H CH2 + + H2 + CH+ + H2 + + H C+ + H2 + + 2H
−1.00 ± 0.01 −0.95 ± 0.02 −0.87 ± 0.05 −0.80 ± 0.10 −1.00 ± 0.01 −0.85 ± 0.10 −0.69 ± 0.11
−1.00 ± 0.01 −0.95 ± 0.02 −0.85 ± 0.03 −0.68 ± 0.05 −1.00 ± 0.01 −0.84 ± 0.10 −0.75 ± 0.10
−1.00 ± 0.01 −0.95 ± 0.01 −0.81 ± 0.02 −0.61 ± 0.03 −1.00 ± 0.01 −0.78 ± 0.08 −0.74 ± 0.10
−1.03 ± 0.17 −1.11 ± 0.18 −0.93 ± 0.16 −0.80 ± 0.16 −1.01 ± 0.17
−1.00 ± 0.02 −0.94 ± 0.04 −0.90 ± 0.04 −0.68 ± 0.06 −1.00 ± 0.02 −0.97 ± 0.08 −0.80 ± 0.08
a
Ward et al.8 also measured the slopes of the ion-ion coincidence map in their experiment of CH4 impacted by 55 eV electron, which is −0.97 ± 0.06 for CH2 + -H+ and 1.01 ± 0.06 for CH+ -H+ .
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TABLE II. Kinetic energy release in different fragmentation channels of the doubly charged methane induced by 55 eV, 75 eV, and 100 eV electrons. Previous measurements of the KER for the fragmentation processes of CH4 2+ dication induced by electron, proton, and photon were summarized and shown in this table for comparison. Unit in eV. Electron impact Present Reaction channel +
H+
CH3 + CH2 + + H2 + CH2 + + H+ + H CH+ + H+ + 2H C+ + H+ + 3H
Proton impact
Photon impact
Ref. 8
Ref. 9
Ref. 10
Ref. 14
Ref. 18 38.5 eV
55 eV
75 eV
100 eV
55 eV
4 keV
10 keV
2 keV
3.0 ± 0.2 3.4 ± 0.2 3.2 ± 0.2 3.6 ± 0.2 3.6 ± 0.2
3. 0 ± 0.2 3.5 ± 0.2 3.3 ± 0.2 4.0 ± 0.2 4.3 ± 0.2
3.1 ± 0.1 3.5 ± 0.1 3.3 ± 0.1 4.2 ± 0.1 4.8 ± 0.1
6.4 ± 0.6 6.8 ± 0.6 7.4 ± 0.4 7.6 ± 0.5
4.34 ± 0.89 5.14 ± 0.71 4.41 ± 0.90 4.33 ± 0.89 5.8 ± 1.27
3.0 ± 0.4 3.5 ± 0.4 5.0 ± 0.8 4.7 ± 0.9 6.5 ± 1.0
6 ± 0.4 6 ± 0.4 5.1 ± 0.4 5.1 ± 0.4
calculated. In the simplest case, if the doubly charged CH4 2+ ion dissociates into two charged fragments, the KER is just the sum of the kinetic energy of those two fragments. If there are neutral atoms/molecules produced in the reaction channel, those neutral atoms/molecules will be considered as a whole molecule in this work. With the principle of momentum conservation, the momentum vectors of the neutral fragment can be estimated. Then the KER for each fragmentation events can be deduced and thus the KER distributions for the Coulomb explosion processes of CH4 2+ were obtained. By fitting the distribution, we use the peak value to represent the KER for various reaction channels shown in Table II. For comparison, the previous experiment results are also summarized in Table II. It should be noted that the values given by Flammini et al.9 are the upper bounds of the kinetic energy distribution, thus are all larger than our measurement. Measuring the KER for the dissociation process would enable us to estimate the excitation state of the precursor CH4 2+ dication. The threshold energy for different dissociation channels of CH4 2+ dications has been theoretically calculated.40 Summing the threshold energy and the KER, the lower energy limit of CH4 2+ dication could be estimated.14 We have summarized the lower limit energy for each ion-ion pair in Table III. The KERs are corresponding to CH4 2+ dication produced in the collision of methane with 55 eV in the table. The possible excited states for CH4 2+ given by Flammini et al.9 were also listed.
5.3 5.3 5 5
be measured directly, and the KER distributions for CH3 + H+ and CH2 + -H2 + are shown in Figs. 2(a) and 3(b), respectively. Increasing the incident electron energy from 55 eV to 100 eV does not change the KER distributions for either reaction channels in the present measurement. It could be explained that 55 eV of the incident electron energy is enough to excite methane which dissociates following those two reaction channels. Higher energy states of precursor CH4 2+ might undergo further fragmentation9 as listed above. The measured KER for the reaction channel CH3 + -H+ induced by 55 eV electrons is 3.0 ± 0.2 eV in this work, which is in good agreement with the data given by Flammini et al.9 and Singh et al.10 The lower limit on the energy of CH4 2+ ion for formation of CH3 + -H+ is 30.8 ± 0.2 eV in this work, which is close to the energy of the excited state of 1 E.9 This indicates that the ion pair of CH3 + -H+ is mainly produced from this state. The measured KER for reaction channel of CH2 + -H2 + in the collision of methane with 55 eV electrons is 3.4 ± 0.2 eV, which is also in good agreement with the data measured by Singh et al.10 According to the theoretical threshold energy for this reaction channel,18, 40 the lower limit on the energy of the CH4 2+ ion is about 33.9 ± 0.2 eV in our measurement, which is close to the energy of the excited state of 3 T1 .9 B. CH2 + + H+
There are four possible reaction channels to produce the ion pair of CH2 + + H+ for CH4 2+ dications, i.e.,
IV. DISCUSSION A. CH3 + + H+ and CH2 + + H2 + +
+
+
CH2 4+∗ → CH2 + + H+ + H,
+
The ion pairs of CH3 -H and CH2 -H2 were produced in the two-body Coulomb explosion processes of CH4 2+ . The momentum of the two charged fragments can
(1)
CH4 2+∗ → CH3 +∗ + H+ → CH2 + + H+ + H,
(2)
TABLE III. The lower limits energies for different fragments produced in the collision of methane with 55 eV electrons. The threshold energy given by Refs.18 and 40, the ionic state of CH4 2+ and Min. energy given by Ref. 9. “Min. energy” is the energy of the minimum of the potential energy surface. Fragments Threshold energy (eV) KER in this work (eV) Lower limits energy (eV) Ionic state Min. energy (eV)
CH3 + + H+
CH2 + + H2 +
CH2 + + H+ + H
CH+ + H+ + H2
C+ + H+ + 3H
27.8 3.0 ± 0.2 30.8 ± 0.2 1E 31.1
30.5 3.4 ± 0.2 33.9 ± 0.2 3T 1 33.8
33.3 3.2 ± 0.2 36.5 ± 0.2 1T 2 35.9
33.4 3.6 ± 0.2 37.0 ± 0.2 1A 1 38.5
37.5 3.6 ± 0.2 41.1 ± 0.2
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FIG. 3. KER distributions for the two-body Coulomb explosion channels: (a) CH3 + -H+ and (b) CH2 + -H2 + produced in the dissociation of CH4 2+ in collision of CH4 with 55 eV, 75 eV, and 100 eV electrons, respectively.
CH4 2+∗ → CH2 + + H2 +∗ → CH2 + + H+ + H, CH4 2+∗ → CH3 2+∗ + H → CH2 + + H+ + H.
(3) (4)
Since there are no distinct counts of CH3 2+ ions in our TOF spectra and also in previous measurement by Ward et al.,8 the reaction channel (4) could be disregarded. Channel (1) is a synchronous concerted dissociation and channels (2) and (3) are two-step fragmentation. In the Coulomb explosion process of multi-charged molecules, the momentum as well as the kinetic energy of the fragments results mainly from Coulomb repulsion. So the final momentum of the neutral H depends on the fragmentation mechanism. For sequential fragmentation, the neutral H will share the momentum with the CH2 + ion in channel (2) and with H+ in channel (3). As stated before, the momentum of the neutral fragments can also be calculated according to momentum conservation. Fig. 4(a) shows the KER distributions for the formation of CH2 + , H+ , and H induced by electron impact with the incident energy of 55 eV, 75 eV, and 100 eV. It can be found in Fig. 4(a) that there is not much difference in the KER distributions induced by different energy of the incident electrons. However, as the incident electron energy increases, the KER slowly increased. So we can still conclude that the incident energy of 55 eV is enough to excite methane to dissociate into CH2 + , H+ , and H. Theoretical threshold energy for formation of CH2 + -H+ -H is 33.3 eV,18, 40 and the lowest KER in the present measurement in the collision of methane with 55 eV electrons is 3.2 ± 0.2 eV. Then the lower limit on the energy of CH4 2+ ions is 36.5 ± 0.2 eV, which is slightly higher than the energy of 35.9 eV in the excited state of 1 T2 .
FIG. 4. Comparison for the fragmentation of CH4 2+ diaction produced in the collisions of methane impacted by 55 eV, 75 eV, and 100 eV electrons, respectively. (a) KER distribution corresponding to the ion pair of CH2 + -H+ , (b) the momentum distribution of neutral H, positive means the momentum of H has the same direction with H+ ions and negative means the momentum of H has the same direction with CH2 + ions.
FIG. 5. (a) Newton diagram for the Coulomb explosion process of CH4 2+ → CH2 + + H+ + H, which induced by 100 eV electron impact. The momentum vector of H+ ion defines the x axis, while the relative momentum vectors of CH2 + ions and neutral H are mapped in the upper and lower half, respectively. (b) Dalitz plot with measured data, the three particles are represented by three edges, the distance of each point from the three edges represents the relative squared momenta, more detail can be found in text.
The slope of the CH2 + -H+ ion pair is constant with different incident electron energy and the value is −0.95 ± 0.02 in the present measurement (in Table I), and Fig. 4(b) shows that there is no difference for the momentum distribution of neutral H with the change of the incident electron energy. We could conclude that the CH2 + -H+ ion pair is produced by the same fragmentation mechanisms for the 55 eV, 75 eV, and 100 eV incident energy of electron. Flammini et al.9 have calculated that the slope for the pure sequential reaction is −0.93, larger than our result. Thus the synchronous concerted fragmentation mechanism has to be taken into account in our case. To identify the fragmentation mechanism of CH4 2+ ions into three bodies, Newton diagram and Dalitz plot41 were used in our analysis. Figs. 5(a) and 5(b) show the Newton and Dalitz plots for the CH4 2+ diaction dissociating into CH2 + , H+ , and H induced by 100 eV electrons. The momentum vector of H+ ion defines the x axis, shown in Fig. 5(a) as an arrow. The relative momentum vectors of CH2 + ions and neutral H are mapped in the upper and lower half, respectively. The momentum of neutral H ranges from −20 a.u. to + 20 a.u. In the fourth quadrant, there are some neutral H atoms with a momentum in the same direction as H+ . While in the third quadrant, there are some neutral H atoms with a momentum in the same direction as CH2 + . It indicates that neutral H shared momentum with H+ and CH2 + , respectively. It should be noted that the arrow in Fig. 5(a) only gives the direction of the momentum, but the amplitude is not constrained. Dalitz plots have been widely used and described in earlier works.42, 43 Briefly, the three-body momentum balance is displayed in an equilateral triangle. Each edge represents one of the final-state fragments. The distances of a given data point from the threeedges are equal to the relative squared momenta πi = pi2 / pj2 , where pj is the momentum of the jth particle. For example, data points in the center of the triangle are at equal distance from all three edges. This region represents equal momentum of all three particles. Combining the Newton diagram and Dalitz plot, we could obtain more information about the fragmentation mechanism. The relationship between the momenta of the three fragments is shown in Fig. 5(b) . In the bottom of the equilateral triangle (in Fig. 5(b)), the momentum of H is very small and
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the momenta of CH2 + and H+ are almost equal to each other, these events correspond to the synchronous concerted fragmentation process (1), as in this fragmentation channel, the momentum was carried by the two charged fragments, and the neutral fragment did not take part in the Coulomb interaction. In fact, we have selected a small region close to the H edge, rebuilt the ion-ion coincidence map, the slope we get is then very close to −1, which is a signature of synchronous concerted fragmentation. On the right part of the equilateral triangle, CH2 + ions shared the momentum with neutral H and must be produced in the sequential fragmentation channel of (2). Namely, in the first fragmentation step, the excited CH3 +∗ ions have the same momentum as H+ but with opposite direction, and then the momentum is shared by CH2 + and neutral H in the second step. While on the left part, H+ ions share the momentum with neutral H and should be produced in the sequential fragmentation channel of (3). On the other hand, stable H2 + peak has been observed in the TOF spectrum with the flight time about 2.9 μs (shown in Fig. 1), it could be an indirect evidence for the formation of CH2 + -H+ ion pair through the reaction channel (3). To be noted, there is no clear boundary on either Newton diagram or Dalitz plot to separate the fragmentation pathway of (1), (2), and (3). C. CH+ + H+
There are many reaction channels to produce the ion pair of CH+ -H+ for the CH4 2+ dication. Since we did not observe the peaks of CH3 2+ , CH2 2+ , and H3 + in the TOF spectrum, here we list the only four possible reaction channels in our measurement CH4 2+∗ → CH+ + H+ + H2 (or 2H) ,
(5)
CH4 2+∗ → CH3 +∗ + H+ → CH+ + H+ + H2 (or 2H) , (6) CH4 2+∗ → CH+ + H2 +∗ + H → CH+ + H+ + 2H,
(7)
CH4 2+∗ → CH2 +∗ + H2 +∗ → CH+ + H+ + 2H.
(8)
Fig. 5(a) shows the KER distributions for CH4 2+ diaction dissociating into CH+ , H+ , and 2H (or H2 ) induced by electron impact with the incident energy of 55 eV, 75 eV, and 100 eV. As the energy of incident electrons increasing, the distributions of KER shift to higher energy as well as the KERs (in Table II). This indicates that the excitation energy of CH4 2+ dication increased. Theoretical threshold energies for formation of CH+ -H+ -H2 and CH+ -H+ -H-H are 33.4 eV and 37.9 eV, respectively.18, 40 The measured KER for formation of CH+ -H+ in the collision of methane with 55 eV electrons is 3.6 ± 0.2 eV, the lower limit on the energy of CH4 2+ ion is 37.0 ± 0.2 eV. The slope corresponding to the CH+ -H+ ion-ion pair increased slowly from −0.87 to −0.81 as the incident energy was raised from 55 eV to 100 eV along with the KER (in Fig. 6(a)), this can be interpreted as the higher excitation energy of the CH4 2+ diaction induced by the higher energy of the incident electrons gives rise to a more dramatic fragmentation in the present measurement. However, the profiles of
FIG. 6. Same as Fig. 4, (a) KER distribution corresponding to the ion pair of CH+ -H+ and (b) the momentum distribution of neutral particle 2H (or H2 ).
the momentum distribution for the neutral particles, which were produced in the dissociation of CH4 2+ induced by the different incident energy of electrons, have almost the same features as seen in Fig. 6(b). So the fragmentation mechanism could be analogous and we will discuss here only the case of 100 eV incident electrons. In previous measurement, Flammini et al.9 has confirmed that the reaction channel (6) has to be taken into account in their collision experiments of methane with 4 keV electrons. In fact, the slope strongly depends on the fragmentation mechanism. And in our measurement, the slopes are larger than −1, which indicates the two-step reaction mechanism can contribute significantly in the formation of the CH+ -H+ ion pair. The Newton diagram (shown in Fig. 7(a)) and Dalitz plot (shown in Fig. 7(b)) for the Coulomb explosion process of CH4 2+ → CH+ + H+ + 2H has been plotted here. One can find that the momentum of the neutral particle varied from −30 a.u. to + 20 a.u. and the direction could be any out of 4π in Fig. 7(a). Fig. 7(b) gives the relationship of the fragments momenta. Apparently, the points in the right part close to the CH+ edge correspond to the neutral particles momentum obtained from the CH2 +∗ ion during a delayed dissociation process of (6), and those points in the left part close to the H+ edge correspond to neutral particles momentum obtained from the H2 +∗ ion during a delay dissociation process of (7). The reaction channel (5) gives the contribution to those points in the bottom of the equilateral triangle. However, there are points in the center of the triangle, which means that both CH+ and H+ shared their momentum with neutral particles. We attribute those points to the reaction channel (8). Namely, both CH+ and H+ were slowed down by the neutral H. Again,
FIG. 7. Same as Fig. 5, (a) Newton diagram and (b) Dalitz plot for the Coulomb explosion process of CH4 2+ → CH+ + H+ + 2H (or H2 ), which induced by 100 eV electron impact.
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FIG. 8. Same as Fig. 4, (a) KER distribution corresponding to the ion pair of C+ -H+ and (b) the momentum distribution of neutral particle 3H.
J. Chem. Phys. 140, 124303 (2014)
FIG. 9. Same as Fig. 5, (a) Newton diagram and (b) Dalitz plot for the Coulomb explosion process of CH4 2+ → C+ + H+ + 3H, which induced by 100 eV electron impact.
there is no boundary on both the Newton and Dalitz plot to separate the different fragmentation pathways. D. C+ + H+
Again, there are many reaction channels which can lead to the production of the ions pair of C+ -H+ , and we are not going to list them here. The threshold energy for the formation of the ions pair C+ -H+ is 37.5 eV and 42.0 eV corresponding to the fragments of C+ -H+ -H-H2 and C+ -H+ 3H, respectively.18, 40 In the present experiment, the measured KER is 3.6 ± 0.2 eV in the dissociation of CH4 2+ dication induced by 55 eV electron impact. Accordingly, the lower limit on the energy of the CH4 2+ ion is 41.1 ± 0.2 eV for the formation of C+ -H+ -H-H2 and 45.6 ± 0.2 eV for the formation of C+ -H+ -3H. Which means the incident energy of 55 eV is enough to excite methane to produce ions pair of C+ -H+ . However, with increasing the incident energy from 55 eV to 100 eV, the KER increased, see Table II. It could be found that, in the KER distribution shown in Fig. 8(a), the higher energy part was absent for the incident energy of 55 eV. The reason is that the CH4 2+ dication obtained more excitation energy in the collision with 100 eV electrons comparing to 55 eV electrons, and then more fragmentation channels were involved with a higher energy release. It is the same situation for the momentum distribution of 3H shown in Fig. 8(b). However, the missed counts mainly appeared in the left part for the incident energy of 55 eV, which is corresponding to the momentum of 3H in the same direction with the momentum of C+ ion. The neutral fragments with the momentum in the same direction as C+ ion are only produced in the sequential dissociation process of CHn +∗ (n = 1, 2, 3) ions. So with increasing the incident energy of electrons from 55 eV to 100 eV, there are more excited CHn +∗ (n = 1, 2, 3) ions produced in the first fragmentation step and then these broke up to form C+ ions in the second fragmentation step. This can be confirmed by the slopes of C+ -H+ ion-ion pair in Table I, which varied from −0.80 to 0.61 with the incident energy increasing from 55 eV to 100 eV. To obtain more information about the fragmentation mechanism, the Newton diagram and Dalitz plot for the Coulomb explosion process of CH4 2+ → C+ + H+ + 3H induced by 100 eV electron impact is presented in Fig. 9. The momentum of neutral fragments ranges from −40 a.u. to 20 a.u. in Fig. 9(a). And the reaction channels could be recognized in Fig. 9(b). The points in the bottom of the triangle
should be produced in a synchronous concerted fragmentation process, and those points closed to the edge of H+ ion in the left part could be produced in a sequential fragmentation of H2 +∗ ion. Since there are more sequential reaction channel to produce C+ ion in the two-step dissociation process of CHn +∗ (n = 1, 2, 3) ions, the distribution of the experiment points in Fig. 9(b) is shifted to the edge of C+ . One also can find some points in the middle of the triangle, which could be produced in the sequential fragmentation of both H2 +∗ and CHn +∗ (n = 1, 2) ions. In fact, by comparing the momentum of neutral fragment shown in Figs. 5(a), 7(a), and 9(a), one could find that the maximum momentum of neutral fragments was always smaller than 20 a.u. with the same direction as the momentum of H+ ion. This could be explained in a simple way. Assuming the neutral H atoms are produced in the sequential fragmentation process, in which the excited H2 +∗ ion broke up into H+ and H. The 20 a.u. momentum of neutral H fragments is just half of the maximum momentum of H2 + , corresponding to the second step happened far away from the Coulomb explosion center. However, the maximum momentum of neutral fragments with the same direction as CHn + (n = 0, 1, 2) increased from 20 a.u. in Fig. 5(a) to 40 a.u. in Fig. 9(a). This cannot be interpreted that the momentum was averagely shared by fragments during the sequential dissociation, because the mass of C+ is much bigger than 3H. To explain the slope of C+ -H+ -3H, Flammini et al.9 have assumed that the fragments CH3 +∗ keeping a pyramidal form after the first fragmentation. We think it could be the case in the present measurement. The three neutral H were launched in the same time with an opposite momentum direction of H+ , and the C+ as the top of the pyramid was decelerated in an asynchronous concerted fragmentation, in which the momentum was mainly carried by the neutral fragments. Nevertheless, to understand this, we need more theoretical analysis in the future.
V. CONCLUSIONS
The fragmentation of CH4 2+ dications induced by 55 eV, 75 eV, and 100 eV electrons was studied using a COLTRIMS. With the measured momentum of each recoil ion, the momentum of neutral particle has been deduced and the kinetic energy release distribution for the different fragmentation channel has been plotted. The KERs for the reactions
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related to the ions pair of CH3 + -H+ , CH2 + -H2 + , and CH2 + H+ -H are constant for different incident energy of electrons in the present measurement energy region. This can be interpreted that the incident energy of 55 eV is enough to excite the CH4 molecule proceeding the fragmentation corresponding to those fragments. With increasing incident energy, the excitation energy of CH4 2+ increases and more reaction channels will be opened. This could be confirmed by comparing the KER for those reaction channels related to the fragments of CH+ -H+ -2H and C+ -H+ -3H in Table II. The fragmentation mechanism for the doubly charged molecular ion breaking up into three or more fragments was studied here. The slopes of the peak in the ion-ion coincidence spectrum have also been estimated. Using the benefits of COLTRIMS, the momentum of the neutral fragment was deduced using the principle of momentum conservation. Based on the momentum of the neutral particles, the fragmentation of CH4 2+ dications in either synchronous concerted or a two-step manner has been distinguished experimentally. Namely, the neutral fragments produced in the two-step fragmentation shared the momentum with the charged fragments. However, in the synchronous concerted fragmentation, the neutral fragments did not take part in the Coulomb interaction, and thus there is no momentum sharing between the neutral fragments and the charged fragments. For the formation of CH2 + , H+ , and H from CH4 2+ , we have clarified the existence of one synchronous concerted reaction channel and two two-step reaction channels. With the help of the Dalitz plot, for the sequential reaction channel, the neutral H produced in the delayed dissociation of CH3 +∗ and H2 +∗ , could be distinguished in our measurement. The same fragmentation pattern has been identified for the dissociation of CH4 2+ dications related to the fragments of CH+ -H+ and C+ -H+ in the present incident energy region. Furthermore, sequential fragmentation processes have been observed in which both of the two charged fragments are in excited states with a delayed dissociation.
ACKNOWLEDGMENTS
This work was supported by the National Magnetic Confinement Fusion Program under Grant No. 2009GB106001, IAEA (CRP; No. 15735) and the National Science Foundation of China under Contract No. 10904019. This work was also supported by Shanghai Leading Academic Discipline Project (Project No. B107). 1 R.
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