Photofragmentation spectra of halogenated methanes in the VUV photon energy range Antonella Cartoni, Paola Bolognesi, Ettore Fainelli, and Lorenzo Avaldi Citation: The Journal of Chemical Physics 140, 184307 (2014); doi: 10.1063/1.4874114 View online: http://dx.doi.org/10.1063/1.4874114 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/140/18?ver=pdfcov Published by the AIP Publishing Articles you may be interested in One-photon mass-analyzed threshold ionization (MATI) spectroscopy of pyridine: Determination of accurate ionization energy and cationic structure J. Chem. Phys. 141, 174303 (2014); 10.1063/1.4900569 Publisher's Note: “Photofragmentation spectra of halogenated methanes in the VUV photon energy range” [J. Chem. Phys.140, 184307 (2014)] J. Chem. Phys. 140, 249901 (2014); 10.1063/1.4884128 Combined vacuum ultraviolet laser and synchrotron pulsed field ionization study of C H 2 Br Cl J. Chem. Phys. 126, 184304 (2007); 10.1063/1.2730829 A vacuum ultraviolet pulsed field ionization-photoelectron study of cyanogen cation in the energy range of 13.2–15.9 eVa) J. Chem. Phys. 123, 144302 (2005); 10.1063/1.2037607 Dissociation of energy-selected c - C 2 H 4 S + in a region 10.6–11.8 eV: Threshold photoelectron—photoion coincidence experiments and quantum-chemical calculations J. Chem. Phys. 123, 054312 (2005); 10.1063/1.1993589

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THE JOURNAL OF CHEMICAL PHYSICS 140, 184307 (2014)

Photofragmentation spectra of halogenated methanes in the VUV photon energy range Antonella Cartoni,1,a) Paola Bolognesi,2 Ettore Fainelli,2 and Lorenzo Avaldi2 1

Dipartimento di Chimica e Tecnologie del Farmaco, Sapienza Università di Roma, P.le Aldo Moro 5, Roma 00185, Italy 2 CNR-IMIP, Area della Ricerca di Roma 1, Monterotondo Scalo (Rm) 00015, Italy

(Received 27 February 2014; accepted 18 April 2014; published online 9 May 2014; publisher error corrected 13 May 2014) In this paper an investigation of the photofragmentation of dihalomethanes CH2 X2 (X = F, Cl, Br, I) and chlorinated methanes (CHn Cl4−n with n = 0–3) with VUV helium, neon, and argon discharge lamps is reported and the role played by the different halogen atoms is discussed. Halogenated methanes are a class of molecules used in several fields of chemistry and the study of their physical and chemical proprieties is of fundamental interest. In particular their photodissociation and photoionization are of great importance since the decomposition of these compounds in the atmosphere strongly affects the environment. The results of the present work show that the halogen-loss is the predominant fragmentation channel for these molecules in the VUV photon energy range and confirm their role as reservoir of chlorine, bromine, and iodine atoms in the atmosphere. Moreover, the results highlight the peculiar feature of CH2 F2 as a source of both fluorine and hydrogen atoms and the characteristic formation of I2 + and CH2 + ions from the photofragmentation of the CH2 I2 molecule. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4874114] I. INTRODUCTION

The emission of greenhouse gases (GHGs) and ozone depleting substances (ODSs) as well as the production of agents which participate in key atmospheric chemical reactions, including formation of cloud condensation nuclei (CCN), are the major cause of climate change.1 Photolysis and the reactions of these substances with hydroxyl radical, the main oxidizing agent of the troposphere, derived from the photolysis of ozone, determine their life-time in the atmosphere. If chemical reactions generate reactive species that, for example, can destroy catalytically the ozone molecule, than the oxidizing capacity of troposphere as well as the climate will be consequently affected. Haloalkanes are a group of molecules involved in these processes. They are emitted in the atmosphere from both human activities and natural sources. In the stratosphere they interact with VUV light and produce halogen atoms which contribute to the ozone depletion process. Moreover in the atmosphere they have large global warming potential as greenhouse gases. Among haloalkanes, halogenated methanes are an important class of compounds with particular and peculiar features used in several fields of chemistry. For instance, CH2 Cl2 is used as a solvent and propellant in industry and its air emission can cause the production of the Cl radical that affects the ozone budget.2 On the other hand CH2 F2 (HFC-32, Freon 32) is a hydrofluorocarbon (HFC) molecule which has replaced chlorofluorcarbons in the refrigerant industry and it is used as etching gas.3 Although CH2 F2 has no ozone depletion potential it is a greenhouse gas a) Author to whom correspondence should be addressed. Electronic mail:

[email protected]

0021-9606/2014/140(18)/184307/13/$30.00

and plays an important role in the terrestrial atmosphere.4 As for organoiodine and organobromine compounds, it is well known that their presence in the atmosphere has predominantly natural, mostly oceanic source, by macroalgae and phytoplankton.5 In particular CH2 I2 is one of the most photolabile iodocarbons emitted in the atmosphere from marine algae.6 It has been suggested that diiodomethane is a source of reactive iodine atoms by the photoreaction with VUV light and that iodine chemistry plays an important role in ozone depletion and in the control of the oxidating capacity of the atmosphere. Furthermore iodine participates to the ultrafine marine aerosol particles formation affecting the Earth’s radiation budget and thus driving the climate change.7 Brominecontaining compounds are also of great interest and they have received considerable attention in the past few years.8 Among bromoalkanes, CH2 Br2 and CHBr3 have been the object of many studies. They are emitted primarily from oceans and are the most important among the so called “very short-lived substances” (VSLSs) bromocarbons. Actually they are rapidly destroyed in the troposphere (atmospheric lifetime shorter than six months) via reaction with hydroxyl radical and photolysis. These molecules together with mainly anthropogenic, long-lived gases like chlorofluorocarbons (CFC), bromofluorocarbons (Halons manufactured for use as fire suppressants) and methyl bromide are thought to contribute significantly to reactive inorganic bromine (Br, BrO, HOBr, HBr) after their degradation. Eventually these reactive species can enter the stratosphere and participate to catalytic destruction of ozone molecules. To understand how the emission of different molecules in the atmosphere can influence the physical properties of the atmosphere and change the climate, a detailed knowledge of

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the thermodynamic and kinetic properties of the atmospheric reactions that involve these compounds as well as of their physical parameters is needed. Up to now much effort has been devoted to the understanding of the chemical physics of halomethanes in order to manipulate and control their use and to explain their behaviour in different processes. In particular photodissociation and photoionization studies are very interesting since different fragmentation channels can lead to different radical and/or cations which can play a role in several fields, from atmospheric chemistry to etching and plasma assisted industrial processes as well as organic synthesis. Therefore, in order to assess the importance of compounds, such as halomethanes, as possible source of reactive species it is fundamental to study their fragmentation under controlled and different experimental conditions.9 Indeed, several works have been devoted to the study of the photodissociation of diiodomethane.10 Moreover other authors have reported detailed investigations on the fragmentation of CH2 Cl2 molecule by proton impact or VUV photon absorption and a dissociative photoionization study of these dihalomethane compounds.11, 12 Although several studies have been carried out to obtain thermochemical data12, 13 (appearance energy, heat of formation, ionization energy, etc.) with different radiation sources, to the best of our knowledge a comparative study of different fragmentation patterns of dihalomethanes observed under VUV radiation is not reported. In this paper we specifically present and discuss the VUV photofragmentation spectra of the series of halomethane compounds where either the halogen atom (dihalomethane, CH2 X2 with X = F, Cl, Br, I) or the number of halogen atoms (chloromethane, CHn Cl4−n with n = 0–3) are changed. The photofragmentation spectra measured at the wavelengths of a rare gas VUV discharge lamp are also compared to the electron ionization (EI) spectra reported in the NIST database.14 The main goals of this work are (i) the study in the VUV range of the two competitive dissociation channels, X-loss 5

5

10

(X=F, Cl, Br, I) and H-loss, observed in the fragmentation spectra of the dihalomethanes even though other channels have also been considered; (ii) the comparison of the fragmentation spectra of CH2 X2 molecules to assess the effect of the particular halogen atom in the photofragmentation of these molecules; and (iii) a brief study of the series of chloromethanes containing one (CH3 Cl) to four (CCl4 ) chlorine atoms to analyze the effect of the number of halogen atoms in the photofragmentation. II. EXPERIMENTAL

The molecular fragmentation spectra have been recorded using the photoelectron-photoion coincidence (PEPICO) technique together with a VUV rare gas discharge lamp and an effusive beam of the molecule under examination. A schematic of the setup is reported in Figure 1. In these experiments, the ions produced by the interaction of the photon beam with the target molecules are extracted from the interaction region by a 700 V/cm DC electric field and accelerated into a Wiley-McLaren time-of-flight (TOF)15 analyser for mass/charge analysis. The ionic fragments are detected with a pair of one inch microchannel plates (MCP) mounted in a chevron configuration. The ion signal is extracted from a 50  impedance matched anode. After being preamplified and shaped into a TTL waveform through a constant fraction discriminator the ion signal is used as the STOP signal of the time-to-amplitude converter (TAC, Ortec 567) NIM unit. The START to the TAC is provided by the signal of an electron channel multiplier (CEM) that detects the electrons produced by the photoionisation of the sample molecules. The CEM, model Dr. Sjuts KBL5RS, has a 10 mm diameter cone and is mounted opposite and coaxial with the TOF spectrometer. A 10 mm aperture in the ion repeller electrode is covered with a 90% transparency gold mesh (the same type used for the ion TOF) and allows the electrons from the interaction region 225

10

Channeltron 20

+3250V

+150V +350 V

0V

E

-350V

A

-1400V

30

MCP

-3500V

-3600V

-1800V

F

FIG. 1. Schematic of the PEPICO set-up, where E, A, and F represent the extraction, acceleration, and free-flight regions, respectively. The typical voltages to the electrodes and detector as well as the geometrical size of the different elements (in mm) are also reported.

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to reach the CEM. The repeller and extractor electrodes of the TOF are oppositely polarized in order to have the interaction region at ground voltage. According to SIMION16 simulations this configuration should provide a 4π detection efficiency for both electrons and ions up to at least 15 eV kinetic energies. The output signal of the TAC is analysed by the 12 bit analog-to-digital converter (ADC) of a NI 6024E multifunction PCI card. The acquisition of the ADC is triggered by the TAC “trigger signal,” being properly delayed. A LabView program is used to acquire the ADC reading and to convert the voltage scale into the time-of-flight scale according to the corresponding length of the TAC window. A TAC window of 10 μs was used for the CH2 Br2 and CH2 I2 molecules, while in all other cases a 5 μs window was enough for the parent ion to reach the MCP detector. A time resolution of about 2.4 and 1 ns/channel is achieved in the TOF spectrum in the 10 and 5 μs TAC time windows, respectively. The photon source is a rare gas discharge lamp used to produce several emission lines. The most intense lines17, 18 are at 21.22 eV (He I), 16.67 eV (Ne I), and 11.62 eV (Ar I). The discharge lamp was operated with a gas pressure in the discharge chamber that varies from 10−1 to 10−3 mbar depending on the rare gas and a driving current of 5 mA. The emission current ranges from 0.1 to 5 pA depending on the rare gas used, as measured by an aluminium Faraday cup collecting the incident beam. The 1 mm bore quartz capillary was terminated with a collimator made of a set of two coaxial 0.5 mm apertures 30 mm apart. The last aperture of the collimator is located outside the TOF electrodes, 55 mm away from the interaction region. The photon beam size at the interaction region is estimated to be about 1.2 mm. The output radiation has not been monochromatised so that unquantified contribution of wavelengths different from the ones listed above cannot be excluded.19 The effusive beam of the target molecules is brought to the interaction region with an electrically insulated needle of 0.5 mm inner diameter and 50 mm length, located about 2 mm below the photon beam. Due to the opposite polarization of the repeller and extraction electrodes in the present experiments, the needle placed at the centre of the extraction zone was not biased, but connected to ground. The base pressure in the experimental chamber is 3 × 10−8 mbar, while a background pressure of about 2 × 10−6 mbar was maintained during the PEPICO experiments. The samples were purchased from Sigma-Aldrich with a purity of 98% or better and used without further purification. Except for CH2 F2 and CH3 Cl, which are gaseous species, all other samples are liquid at room temperature. They were maintained in a test tube outside the vacuum chamber and connected to the gas line to be admitted to the interaction region via the hypodermic needle. The test tube and gas line can be gently heated to prevent condensation or baked for cleaning up. The spectra were measured at the wavelengths of the He, Ne (all samples), and Ar (CH2 I2 , CHCl3 , and CCl4 ), gases in the discharge lamp. The raw TOF spectra have been transformed into mass spectra according to the following equations: m/z = A + Bt 2 ,

(1)

 Icorrected

1 = Imeasured 2B

B , m/z − A

(2)

and normalized to the intensity of the most intense fragment ion. In Eq. (1), representing the time (t) to mass-over-charge (m/z) conversion, A and B parameters are obtained in a calibration procedure where a quadratic fit is used to represent the peak positions of known fragments in the TOF spectrum. In Eq. (2), I represents the measured/corrected intensities in the mass spectrum. The areas of the peaks corresponding to the four fragments relevant to the present work (CH2 X2 .+ , CHX2 + , CH2 X+ , and CHX+ ) and of the peaks relative to the I+ /I2 + ions in the CH2 I2 molecule have been evaluated by a fitting procedure using gaussian functions where the peak position, width, and area are used as fitting parameters. For each molecule and photon energy the sum of the considered areas has been normalized to 100. Tables I and II report their relative intensities. The errors in the relative intensity are calculated by the error on each area given by the fitting procedure and the formulas of error propagation. A similar approach has been used for the chloromethanes. The results are reported in Tables III. In the evaluation of the areas the contributions of C, Cl, and Br isotopes of fragment ions have been taken into account. Since the TOF mass resolution is less than one unit for m/z > 100, in the CH2 Br2 , CH2 I2 , and CHCl3 cases the peaks of the M.+ and [M-H]+ ions as well as of [M-X]+ and [M-HX]+ ions could not be resolved or deconvolved from the mass spectra. Consequently in Tables I and III the areas indicated with * are the sum of these nearby contributions, i.e., area (CH2 X2 .+ + CHX2 + for Br and I) and area (CH2 X+ + CHX+ for I). The notation of HX-loss channel that produce CHX+ ions, if not specifically stated, indicates the loss of either HX molecule or H and X separated atoms. III. RESULTS AND DISCUSSION

The mass spectra of the dihalomethanes CH2 X2 (X=F, Cl, Br, I) and chloromethanes CHn Cl4−n (n = 0–3) measured using the vacuum ultraviolet (VUV) He, Ne, and Ar discharge lamps as ionizing sources together with the spectra reported by the NIST database measured at 70 eV electron impact (EI) energy are shown in Figures 2, 4, 6, and 8. For all cases, in the region of interest the mass spectra display a similar general structure, with two main groups of peaks corresponding to the molecular ion and the loss of one halogen atom, respectively. The substructure of each group is due to the isotopic distribution of the halogen atom and the additional hydrogenloss channel. I+ ions and very weak signals due to Cl+ and Br+ cations are also observed in the mass spectra of their respective dihalomethane, while no F+ ions are detected in the case of the difluoromethane molecule. Only in the case of the CH2 I2 molecule the I2 + and CH2 + fragments have been observed (Figure 2 and Table II). Due to the different nature of the photon and electron interaction with matter the mass spectra recorded using VUV radiation display less fragmentation than those obtained with EI sources. Moreover in the case of the rare gases discharge lamp, the photon energy is definitely lower than the typical

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1.3 ± 0.1 3.9 ± 0.3 4.9 ± 0.3 3.5 ± 0.1 6.1 ± 0.2 5.1 ± 0.3 4.9 3.9 3.0 3.5 13.30c ,17.7 12.46d 12.59f , 16.0 ...

I+ I2 + I+ /I2 +

72.3 ± 0.7 56.3 ± 0.8 33.7 ± 0.4 41.2 ± 0.7* 46.2 50.4 44.4 40.0 45.3 ± 0.8 1.2 ± 0.1 20.8 ± 0.9 1.6 ± 0.2 44.7 1.5 0.7 0.9

He (21.22 eV)

Ne (16.67 eV)

76.3 23.7 3.2

70.3 ± 1.8 29.7 ± 0.9 2.4

63.1 ± 3.8 36.9 ± 2.4 1.7

A. Mass spectra of CH2 F2

b

a

IE and AE (in bold) not labelled are from Ref. 14. Reference 13(c). c Reference 4(c). d Reference 13(e). e Reference 13(h). f Reference 13(g).

13.11 13.00 12.64f ... 8.7 ± 0.2 41.9 ± 0.9 54.1 ± 0.7* 57.7 ± 1.2* 3.4 ± 0.3 36.0 ± 0.9 61.2 ± 1.0* 58.8 ± 1.0* 4.2 44.2 51.9 55.6 12.71 11.33 10.24e 9.46 CH2 F2 CH2 Cl2 CH2 Br2 CH2 I2

EI (70 eV)

70 eV used in EI. Indeed depending on the gas used, the photon energy can be in the near threshold region for the photoionisation, where more significant changes in the cross section occur, and even below the appearance potentials of some fragmentation channels. Clearly, these effects are reduced with the He lamp. At 21.22 eV photon energy all the ionization channels of the valence and inner valence states of most molecules can be accessed.20,11 In Table I the branching ratios of the four ions of the dihalomethane molecules investigated in this work as well as the ones from the EI14 experiments are reported, together with the ionization energy (IE) for each molecule and the appearance energy (AE) of the fragment ions. The same information is also displayed in Figures 3 and 5. The results of the study on the chloromethanes are collected in Table III and Figure 7. In Sec. III A–III C a detailed description and discussion of the mass spectra of each CH2 X2 dihalomethane obtained with the different ionising sources is reported with the main focus on the H-loss versus X-loss dissociation in the CH2 F2 and CH2 Cl2 molecules. In Sec. III D a comparison of the fragmentation spectra of the different CH2 X2 molecules is discussed together with their implication for atmospheric science. In Sec. III E the photofragmentation spectra of different chlorine containing molecules CH3 Cl to CCl4 are briefly analyzed and compared.

14.30b 12.10 11.27 10.42

He EI AE (eV) Ne He EI Ne EI IEa (eV) Molecule M

TABLE II. Relative % intensities of I+ and I2 + ions and the I+ /I2 + ratio from fragmentation of the CH2 I2 molecule with EI, He and Ne ionization source.

44.7 ± 1.0 53.0 ± 0.9 41.0 ± 0.5 42.3 ± 0.7*

EI AE (eV)

He

J. Chem. Phys. 140, 184307 (2014)

He

AE (eV)

Fragmentation channel % H-loss ( CHX2 + )

% X-loss (CH2 X+ )

Ne

% HX-loss (CHX+ )

Ne

Cartoni et al.

% M.+ (CH2 X2 .+ )

TABLE I. Relative intensities of the parent (M.+ ) and the fragment ions of CH2 X2 molecules (X=F, Cl, Br, I) in EI, and VUV photoionization with He and Ne discharge sources. For each molecule the sum of the four considered channels is normalized to 100%. The areas indicated with * correspond to the sum of the areas of the peaks due to (CH2 X2 .+ + CHX2 + , X = Br and I) ions and (CH2 X+ + CHX+ , X = I) ions (see text).

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The mass spectra of the CH2 F2 molecule measured with the three different ionising sources, EI, He, and Ne lamps, are shown in Figures 2(a)–2(c), respectively. In all spectra the molecular parent ion (CH2 F2 .+ , m/z = 52), the H-loss product (CHF2 + , m/z = 51), the F-loss product (CH2 F+ , m/z = 33), and the HF-loss product (CHF+ , m/z = 32) are observed. CHF2 + and CH2 F+ are the most prominent ionic fragments detected in the present study as well as in previous EI4(c), 14 and photoionisation experiments.13(c) The dissociation channel yielding the CF+ and CF2 + ions (m/z = 31 and 50, respectively) are also observed, but they will not be discussed in this work. The small peak at m/z = 28 is due to nitrogen impurity. The relative intensities of the CH2 F2 .+ , CHF2 + , CH2 F+ , and CHF+ ions are reported in Table I and shown in Figure 3(a). The IE of CH2 F2 and the AE of the ionic fragments, ranging from 12.71 eV to 14.30 eV are below the lowest photon energy used in this study (NeI at 16.67 eV)18 and consequently all the peaks relative to these species are observed. At all the investigated energies the intensity of the parent ion

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0.7 ± 0.1 3.9 ± 0.3 1.7 ± 0.1 6.1 ± 0.2 7.1 ± 0.4 3.8 3.9 4.7 14.6 12.46c 12.2 55.0 ± 3.5 53.0 ± 0.9 94.1 ± 0.8* 100 51.5 ± 3.8 56.3 ± 0.8 88.1 ± 0.7 100 32 50.4 90.6 100 13.33b 12.10 11.49 11.28 3.0 ± 0.2 1.2 ± 0.9 1.3±0.1 2.0 ± 0.1 1.6 ± 0.2 1.0±0.1 6.2 1.5 1.9

IE and AE (in bold) not labelled are from Ref. 14. Reference 39. c Reference 13(e). b

41.3 ± 1.2 41.9 ± 0.9 4.6 ± 0.1 44.8 ± 2.4 36.0 ± 0.9 3.8 ± 0.1 58 44.2 2.8 11.26 11.33 11.37 11.47 CH3 Cl CH2 Cl2 CHCl3 CCl4

a

AE (eV) Ne He EI AE (eV) Ne He EI AE (eV) Ne Molecule M

IEa

(eV)

EI

He

12.84b 13.00 11.7

He

Ne

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EI

% HX-loss % X-loss Fragmentation channel % H-loss % M.+

TABLE III. Relative intensities of the parent ions (M.+ ) and the fragment ions of chlorinated molecules in EI, and VUV photoionization with He and Ne discharge sources. For each molecule the sum of the four considered channels is normalized to 100%. The areas indicated with * are the sum of CHCl2 .+ + CCl2 + ions.

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CH2 F2 .+ is less than 10% of the total intensity. This suggests a large instability of the parent ion, at least on the time scale of the present experiments ( 26.81 eV), a fraction of the photon beam might have an energy higher than the quoted 16.67 eV. The characterisation of a rare gas discharge lamp described by Schonhrnse and Heinzmann19 suggests the Ne II component to be in the order of 10% or less with respect to Ne I. As a consequence, the small m/z = 32 fragment observed in Figure 2(c) might be generated by this higher photon energy beam. Therefore, we cannot rule out its attribution to the CHF+ + H + F channel. Therefore whether the lowest AE of the CHF+ fragment observable in photofragmentation studies has to be attributed to the CHF+ + HF or to the CHF+ + H + F channel still remains an open question. The CHF+ fragment is observed with a small relative intensity at all the investigated energies, being 4.9% in EI, 3.5% and 1.3% with He and Ne discharge lamps, respectively (Table I). The low probability of formation of CHF+ may be due to (i) unfavorable Franck-Condon factors from neutral to intermediate states with different structural features and/or (ii) simultaneous bond-breaking/bond-making processes involved in this channel as well as (iii) fragment ion instabilities.

EI He Ne

(a)

60 40 20

0

.+ H-loss CH2F2

F-loss

HF-loss

Fragmentation Channels

80

Ionic yield (%)

Ionic yield (%)

80

EI He Ne

(b)

60 40 20 0 CH2Cl2.+

H-loss

Cl-loss

HCl-loss

Fragmentation Channels

FIG. 3. Comparison of the relative intensities reported in Table I versus the analyzed fragmentation channel for CH2 F2 (a) and CH2 Cl2 (b) with EI source (, black dashed line), He (, full red line), and Ne (●, dotted blue line) lamps.

As shown in Figure 3(a) the fragmentation channels leading to CHF2 + and CH2 F+ ions are predominant in the CH2 F2 dissociative ionization. These processes involve the breaking of the C–H and C–F single chemical bonds, whose dissociation energies from termochemical data amount to 431.8 kJ/mole and to 496.2 kJ/mole, respectively, for the CH2 F2 molecule24 and 56.0 kJ/mole and 139.4 kJ/mole, respectively, in CH2 F2 + .14, 25 This higher dissociation energy of the C–F bond with respect to the C–H one might explain the higher AE of the CH2 F+ (14.30 eV) with respect to the CHF2 + fragment (13.11 eV). As we can see in Table I and Figure 3(a) the relative intensities of the CHF2 + and CH2 F+ ions are very similar to each other in both EI and Ne discharge lamp experiments. However, a large difference is observed in the spectrum measured with the He discharge lamp, where the F-loss product (72.3%) largely prevails over the H-loss product (20.8%). This is somehow surprising because one expects the photoion mass spectra should becoming more and more similar to the EI ones as the photon energy increases as already mentioned in Sec. III. In order to explain this “unusual” intensity behaviour we have analysed the accessible electronic states of the ion that may contribute to the mass spectra measured with the different ionizing sources. The HeI photoelectron spectra, PES, of the CH2 F2 molecule, its threshold photoelectron, TPES, and threshold photoelectron-photoion coincidence, TPEPICO, spectra have been measured with synchrotron radiation26 and several ab initio calculations of molecular orbital energy are reported in the literature.26(a), 27 In particular, the TPEPICO technique allows to associate the fragmentation channels observed in the mass spectrum to selected electronic states of the ions. It is worth of note that the energy ordering of the molecular orbitals in the CH2 F2 compound is not always consistent among different authors. In the TPES of CH2 F2 reported by Tuckett et al.,21 four bands are observed at 13.3, 15.5, 19.1, and 24 eV. Among them the X2 B2 band at 13.3 eV shows a vibrationally resolved structure from 12.6 to 14.0 eV. The more selective TPEPICO

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measurements reveal that the ground electronic state of the ion remains stable and bound only in the lowest vibrational states and up to the opening of the H-loss dissociation channel at 13.08 eV, thereafter efficiently dissociating into CHF2 + + H. Indeed, the calculated molecular orbital energy of the highest occupied molecular orbital (HOMO) and its strong C–H bond character reported by several authors show clearly that the removal of an electron from this orbital can be expected to result in the weakening of this bond and subsequent H-loss.4(c), 26(a), 26(d) This explains, as a direct consequence, the small intensity of the molecular ion CH2 F2 .+ compared to the CHF2 + fragment reported in the present as well as in previous works (Table I and Figure 3(a)). The second band observed in the PES of CH2 F2 in the energy range from about 14.5 to 17 eV, is associated to three ionic states and mainly dissociates to CHF2 + and CH2 F+ fragments with the loss of the H and F atoms, respectively.21 Moreover Castaño et al.4(c) show that their measured AE for CH2 F+ ions of 14.9 eV is comparable with the energy required to remove one electron from the pure C–F bonding 4b1 orbital. This may explain the mass spectra recorded with Ne discharge lamp, where the comparable intensity of these two fragment ions (Figure 3(a)) suggests that the H-loss and F-loss are two strongly competing fragmentation channels. Interestingly, the F-loss pathway appears to be predominant over the H-loss at 21.22 eV, the energy of the He discharge lamp, returning to a more balanced situation at 70 eV in the EI experiments. Indeed, as shown in the photoelectron spectra, the HeI radiation enables to access new electronic states in the binding energy range 18–20 eV which can be involved in the photofragmentation process. These molecular orbitals have a predominant C–F bonding character, thus making the F-loss channel favourable. A large photoionisation cross section in the near threshold region may explain why the F-loss channel is enhanced due to the contribution of these orbitals in the He discharge lamp spectra, while being comparable in intensity with respect to the H-loss channel in the Ne discharge lamp and EI spectra (Figure 3(a)), where their contribution is either absent (at 16.67 eV) or mixed with many other processes (at 70 eV). B. Mass spectra of CH2 Cl2

The mass spectra of the CH2 Cl2 molecule measured with the three different ionization sources are shown in Figures 2(d)–2(f) while the relative intensities of the four fragmentation channels of interest are reported in Table I and Figure 3(b). The three mass spectra do not show major changes suggesting that the molecular fragmentation is dominated by processes already occurring in the lower energy region, i.e., CH2 Cl2 .+ > CH2 F2 .+ regardless of the radiation source used and reveals the higher stability of the radical cation containing the halogen atom with lower electronegativity. This result is also consistent with the calculated difference of IE of the CH2 X2 molecule with respect to the AE of its lowest energy dissociative channel, i.e., the H-loss for CH2 F2 and Xloss for the other dihalomethanes. From Table I we can calculate (AE − IE) = 0.4, 0.77, 1.03, and 0.96 eV for the difluoro, dichloro, dibromo, and diiodomethane, respectively. This quantity decreases passing from X = I and Br to Cl and then F, showing a similar trend as for the relative intensity of the molecular ions and supporting the observation that more energy is required to fragment the molecular ions with less electronegative halogen atoms. In all dihalomethanes the CHX+ ion displays a very low intensity, showing that the HX-loss fragmentation channel, where more than one bond is broken, is not favoured compared to other fragmentation patterns. The X-loss channel, on the other hand, is an important dissociation channel for all dihalomethanes and is always observed with significant intensity. It can be asserted that it is the only important dissociative pathway for the CH2 Br2 and CH2 Cl2 molecules, whose implication in atmospheric field has been extensively studied.34 Moreover, in the case of CH2 Cl2 , the CH2 Cl+ ion associated to the loss of the chlorine atom is always the most intense peak in the mass spectra. The loss of the halogen atom is also the main fragmentation pathway for CH2 I2 .+ radical cations, but in this case two other small, but not 80

He

CH2Br2 60

40

20

60 Ionic yield (%)

CH2I2 Ionic yield (%)

Ionic yield (%)

60

40

20

0

0 molecular ion H-loss

X-loss

HX-loss

Ne

40

20

0

molecular ion

H-loss

X-loss

HX-loss

molecular ion H-loss

X-loss

HX-loss

FIG. 5. Graphical representation of experimental data collected in Table I for CH2 F2 (, black); CH2 Cl2 (●, red); CH2 Br2 (, green); and CH2 I2 (, blue).

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negligible peaks are observed (Figures 2(l)–2(n) and Table II) assigned to I+ and I2 + ions. Moreover the loss of the I2 molecule, leading to CH2 + ion, is another peculiar feature of diiodomethane in our experimental conditions (Figures 2(m) and 2(n)). The CH2 F2 molecule has also a “unique” behaviour among the dihalomethanes, being the only one to fragment importantly via the H-loss channel, leading to CHF2 + ions and H atom. Indeed, as can be seen in Figure 5, the F-loss product prevails when the He discharge lamp is used, while it is comparable to the H-loss product when the Ne discharge lamp and EI are used. These findings have been explained on the basis of the molecular orbitals that can take part in the photofragmentation process and demonstrate how fluorine atom makes the physical chemistry of the molecule different from those containing the other halogen atoms. Hence in the VUV photodissociation experiments the CH2 F2 molecule is not only a source of fluorine atom but also of hydrogen atom. These degradation processes should be taken into account in those fields where CH2 F2 is involved, as in atmospheric chemistry. Indeed, the CH2 F2 molecule is a simple HFCs used for different purposes, mainly as a refrigerant, and it can be released into atmosphere. Its atmospheric lifetime is long enough to allow CH2 F2 to reach the upper atmosphere.35 Here, although the reaction with the O(1 D) radical is possible, difluoromethane might also interact with VUV radiation producing fluorine and hydrogen atoms, as clearly shown in the present work. Fluorine is rapidly transformed in HF,36, 37 whose contribution to aerosols particulates formation and acid rain should be considered. On the other hand, the hydrogen atom is a very reactive species in the atmosphere and may affect the chemistry of different molecules like ozone.38 It is important to point out that the degradative oxidation studies together with the photofragmentation investigations of molecules of environmental interest are fundamental to understand the complex processes happening in the atmosphere and to propose suitable alternative compounds to harmful chemicals produced by human activity. HFCs as CH2 F2 seem to be harmless for the ozone layer but they can release potentially highly reactive hydrogen atoms and have a dangerous global warming potential, which makes these molecules not completely safe. Indeed, much effort is already underway and more still must be made to find the really suitable “green” chemicals, with the lowest environmental impact.1(b), 4(e) E. The fragmentation mass spectra of chloromethanes: CH3 Cl, CH2 Cl2 , CHCl3 , CCl4

The dissociative photoionization of chloromethanes has been the object of many studies since the 1960s. The competitive pathways of hydrogen versus halogen loss have been extensively investigated by many authors, reporting that the most intense signal in the photofragmentation spectra is due to ions produced by the chlorine loss. These former results have also highlighted that in the case of the CH3 Cl molecule, even though the hydrogen loss is energetically favoured (AE = 12.8439 or 13.0240 eV) to the chlorine loss (AE = 13.33 eV39 or 13.3840 ), the Cl-loss product ion CH3 + is the main or the only fragment ion detected.13(a), 13(d), 13(f), 40 The issue has been resolved by theoretical and experimental

J. Chem. Phys. 140, 184307 (2014)

investigations which concluded that the H-loss channel comes from the contribution of an autoionization process to the ionic ground state of CH3 Cl+ ion.41 It is interesting to note that chloromethane, CH3 Cl, is the only halogenated molecule considered in this work in which the higher bond energy reported for the C–H (419.0 kJ/mole) with respect to the C–Cl bond (350.2 kJ/mole) in the neutral molecule24 becomes lower in CH3 Cl.+ parent ion, i.e., 177.0 kJ/mole and 219.3 kJ/mole, respectively.14, 25 A possible explanation is that in the CH3 Cl+ the charge mainly localized on Cl atom13(c) makes the C–Cl bond stronger. As a result, the loss of the hydrogen atom is a lower energy process for the parent ion, as observed both experimentally and theoretically. In this section the VUV photofragmentation mass spectra of chlorinated methanes recorded with He and Ne discharge lamps as photoexcitation source are presented and compared also with the EI mass spectra (Figure 6). In Table III the relative intensities of the studied fragment ions of chloromethanes CH3 Cl, CH2 Cl2 , CHCl3 , and CCl4 obtained using VUV lamps are reported together with the data of EI ionization.14 The values are also graphically represented in Figure 7. In the case of CCl4 and CHCl3 molecules the photofragmentation spectra with Ar discharge lamp (Figure 8) have been also recorded because they are the only chlorinated methanes having fragment ions with an appearance energy lower that the most intense photoline of Argon (11.62 eV). The present results, in good agreement with previous experimental findings, show the dominant Cl-loss fragment ions in all mass spectra even though the molecular radical cations seem to have different stabilities against dissociation depending on the radiation sources used.40,41(a),42 Focusing on the molecular ions, their intensities decrease in the following order CH3 Cl > CH2 Cl2 > CHCl3 > CCl4 (Figure 7). In particular, although a tiny signal of CCl4 .+ has been observed in some earlier experiments42(a), 42(b) the CCl4 .+ parent ion was not observed neither in the present photoionization experiments nor in the electron impact. The Cl-loss is the main fragmentation pathway and the intensity of the associated fragment ions decrease in the following order: CH3 + (CH3 Cl) < CH2 Cl+ (CH2 Cl2 ) < CHCl2 + (CHCl3 ) < CCl3 + (CCl4 ) (Table III and Figure 7). These results reveal that the increase of the halogen substitutions leads to (i) the increase of the molecular ion instability, (ii) the decrease of the C–Cl bond energy,24 and finally (iii) the decrease of the AE of the Clloss fragmentation channel (Table III). Probably the high electronegativity of the chlorine atom makes the ion more and more unstable with the increase of the number of the chlorine atoms in the molecule and hence the loss of chlorine become more and more energetically favourable up to the CCl4 case with no evidence of the CCl4 .+ parent ion. In the CCl4 mass spectrum obtained with the Ar discharge lamp (Figure 8), no parent ion CCl4 .+ is observed and CCl3 + is the only fragment (AE = 11.28 eV, Table III). In the Ne spectrum (Figure 6) also the CCl2 + ion (AE = 14–16 eV)14 is detected, while the CCl+ ion (AE = 17– 19.4 eV)14 appears only in the He spectrum. In all cases the CCl3 + fragment dominates the spectrum. At variance with the CCl4 , the CHCl3 molecule displays a contribution from the molecular ion, CHCl3 .+ , together with

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CCl4

(a)

J. Chem. Phys. 140, 184307 (2014)

CHCl 3

CCl3+

(d)

CH3Cl

CH2Cl2

+ CHCl2

(g)

CH2Cl2

EI

He

Intensity (arb. units)

CCl+ Cl+

CH3Cl

(l)

+ CH2Cl

.+

CH3

.+

+

+ CCl

CCl2+ Cl

+ CHCl3

.+

(b)

(e)

(h)

(c)

(f)

(i)

(m)

(n)

Ne impurity

40

60

80

100 120 140 160

40

60

m/z

80 m/z

100

120

20

40

60

100 10

80

20

30

40

50

60

m/z

m/z

FIG. 6. Mass spectra of chloromethanes CCl4 (a)–(c), CHCl3 (d)–(f), CH2 Cl2 (g)–(i) and CH3 Cl (l)–(n) obtained with EI source (top row) and He and Ne lamps (middle and bottom rows, respectively).

the dominant CHCl2 + ions (AE = 11.49 eV, Table III and Figure 8). In the mass spectra recorded with He and Ne discharge lamps (Figure 6), also an intense signal due to the CCl+ (AE = 16.3 eV)14 ions associated to more complex fragmentation processes together with weak signals due to CCl3 + , CCl2 + , CHCl+ ions are present. The H-loss and HClloss channels can be neglected in all chlorinated methanes. These results support earlier studies on the CH3 Cl molecule, where a tiny or no H-loss product was observed although this channel is energetically favoured. In order to explain the features of chlorinated molecules extensive experimental and theoretical investigations have been performed. Much effort has been devoted by several research groups to explain the absence or very weak signal due to CCl4 .+ radical cation in the mass spectrometric studies, as in the case of CF4 molecule.43, 44 PES spectra together with PEPICO, TPEPICO, and velocity imaging photoionization

coincidence VIPCO measurements showed that the three lowest energy bands of CCl4 .+ , obtained removing non-bonding p electrons, all dissociate to CCl3 + + Cl.28(a), 44 Moreover, the thermochemical measurements: (i) D0 (CCl4 + → CCl3 + + Cl) = AE(CCl3 + ) − AE(CCl4 + ) = −0.13 eV44(c) (or −0.19 Table III) or (ii) Hf ◦ (CCl3 + ) + Hf ◦ (Cl) − Hf ◦ (CCl4 + ) = −0.60 eV14, 25 indicate that the Cl-loss dissociation channel is exothermic and hence favoured. Recent theoretical studies13(d) found more than one distorted geometrical structure for the cations of chloromethanes and, in the case of the CHCl3 and CCl4 molecules, the calculations indicate the [CCl2 -ClH]+ and [CCl3 -Cl]+ ions, in the C1 and Cs symmetry, respectively, as more stable geometries than their tetracovalent isomers CHCl3 + and CCl4 + in the C3v and C2v symmetry, respectively. As reported by those authors the CCl4 + ion with C2v symmetry isomerizes rapidly and with a very low energy barrier (0.13 eV) into the more stable [CCl3 -Cl]+

CH3Cl CH2Cl2

EI

CCl4

60 40 20

He

100

80 Ionic yield (%)

80 Ionic yield (%)

100

CHCl3

60 40 20

0 Cl-loss

HCl-loss

60 40 20

0 molecular ion H-loss

Ne

80 Ionic yield (%)

100

molecular ion H-loss

Cl-loss

HCl-loss

0 molecular ion H-loss

Cl-loss

HCl-loss

FIG. 7. Graphical representation of the experimental data collected in Table III for CH3 Cl (, black); CH2 Cl2 (●, red); CHCl3 (, green); and CCl4 (, blue).

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Ar (11.62 eV) CCl3+

Intensity (arb. units)

CCl4

No CCl4

CHCl

CHCl2

+

3

CHCl3 40

.+

60

80

.+

100 120 140 160 m/z

FIG. 8. Mass spectra of CCl4 (top) and CHCl3 (bottom) recorded with Ar discharge lamp (11.62 eV).

process, which might be relevant in the upper atmosphere. In addition to atmospheric field, the study of CH2 I2 molecule finds an increasing interest in nuclear chemistry where the understanding of the mechanisms involved in its degradation is of crucial importance. For this reason further studies of this important compound have been undertaken. The fragment ions associated to the loss of the halogen atom dominates all the mass spectra of halomethanes and it is the only intense fragmentation channel observed for CH2 Br2 and chloromethanes, confirming the role of these compounds in atmospheric science as a source of halogen atoms. Moreover, the CCl4 .+ molecular ion has not been detected in our VUV photofragmentation experiments in agreement with its instability towards the exothermic loss of chlorine atom. This result is in line with some published results and does not support early studies, where a very low, but detectable signal of CCl4 .+ ions has been observed also in time scale of 20 μs. ACKNOWLEDGMENTS

This work is supported by the MIUR FIRB RBFR10SQZI project 2010. The authors thank P. Cafarelli for technical support. This work is dedicated to Professor Marina Attinà. 1 (a)

structure, where a C–Cl bond is stretched so much (3.569 Å) to lead to a fast dissociation into CCl3 + + Cl, therefore hampering the detection of CCl4 .+ parent ion within the time scale of our experimental set-up. On the other hand the CHCl3 + ions in the C3v symmetry have a high barrier to overcome (0.6 eV) in order to form the [CCl2 -ClH]+ ion that would naturally lead to the HCl loss. Furthermore, the AE of the Cl-loss dissociation channel is very close in energy to the IE of the CHCl3 molecule, this being only 0.03 eV away (0.12 eV Table III) and therefore favoured from the energetic point of view with respect to the HCl loss channel.13(d) The loss of chlorine atom is an endothermic fragmentation process and this might be the reason why a tiny but detectable CHCl3 .+ molecular ion is observed in our experiments with the different light sources as well as in the EI experiments (Table III). IV. CONCLUSIONS

This work reports a study on the photofragmentation of CH2 X2 molecules (X=F, Cl, Br, I) and chlorinated methanes (CHn Cl4−n with n = 0–3) using He, Ne, and Ar VUV discharge lamps. The results show the peculiar behaviour of the CH2 F2 molecule which is the only dihalomethane to dissociate via the H-loss channel yielding CHF2 + ion and H atom, with relevant atmospheric implications. Indeed, as fluorine reacts rapidly with atmospheric molecule to form HF, the reactive H atom can also affect the atmospheric environment via different chemical reactions with important molecules like ozone. As far as the CH2 I2 molecule is concerned, this is the only dihalomethane that shows small but measurable signals due to I2 + ions probably formed from molecular detachment. This fragmentation pathway is generally a high energy

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