2)6s Rydberg State.

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Photodissociation of Methyl Iodide via Selected Vibrational Levels of the B (E )6s Rydberg State 2

3/2

Hong Xu, and Stephen T. Pratt J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.5b00860 • Publication Date (Web): 06 May 2015 Downloaded from http://pubs.acs.org on May 20, 2015

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The Journal of Physical Chemistry

Photodissociation of Methyl Iodide via Selected Vibrational Levels of the

(2E3/2)6s

Rydberg State Hong Xu* and S. T. Pratt Argonne National Laboratory, Argonne, Illinois 60439, USA

The submitted manuscript has been created by UChicago Argonne, LLC, Operator of Argonne National Laboratory (“Argonne”). Argonne, a U.S. Department of Energy Office of Science laboratory, is operated under Contract No. DEAC02-06CH11357. The U.S. Government retains for itself, and others acting on its behalf, a paid-up nonexclusive, irrevocable worldwide license in said article to reproduce, prepwere derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government.

* Current Address Chemistry Department, Building 555, Brookhaven National Laboratory, P.O. Box 5000, Upton, NY 11973-5000 Tel: 630/252-4199 Email: [email protected]

ACS Paragon Plus Environment


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ABSTRACT We have determined the I 2P3/2 and 2P1/2 branching fractions following the photodissociation of methyl iodide (CH3I) via a number of vibronic bands associated with the

(2E3/2)6s Rydberg

state at excitation wavelengths between 201.2 nm and 192.7 nm. Vacuum ultraviolet light at 118.2 nm was used to ionize both the product iodine atoms and the methyl radical co-fragments, and velocity map ion imaging was used to determine the product translational energy distributions and angular distributions. The known relative photoionization cross sections for I 2

P3/2 and 2P1/2 at 118.2 nm were used to determine the corresponding branching fractions. The

results extend our earlier work at 193 nm by Xu et al. (J. Chem. Phys. 2013, 139, 214310), and complement the closely related work of González et al. (J. Chem. Phys. 2011, 135, 021102). We find that for most of the excited vibronic levels of the

state studied, the I 2P3/2 branching ratio

is small, but non-zero, and that this channel is associated with internally excited CH3 radicals. The results are discussed in relation to the recent theoretical results of Alekseyev et al. (J. Chem. Phys. 2011, 134, 044303).

2


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I . INTRODUCTION The photodissociation of polyatomic molecules often results in complex behavior that is reflected in product branching fractions, state distributions, and angular distributions.1,2 Even "simple" systems can display non-intuitive behavior that reflects the multidimensional character of the relevant potential surfaces. Such behavior places a high value on well-characterized prototypical systems for which the detailed dynamical processes are understood. The photodissociation of methyl iodide, CH3I, via the

band in the near ultraviolet (~300 nm to 220

nm) provides one such system.1-21 In recent years, there has been a significant effort to extend that understanding to photodissociation in the first Rydberg bands of CH3I,22-36 which lie between 202 nm and 175 nm and result from excitation from the lone pair 5pπ (e) orbital of the iodine atom into the 6sa1 Rydberg orbital.37-47 In the C3v symmetry of the CH3I ground state, the removal of an electron from the highest occupied molecular orbital (e symmetry) results in 2E3/2 and 2E1/2 states that are split by approximately 0.6 eV as a result of the spin-orbit interaction.37 Coupling a 6sa1 Rydberg electron to these ion core states results in five Rydberg states: the lower energy 3E2 and 3E1 states associated with the 2E3/2 core, and the 3E0(A1), 3E0(A2), and 1E1 states associated with the 2E1/2 core.29 We will refer to these states as follows: 3E2 = 3R2; 3E1 = 3R1; 3

E0(A1 and A2) = 3R0+,0-; and 1E1 = 1R1. The lower and higher energy groups of states are known

as the

and

states, respectively. The splitting between the two groups of states is similar to

the splitting of the ion core, while the splittings within each group are small (~0.07 - 0.09 eV). The A1 and A2 states are nearly degenerate.29 Figure 1 shows cuts through the potential surfaces of these states along the C-I stretching coordinate from the calculations of Alekseyev et al.29 Figure 1a also shows the curves responsible for the also play a role in the predissociation of the of the

and

band in the near ultraviolet (UV), which states. Figure 1b shows an expanded view

state, along with curves for the states thought to be responsible for its predissociation.

Figure 2 shows the relevant portion of the

←

Eden et al.43 Single-photon transitions from the

absorption spectrum of CH3I recorded by 1

A1, Ω = 0 ground state of CH3I to the 3R2, Ω

3



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= 2 electronic state are forbidden by the ∆ Ω = 0, ±1 dipole selection rule,48 so that most of the oscillator strength for the

state goes to the 3R1 state. The Franck-Condon factors between the

ground state and the 3R1 Rydberg state are strongly peaked at the origin band, and this feature dominates the spectrum. Weaker transitions involving vibronic levels with excitation of ν2 (CH3 umbrella mode with a1 symmetry), ν3 (C-I stretch, a1), ν5 (CH3 degenerate deformation, e), ν6 (CH3 degenerate rock, e), and their combinations are also observed in this region.38,43 The bands associated with the totally symmetric ν2 and ν3 vibrations have relatively simple shapes, while the bands associated with the degenerate ν6 vibration (i.e., 610 and 210 610 ) are broader, with partially resolved substructure that has been analyzed by Felps et al.38 and Dagata et al.40 Although the band involving the degenerate ν5 vibration is quite weak, it also shows some substructure.

Predissociation of the vibronic levels of the

state can result in two distinct product channels:

CH3I → CH3 + I(2P3/2)

(1)

CH3I→ CH3 + I(2P1/2) ,

(2)

and

with dissociation thresholds, ED, of 2.366 ± 0.013 eV and 3.309 ± 0.013 eV, respectively.49 Within each channel, the CH3 can be left in a range of different rovibrational levels. The 3A1(A2, E) states cross the

state curves at approximately the equilibrium geometry of the molecule.

The two 3A1 states are thought to be the primary states responsible for the predissociation of the state, and both are correlated with I 2P1/2 fragments.29 The next closest state that could predissociate the

state corresponds to the 1Q1 state, which correlates with the I 2P3/2

fragment.29 Interestingly, as seen in Figure 1a, the 1Q1 curve also crosses the 3Q0+ curve on the way to dissociation products. Thus, predissociation of the

state by the 1Q1 state, followed by

crossing to the 3Q0+ state, could produce I 2P1/2 fragments as well as the diabatic I 2P3/2 fragments. However, at least in the calculations of Alekseyev et al.,29 the 1Q1 state only comes close to the 3R1 and 3R2 curves at energies above ~6.8 eV, that is, ~0.6 eV above the 4


state

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origin. These observations suggest that the I 2P1/2 quantum yield for the predissociation of the state should be close to unity. Over the years, however, several studies22,33-35 of the

state

predissociation have displayed evidence for I 2P1/2 quantum yields less than 1.0. The present study provides new experimental evidence on the vibronic dependence of the branching to the weak I 2P3/2 dissociation channel, as well as insight into the predissociation mechanism.

A variety of techniques have been used to characterize the vibrational dependence of the

state

lifetime. Ziegler and coworkers50-54 performed a series of Raman and hyper-Raman experiments on the lifetimes of several vibronic levels. They concluded that the bandwidths, and thus the lifetimes, showed significant mode-dependent behavior.54 In particular, although the lifetimes of levels with one quantum excited in ν2 or ν6 were similar to that of the vibrationless level, the lifetime of the level with one quantum in the ν3 (i.e., the reaction coordinate) actually lengthened the lifetime by a factor of ~3. In addition, they found that excitation of the level with one quantum in the ν1 C-H symmetric stretch led to a decrease in lifetime by a factor of ~10.54 In contrast, Syage55 studied the mode dependence of widths determined in resonant multiphoton ionization, and concluded that the lifetimes were insensitive to the vibronic level for the 0 00 , 210 ,

310 , and 610 bands. Somewhat later, Baronavski and Owrutsky used a femtosecond pump-probe photoionization techniques to perform direct lifetime measurements of vibronic levels of the state. Their results were in qualitative agreement with the results of Wang and Ziegler,54 showing a significant mode-dependence of the predissociation rates and a significantly longer lifetime for the 310 band. Quantitatively, the decay rates measured by Baronavski and Owrutsky26 were typically a factor of 2 - 3 larger than those measured by Wang and Ziegler.54 Baronavski and Owrutsky26 stressed that the observed vibronic mode dependence of the

state lifetimes, in

particular the non-intuitive increase in lifetime for excitation of the ν3 C-I stretch, was evidence for the multidimensional nature of the predissociation mechanism for the

5


state.


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Since that time, there have been a number of femtosecond pump-probe experiments on the origin band of the

state.27,28,30-33 The techniques employed include mass spectrometry, state-selective

detection of CH3 and I fragments, velocity map imaging, and time-resolved photoelectron spectroscopy, providing insight into the predissociation lifetime, product state distributions, and product angular distributions. Recently, Gitzinger et al. have reported a femtosecond velocity map imaging study of the 210 and 310 bands as well.34 All of these studies provide lifetimes that are consistent with the work of Baronavski and Owrutsky,26 including the significantly longer lifetime of the 310 band. The femtosecond imaging studies of the 310 band by Gitzinger et al.34 also confirmed an earlier nanosecond imaging study of González et al.,33 that showed a small, but non-zero, branching fraction for the dissociation process producing CH3 + I 2P3/2. As discussed below, while the I 2P3/2 branching fraction34 was only estimated to be φI = 0.07 ± 0.05, this result is surprising in light of earlier photodissociation experiments at 193 nm (accessing higher vibronic levels of the

state) as well as theoretical calculations of the relevant potential

energy surfaces.29

Much of the earlier work on the photodissociation of CH3I via the

state was performed at 193

nm by using the output of an ArF excimer laser.22-25 The principal target of these experiments was the quantum yield for producing I 2P3/2 or 2P1/2 (φI or φI*, respectively). These studies led to conflicting results, with the early experiments based on I/I* absorption/gain measurements22 yielding φI* = 0.70 ± 0.04, while experiments based on translational spectroscopy23,24 yielded φI* = 1. The conclusions of the translational spectroscopy experiments23,24 were based on the expectation that the fragments in the CH3 + I 2P3/2 channel would have significantly higher kinetic energy than those in the CH3 + I 2P1/2 channel, and the observation that no such fast CH3 + I 2P3/2 was observed. A more recent absorption measurement was consistent with the earlier absorption result, but a more detailed analysis suggested that the observed I 2P3/2 was produced by photodissociation of I2 that built up in the reaction vessel.25 Concurrent measurements using

6


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Doppler spectroscopy showed no evidence for the fast I 2P3/2 expected from the dissociation process,25 and were consistent with φI* = 1.

Recently, we have studied the 193-nm photodissociation of CH3I in a molecular beam by using velocity map ion imaging.35 In these experiments, the I and CH3 fragments were ionized by a single-photon of vacuum ultraviolet (VUV) light. Only the I 2P1/2 atoms were detected when the light was tuned below the I 2P3/2 threshold, while both species were detected above the threshold; comparison of the images obtained at the two wavelengths thus allowed the state-resolved detection of the I atoms. The relative photoionization cross sections for I 2P3/2 and 2P1/2 were also characterized in this region, allowing a determination of φI*. Furthermore, the absolute photoionization cross section for I 2P3/2 is quite large at threshold, making this an extremely sensitive detection method for this state. The results of these measurements were surprising. First, measurements of the I signal above and below the 2P3/2 threshold clearly demonstrated the production of I 2P3/2 atoms,35 albeit with a quantum yield of only φI = 0.07 ± 0.01. Second, the images and resulting translational energy distributions indicated the I 2P3/2 atoms were actually slower than the I 2P1/2 atoms, which implied the CH3 radicals in the I 2P3/2 channel were formed with considerable internal energy.35 This observation would explain the missing I 2P3/2 and associated CH3 in the earlier translational spectroscopy experiments, as they would have appeared at much longer flight times than expected, and because the branching fraction is small, they would likely have been obscured by the background signal. Furthermore, the slow I 2P3/2 would also have been difficult to detect in the recent Doppler measurements, where the signal would have also been weak and buried in the line center of the much stronger I 2P1/2 signal.

The new observation35 of a non-zero φI value at 193 nm supports the earlier observation of nonzero φI for excitation of the 310 band at 199.11 nm.33,34 Interestingly, the latter I 2P3/2 signal is also observed in association with vibrationally excited CH3, although not nearly as hot as that produced at 193 nm. One difficulty with the measurement at 193 nm is that the bandwidth of the 7



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laser is large (~120 cm-1), making it difficult to characterize which vibronic bands of the

state

contribute to the observed signal. In particular, the absorption spectrum43 shows that the weak 2 210 610 band is within this bandwidth, and that the much more intense 2 0 band lies close to the

edge of the bandwidth. Lao et al.56 suggested that the 2132 vibronic state also contributes to the dissociation process, although this band was not assigned in the absorption studies.

The velocity map imaging results clearly provide insight into the predissociation mechanism of the

state,33-35 and the unexpected observations reinforce the conclusion of Baronavski and

Owrutsky26 that the dissociation process involves multidimensional characteristics of the relevant potential energy surfaces. The observation33-35 of the CH3 + I 2P3/2 channel at 199.11 nm and 193 nm is also at odds with the best theoretical potential energy surfaces29 for the

state and the

states responsible for its predissociation. In particular, as discussed above, those calculations29 indicated that, at least along the C-I stretching coordinate, the dissociative states correlated with CH3 + I 2P3/2 products cross the

state curve at energies significantly higher than those

accessed in the experiments. One possibility is that the relevant surface crossing(s) occurs away from the reaction coordinate for simple C-I bond scission. This possibility might also help explain the high internal energy of the CH3 fragment in the I 2P3/2 channel.

To address this problem, we have performed new velocity map imaging experiments using narrow-band tunable UV light between 201.2 nm and 192.7 nm (6.162 eV to 6.434 eV) for the photodissociation step, and using single-photon ionization of the I and CH3 fragments. These measurements allow the characterization of φI* and the internal energy of the CH3 for selected vibronic levels, providing more details about the predissociation mechanism. Our experimental approach is described in Section II, and some background material on energetics and photoionization cross sections is presented in Section III. The results are presented and discussed in Section IV, where we also consider more general implications of our findings. The conclusions are presented in Section V. 8


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II. EXPERIMENT The experimental set up has been described in previous works and only the details relevant to the present experiments are given here.57-59 In this experiment, we photodissociate methyl iodide via selected vibronic levels of the

state by using tunable UV light between 201.2 nm and 192.7

nm, and photoionize the photolysis products by using single-photon, VUV light at 118.2 nm. We use velocity-map ion imaging to determine the CH3 and I fragment translational energy distributions and angular distributions.60,61

Two schemes were used to generate the tunable UV light. For the region between of 202 nm and 197 nm, the frequency-doubled output of a Nd:YAG-pumped dye laser was mixed with the dye laser fundamental in a β-barium borate (BBO) crystal. For the region between 197 nm and 192.7 nm, the fourth harmonic radiation of a Nd:YAG laser was combined with the idler output of a Nd:YAG-pumped optical parametric oscillator/amplifier (OPO) system (Continuum Sunlite EXOPO) in a BBO crystal to generate the sum frequency at the desired wavelength. The wavelengths of the output of OPO and dye laser were measured with a commercial wavemeter (Coherent Wavemaster). The resulting beam from either process was attenuated to minimize multiphoton effects and loosely focused into the experimental chamber. Just before entering the chamber, the UV beam passes through an air-spaced MgF2 Rochon polarizer to purify the linear polarization of the light, and then attenuated by an adjustable iris and neutral density filters. The typical energy of the UV pulse was ~10 µJ. The bandwidth of the light from the tripling process is expected to be ~0.3 cm-1, while the bandwidth from the sum frequency mixing is limited to ~1.0 cm-1 by the unseeded Nd:YAG bandwidth. The widths of the vibronic levels of the

state

are significantly larger than either of these. The VUV light at 118.2 nm was generated by focusing the third-harmonic output of a Nd:YAG laser into a Xe gas cell to produce the thirdharmonic output at 118.2 nm. This output was refocused into the experimental chamber by a LiF lens. By positioning the lens slightly off axis, it also served to spatially separate the VUV light 9



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from the 355 nm Nd:YAG harmonic. The UV photodissociation beam and VUV photoionization beam were counter-propagating through this chamber.

The sample corresponded to a mixture of ~5% CH3I in He, with a total pressure of ~1500 Torr. The gas was introduced into the source region by using a pulsed valve (General Valve, Series 9) with a 0.5 mm diameter nozzle. The resulting molecular beam was skimmed by using a 1 mm skimmer and then passed through a small hole in the repeller plate of the velocity map imaging spectrometer. The molecular beam travels co-linear with the time-of-flight axis of the spectrometer. The timing of the gas pulse and laser pulses was controlled by a series of digital delay generators, and is set so that only the initial part of the gas pulse interacts with the laser pulses. This setting minimizes the potential for dimer formation, and no masses higher than that of the parent CH3I+ were observed under the conditions of the experiment. During the experiment, the pressure in the source region was ~3.5x10-5 Torr, while the pressure in the detector region was ~10-7 Torr.

The molecular beam was crossed at 90o between the repeller and extractor plates of the velocity map imaging spectrometer by the counter-propagating photodissociation (pump) and photoionization (probe) beams. The VUV probe pulse was set to arrive just after the UV pump pulse, with a typical delay of ~35 ns. The linear polarization of the VUV beam was parallel to the polarization of the pump beam; both polarizations were perpendicular to the spectrometer axis and parallel to the face of the detector to allow simple reconstruction of the images. Ions produced in the interaction region were accelerated by the repeller and extractor plates to an 80 mm diameter dual channelplate detector that was coupled to a phosphor screen. A standard video camera synched to the experiment was gated to allow the detection of only the ions with the flight time (mass) of interest. The images were centroided on each shot and the recorded image consists of a sum of 10,000 individual images. Background images were also recorded with either the pump UV beam or the probe VUV beam blocked. Ion images resulting from the 10


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photoionization of both the iodine and methyl fragments were recorded. The final images were obtained by subtracting the sum of the background images from the image recorded with both laser beams present. The subtracted images were then reconstructed by using the pBASEX program62 to provide total translational energy distributions and angular distributions.40 The energy scale of the ion images was calibrated by using methyl images recorded at 118.302 nm following the well-characterized photodissociation of CH3I at 266 nm.

III. BACKGROUND CONSIDERATIONS For single-photon dissociation of an unaligned sample, the photofragment angular distributions will have the form:63,64 I(θ) ∝ 1 + β2P2(cosθ) ,

(4)

where θ is the angle between the linear polarization of the laser and the fragment velocity vector,

β2 is the angular distribution parameter, and P2(cosθ) is the second Legendre polynomial. In the present experiments, the VUV probe beam is also linearly polarized (parallel to the photodissociation laser's polarization). Under these conditions, the probe can add anisotropy to the signal, and the angular distribution has the more general form:63-65 I(θ) ∝ 1 + β2P2(cosθ) + β4P4(cosθ) .

(5)

If the ionization step is saturated, the form of the angular distribution reverts back to that of Equation 4. However, the large differences in detection efficiencies observed for the I 2P3/2 and 2

P1/2 indicate that the ionization process in the present experiments is not saturated, so that

Equation 5 is in principle more appropriate. Because the photodissociation and detection are performed with linearly polarized light, the fragments can be aligned (i.e., MJ distributions with equal populations of levels with the same |MJ|) but not oriented. For the I 2P1/2 fragment, alignment is not possible, and the corresponding observed angular distributions will be described by Equation 4. (Note that the corresponding CH3 co-fragment can still be aligned, so that its angular distribution will still follow Equation 5.)

11



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The

→

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transition is a perpendicular transition with ∆Ω = +1. For prompt dissociation

following a pure perpendicular transition, the angular distribution for the photodissociation step is expected to have β2 = -1. If the dissociation is slower, as in the case of predissociation of the state, rotational motion can reduce the observed anisotropy.66,67

Upon photodissociation, the excess energy (Eavail) available for either relative translation of the two products (ET), or excitation (rotational or vibrational) of the CH3 radical (Eint) is given by: Eavail = ET + Eint = Ehν - ED(2P3/2 or 2P1/2) ,

(3)

where Ehν is the photon energy and ED is the dissociation energy to produce ground state CH3 and I 2P3/2 or 2P1/2. With nearly 1 eV greater Eavail, channel 1 is at least simplistically expected to produce fragments with higher translational energy than channel 2. However, the imaging experiments35 at 193 nm have shown that the weak I 2P3/2 channel actually produces fragments with lower translational energy than the I 2P1/2 channel, indicating very high internal excitation of the CH3 fragments in channel 1.

In our earlier experiments,35 the fragments were ionized with tunable VUV light, allowing the clear separation of the I 2P3/2 and 2P1/2 signals. In the present experiment, fixed frequency VUV light at 118.2 nm is used, which is sufficient to ionize either spin-orbit state of I. A determination of the relative photoionization cross sections of the two spin-orbit states indicated the cross section σ118.2nm(I 2P3/2) was a factor of 16.6 ± 2.0 larger than the cross section σ118.2nm(I 2P1/2).35,57 Note that our original determination of this cross section ratio57 gave a value of 19.2 ± 2.0, which was extracted by using a value for the I* 2P1/2 branching fraction of 0.73 at 266 nm. In a subsequent paper,35 the value of the cross section ratio at 118.2 nm, as well as the values at other wavelengths near 118.2 nm, were modified by using a slightly different value (0.70) of the I* 2

P1/2 branching fraction at 266 nm, which was recommended by a referee. The use of this

recommended branching fraction at 266 nm yields the I 2P3/2 to 2P1/2 photoionization cross section ratio at 118.2 nm of 16.6 ± 2.0 used here. A better measurement of the 266-nm branching 12


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fractions would directly allow a better determination of the cross section ratio.

In contrast to I 2P3/2 and 2P1/2, the photoionization cross section of CH3 at 118.2 nm is expected to show a relatively small dependence on internal energy for low levels of excitation,35,57 but is expected to decrease considerably for high internal energy. As a result, comparison of the translational energy distributions determined from the CH3 and I fragments are expected to show clear differences, making the identification of the I 2P3/2 and 2P1/2 contributions relatively straightforward to assess.

IV. RESULTS AND DISCUSSION A. Photofragment Velocity Distributions The photodissociation of CH3I was examined at selected energies across the

state band

system. In general, the energies used to record the present data correspond to the peak positions of Felps et al.,38 and are indicated by arrows in Figure 2. These energies are also provided in Table I, along with the available energies for the two dissociation channels. Note that the peak positions reported by Felps et al.38 are slightly different from those reported by Eden et al.,43 whose spectrum is shown in Figure 2. Thus, the arrows indicating the former peak positions are slightly displaced from the lines identifying the bands in at the latter positions. Note, however, that because the vibronic bands are broad, the resulting images do not appear to be strongly sensitive to the photon energy around each position. We begin by considering the photodissociation processes at wavelengths near 193 nm (6.424 eV), rather than those for the origin band and other low-lying vibrational levels of the

state. In doing so, we can refer back

to the results and conclusions of Reference 35, where the photodissociation was performed with a 193-nm excimer laser and the photoionization was performed by tunable VUV probe light. This approach allows a more convincing explanation of the observed data. Figure 3 shows reconstructed images for both I and CH3 fragments formed by photodissociation at 6.360 eV, 2 1 1 2 6.403 eV, and 6.434 eV, corresponding to the 6 0 or 2 0 30 band, 210 610 band, and the 2 0 band,

13



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respectively.43 Also shown are the corresponding translational energy distributions obtained from the CH3 and I images. The distributions have been scaled so that the I distribution intensity is matched to the CH3 distribution intensity at the maximum of the latter. These translational energy distributions are very similar to those observed for 193-nm excimer laser photodissociation.35 In particular, the CH3 distributions show a dominant feature corresponding to CH3, v1 = 0 + I 2P1/2 atoms, along with a short progression of decreasing intensity corresponding to CH3, v1 = 1, 2 + I 2P1/2. As has been discussed previously, each CH3, v1 + I 2P1/2 feature may contain substructure corresponding to a progression in the ν2 umbrella vibration, but this progression is not visible with the present resolution.

To simplify the calibration of the translational energy distributions, the I images were recorded with the same repeller and extractor voltages as the CH3 images. This arrangement results in relatively low resolution for the I images, as can be seen in Figure 3. Nevertheless, the translational energy distributions are similar to those observed with excimer laser photolysis. Two intense broad features are present. The higher energy feature follows the envelope of the CH3 + I 2P1/2 distribution in the CH3 image, whereas the lower energy feature has no counterpart in the CH3 image. For 193-nm photodissociation,35 iodine images recorded above and below the 2

P3/2 ionization threshold show conclusively that the lower energy feature is associated with I

2

P3/2. This I 2P3/2 must be formed in conjunction with highly vibrationally excited CH3 fragments.

The three distributions shown in Figure 3 show similar behavior, although the intensity of the low energy feature varies considerable for the three pump transitions.

In viewing the I distributions of Figure 3, the reader is reminded that the photoionization cross section35,57 for I 2P3/2 is 16.6 ± 2.0 times greater than that for I 2P1/2, so the intensity of the lower energy peak in the translational energy distributions appears much stronger than expected. Figure 3 also shows the low-energy peak scaled by this factor to give a more direct visual impression of the relative branching fractions of the two channels. Each of the I distributions show two 14


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components that are at least partially resolved. For each pump transition, one component of the I distribution is well-correlated with the CH3 distribution. This component can be associated with the I 2P1/2 channel, which has the dominant branching fraction. This assignment is also consistent with earlier work.35 The second component of the I distribution, which in general is associated with a very weak CH3 signal, is correlated with I 2P3/2. Quantitative branching fractions were determined in two manners. The first involved simple numerical integration68 of the two components of the I translational energy distribution, and scaling of the I 2P3/2 component. The second involves Gaussian fits of the two features, integration of the resulting areas, and scaling of the I 2P3/2 component. Although the features are obviously not Gaussian, the fits reproduce the distributions quite well. The two approaches yield very similar branching fractions, and the differences have been used to assess the uncertainty in the branching fractions. The resulting values are included in Table I. Note, however, that, at least in principle, the I 2P3/2 signal could also contribute to the feature associated with the I 2P1/2 fragment. While this contribution is expected to be small, this possibility means that the I 2P3/2 branching fractions determined here are really lower limits to the true value.

The branching fractions for the three bands in Figure 3 range from 0.05 ± 0.02 to 0.07 ± 0.02, and are in line with the value of 0.07 ± 0.01 determined with the ArF excimer laser. The present 2 measurements are consistent with either or both the 210 610 and 2 0 bands contributing to the

distributions determined with the excimer laser.35

As discussed previously, the absence of the CH3 + I 2P3/2 channel in the translational distributions derived from the CH3 images is attributable to two factors. First, the actual branching fraction of this channel is relatively small. If the CH3 photoionization cross section at 118.2 nm was independent of the internal energy of the CH3, the corresponding I 2P3/2 feature in the CH3 distribution would have the intensity of the scaled I 2P3/2 signal in Figure 3. Such a signal is almost at the level of the noise in the CH3 distributions. Second, for very highly excited 15



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Page 16 of 40

CH3, the photoionization cross section at 118.2 nm is expected to be considerably smaller than that for cold CH3.35,57 This situation arises from a spreading of the Franck-Condon envelope for photoionization from higher vibrational levels that results in a significant fraction of the envelope being energetically inaccessible at 118.2 nm. When taken into account, this additional factor will reduce the intensity of the I 2P3/2 component of the CH3 distribution even further.

Figure 4 shows the reconstructed images and translational energy distributions for three 1 1 1 somewhat lower energy bands, corresponding to 6 0 , 2 0 , and 50 transitions at 6.267 eV, 6.298 1 eV, and 6.316 eV, respectively.43 As seen in Figure 2, the 6 0 band is quite broad, with several

partially resolved sub-bands. Images recorded for several sub-bands across the broad profile are 1 similar in appearance to those shown for the 6 0 in Figure 4, and these images all yield similar

branching fractions for I 2P3/2. The three CH3 distributions are similar to those in Figure 3, displaying strong CH3, v1 = 0 + I 2P1/2 peaks and short progressions in ν1. These progressions are somewhat shorter than in the shorter wavelength distributions, as might be expected. The three corresponding I distributions all show two components, as in the higher energy distributions. In Figure 4, however, the intensity of the slower I 2P3/2 component is significantly smaller than the 2

P1/2 component, especially when the intensity of the former is corrected for the relative

photoionization cross sections. For all three bands in Figure 4, the I 2P3/2 branching fraction is quite small, ranging from 0.02 ± 0.02 to 0.04 ± 0.02. Interestingly, all three distributions in Figure 4 appear to show some signal on the high energy side of the I 2P1/2 peak, which may be associated with I 2P3/2 and CH3 in relatively low vibrational levels. When scaled for the relative photoionization cross sections, this component of the I 2P3/2 distribution is quite small. Nevertheless, there appears to be a very weak corresponding component in the CH3 distribution. 1 Gitzinger et al.34 recently studied the photodissociation of CH3I via the 2 0 band, but did not

observe the I 2P3/2 product seen here. However, the present branching fraction is small, and it is not clear that their detection scheme would have been sufficiently sensitive to observe the I 2P3/2 or the very hot CH3 conclusively. 16


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Figure 5 shows the reconstructed images and translational energy distributions for photodissociation at 6.163 eV, 6.203 eV, and 6.223 eV, corresponding to the

3

R1 state origin

1 1 1 band, the 3R2 6 0 band, and the 3R1 30 band, respectively.43 Whereas the 3R2 6 0 band was not

assigned in the original single-photon absorption spectrum,43 it was assigned by Dobber et al.,46 based on two-photon resonant multiphoton ionization and photoelectron spectroscopy.

As expected from the earlier work,27,28,30-32 the CH3 distribution for the origin band shows a single strong feature that can be assigned to ground state CH3 + I 2P1/2, and a single weak feature at lower energy can be assigned to CH3, v1 = 1 + I 2P1/2. Although somewhat broader, the distribution from the I image follows that of the CH3 image. An additional small shoulder extending to lower energy is observed in the distribution from the former image. This shoulder is similar to, but much weaker than, the CH3 + I 2P3/2 features in Figures 3 and 4. If that assignment is correct, the I 2P3/2 branching fraction is still less than 0.01 ± 0.01 when corrected for the 1 photoionization cross section of I 2P3/2. The distributions observed for the 3R2 6 0 band are

similar to those observed for the origin band, although the low-energy shoulder in the former is somewhat larger, resulting in a branching fraction of 0.03 ± 0.02 for CH3 + I 2P3/2.

The translational energy distributions extracted from the CH3 and I images obtained via the 3R1

310 band are substantially different from the distributions at the other wavelengths reported here. The distribution from the CH3 image shows four significant peaks. The three lowest energy peaks can be readily assigned to a progression with v1 = 0 - 2 in the CH3, v1 + I 2P1/2 channel. This channel clearly dominates the dissociation process, and the observed progression is consistent with the results obtained previously by González et al.33 and Gitzinger et al.34 The fourth peak in the distribution derived from the CH3 is much smaller than the other three, and is too fast to be associated with the I 2P1/2 channel. Rather, it is assigned to CH3, v1 = 1 + I 2P3/2; a much weaker, slightly faster peak is consistent with CH3, v1 = 0 + 2P3/2 products. Again, these 17



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Page 18 of 40

observations of I 2P3/2 are also consistent with the observations of González et al.33 and Gitzinger et al.34 In particular, Gitzinger et al. found an I 2P3/2 branching fraction of 0.07 ± 0.05, and also concluded that the I 2P3/2 was primarily associated with vibrationally excited CH3. Interestingly, the present results indicate that whereas the CH3 associated with I 2P3/2 is vibrationally excited, the internal energy is considerably smaller than for excitation via the vibronic bands shown in Figures 3 and 4.

As discussed above, the photoionization cross section of CH3 at 118.2 nm is expected to decrease with substantial vibrational excitation;35,57 however, the cross sections for CH3, v1 = 0 and 1 are expected to be only weakly dependent on v1. If we assume they are equal, we can integrate the peaks in the distribution for the CH3 image associated with the I 2P1/2 and 2P3/2 and use the areas to determine the branching fractions directly. The distribution of Figure 5 thus yields φI = 0.11 ± 0.02 for excitation of the

310 band. This value is in reasonable agreement with the value of

Gitzinger et al.34

1

The translational energy distribution for the 3R1 30 band obtained from the I image is also quite different from the other distributions presented here. This distribution shows an intense high energy peak and a moderately intense lower energy feature. The latter follows the envelope of the CH3 + I 2P1/2 process in the distribution determined from the CH3 image. The former feature follows the CH3 + I 2P3/2 distribution, but is broader and extends to somewhat higher energy. Assuming this feature corresponds to the 2P3/2 channel, and correcting the intensity for the relative photoionization cross section, this component is reasonably consistent with the feature in the distribution from the CH3 image. The branching fractions determined from the I image yield φI = 0.13 ± 0.02, also in reasonable agreement with the other determinations.34

As summarized in Table I, the I 2P3/2 branching fraction following excitation into the

state

depends on the specific vibronic level accessed. Nevertheless, for all vibronic bands above the 18


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origin, this branching fraction is non-zero, though typically quite small. It remains to be seen how these branching fractions can be explained in terms of the potential surfaces involved in the dissociation dynamics.

Alekseyev et al.29 have reported cuts through the theoretical potential energy surfaces of the relevant states of CH3I. As shown in Figure 1, these curves were calculated along the C-I stretching coordinate, while remaining on the minimum energy path with respect to the CH3 umbrella vibration. These calculation indicate that, in the energy region of the experiments, none of the theoretical curves predissociating the

state are correlated with CH3 + I 2P3/2 products.

The 1Q1 potential, which is the closest curve associated with the CH3 + I 2P3/2 asymptote, only approaches the repulsive walls of the 3R1 and 3R2 states at ~6.8 eV. The clear observation of 2P3/2 products in both the earlier measurements33-35 and the present ones suggests that the theoretical curves are not providing the full picture. It appears that either the relative energies of some of the states in Figure 1 must be shifted, or that, away from C3v symmetry, the relevant surface crossings or interactions occur at somewhat lower energy.

The translational energy distributions in the CH3 + I 2P3/2 channel show some dependence on the vibronic level of the

1 state that is excited. For excitation of the 30 band, the I 2P3/2 channel

produces CH3 radicals with moderate internal excitation, whereas for excitation of most of the other bands, the CH3 radicals are produced with quite substantial internal excitation. The different CH3 distributions could result from two distinct predissociation mechanisms, or simply from accessing different parts of the same surfaces.

Certainly from Figure 1 it appears that the 1Q1 surface is most likely responsible for dissociation to CH3 + I 2P3/2 products. Interestingly, on its way to these products, the 1Q1 potential crosses the 3

Q0+ potential, which is associated with CH3 + I 2P1/2 products. Given the high internal energy of

the CH3 observed here, this curve crossing might be expected to produce some internally hot 19



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Page 20 of 40

CH3 associated with I 2P1/2 as well. However, the importance of this crossing at geometries away from C3v remains to be assessed. At the moment, we have no evidence for highly excited CH3 in the I 2P1/2 channel.

Previous work on the lifetime of the 3R1, v3 = 1 level yielded values a factor of 3-4 larger than that of the origin level.26,34 This observation suggested that excitation of ν3 moved the system away from the intersection with the predissociating 3A1 surfaces. Whereas the lifetimes of many of the other vibronic levels are comparable to, or shorter than, the lifetime of the origin level, vibrational excitation may change the relative importance of surface crossings or influence the location at which the crossings occur. These differences would then be reflected in the product translational energy distributions. At the present time, there is insufficient information available on the multidimensional potential surfaces to extract the implications of the data reported here. We hope that the present results stimulate new calculations at non-C3v geometries.

B. Photofragment Angular Distributions As discussed above, the

→

transition is a perpendicular transition, and for prompt

dissociation the angular distribution is expected to have β2 = -1. For predissociation that takes place on the timescale of the rotational motion, the observed anisotropy will be reduced. Indeed, femtosecond time-resolved measurements by Gitzinger et al.30 of the photofragment anisotropy for the origin band of the CH3I

state have shown how the β2 parameter changes from the

asymptotic value of -1 to a value of approximately -0.5 in 4 - 5 ps. Similar behavior was also observed for the origin band by Thire et al.,32 and for the 210 and 310 bands by Gitzinger et al.34

Our experiments were performed with long pulses (~10 ns), and thus probed the long-time behavior of the anisotropy. There have been several previous measurements of the photofragment angular distributions following excitation of the

state with ns laser pulses.

Using an excimer lasers operating at 193.3 nm to initiate photodissociation, Van Veen et al.23,68 20


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and Gilchrist et al.25 found β2 values of -0.72 ± 0.10 (from CH3) and -0.71 ± 0.09 (from I 2P1/2) for the I 2P1/2 channel, respectively. The excimer laser bandwidth is relatively broad so that these measurements reflect the excitation of a convolution of several vibronic levels including the 2 20 and 210 610 bands. González et al.33 have used two-photon resonant, three-photon ionization to detect CH3 in the vibrationless ground state and in the v1 = 1 state following photodissociation via the 310 band, and found β2 values of -0.4 - -0.5 and β4 values of -0.10 - -0.2 for both vibrational levels and for both the I 2P3/2 and 2P1/2 channels. (Here, the detection process can lead to a nonzero β4 value even in the 2P1/2 channel.) More recently, González et al.36 have performed a very detailed study of the photofragment anisotropies for CH3 + I 2P1/2 produced via the origin band of the

state.

While more detailed studies similar to that of González et al.36 of the angular distributions and vector correlations will ultimately be necessary to unravel the dissociation dynamics of the excited vibronic levels of the B state, we now briefly discuss the angular distributions parameters determined here. The angular distribution parameters extracted from the present data are shown in Table II. Especially for the weaker bands, the angular distribution data are somewhat noisy, and we have not attempted to extract parameters for individual CH3 vibrational levels. Thus, all of the β 2 and β 4 values reported here are vibrationally averaged. Furthermore, the CH3 signal associated with the I 2P3/2 signal is too weak to produce useful angular distribution data.

0 The present β2 values determined from the I 2P1/2 and CH3 distributions for the 0 0 band are

nearly identical, and the β 4 value from the CH3 distribution is negligible. The value, β 2 = -0.330 34, is somewhat more negative than the value determined at the peak of the 0 0 band by González

et al. of β 2 = -0.23. The β 2 values determined by González et al. do fall away from the band center, and the somewhat more negative value observed here may reflect a slight difference in excitation wavelength. The deviation from the limiting value of β

21


2

= -1 for a perpendicular


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Page 22 of 40

transition has been discussed previously, and reflects rotational motion of the CH3I on a timescale of the dissociation.

1 1 The 30 band also deserves special mention. The I 2P1/2 component of the 30 data shows behavior

similar to that of the origin band. The β2 values extracted from the I 2P1/2 and CH3 images are ~0.45-0.46, and the β4 value from the latter image is essentially zero. These values are in good agreement with the data of González et al.33 The β2 values are slightly more negative than our values for the origin band, although the longer lifetime of the v3 = 1 level might have been expected to result in a isotropic distribution. This same trend is observed by González et al.33,36 1 The β2 value for the I 2P3/2 component of the 30 distribution is somewhat more isotropic than the

value for the I 2P1/2 component, but shows a small negative β4 value. The present 2P3/2 distribution is somewhat more isotropic than that observed by González et al.

The angular distributions for most of the other bands show similar behavior. In particular, for the 1 1 1 [1] 6 0 , 6 0 , and 50 bands, the β2 values range from ~-0.4 - -0.8, and the values determined from 1 2 1 1 2 the I 2P1/2 and CH3 data are similar. The data for the higher energy bands ( 2 0 , 6 0 , 2 0 6 0 , and 2 0 )

are somewhat different. Here the I 2P1/2 distributions yield β2 values of ~-0.2 - -0.4, while the CH3 distributions yield β2 values close to the limiting value of -1, and show significant β4 values as well. The β2 values from the corresponding I 2P3/2 distributions are similar to those from the I 2

P1/2 distribution. The smaller than expected values of β2 for the I 2P1/2 distribution thus may

reflect some contribution from the I 2P3/2 channel. More detailed polarization studies with better resolved I 2P3/2 and 2P1/2 distributions will be required to improve the characterization of these angular distributions.

IV. CONCLUSION We have studied the photodissociation dynamics of a series of vibronic levels in the

state of

methyl iodide, and measured the branching fraction for the I(2P3/2) channel. We find a non-zero 22


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branching fraction of I(2P3/2) for most vibronic levels of the

-state. The non-zero fraction is

most dramatic for the 310 level of the 3R1 Rydberg state, which has been previously characterized by Gitzinger et al. We find the φI branching fraction varies strongly depending upon the vibrational mode excited in the

Rydberg state, reaching a value of 0.11 to 0.13 for the 310 level,

but remaining consistently between 0.03 and 0.07 for many of the other vibronic bands. As discussed by González et al.33 and Gitzinger et al.,34 the I 2P3/2 produced from the 310 level is associated primarily with vibrationally excited CH3. Nevertheless, the internal energy is still relatively small. For the other vibrational modes, the I 2P3/2 fragment is actually slower than the I 2

P1/2 fragment, implying substantial (> 1 eV) vibrational excitation in the associated CH3

fragments.

The present measurements suggest a crossing of the Rydberg surface by a dissociative surface or surfaces correlated with I 2P3/2 at lower energy than previously predicted by theoretical calculations along the C-I coordinate.29 It is possible that this crossing occurs away from the symmetric geometry, for example, at rocking or bent geometries of the molecule. A crossing at such geometries may help explain the high internal energy of the CH3 fragment associated with I 2

P3/2. We hope that the mode-dependence of the I 2P3/2 product observed here will shed some

light on where the crossing likely occurs. However, as originally concluded by Baronavski and Owrutsky,26

the

complex

mode-dependence

provides

significant

evidence

for

the

multidimensional character of the predissociation process, and it appears likely that full dimensional calculations such as those performed at lower energy by Amatatsu et al.13 will be necessary to provide a convincing description of the dissociation process.

ACKNOWLEDGEMENTS This material is based on work supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences under contract No. DE-AC02-06CH11357. 23



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3.

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Riley, S. J.; Wilson, K. R. Excited Fragments from Excited Molecules: Energy Partitioning in the Photodissociation of Alkyl Iodides. Disc. Faraday Soc. 1972, 53, 132-137.

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32. Thiré, N.; Cireasa, R.; Staedter, D.; Blanchet, V.; Pratt, S. T. Time-Resolved Predissociation of the Vibrationless Level of the B State of CH3I. Phys. Chem. Chem. Phys. 2011, 13, 18485-18496. 33. González, M. G.; Rodríguez, J. D.; Rubio-Lago, L.; Bañares, L. First Observation of Ground State I(2P3/2) Atoms from the CH3I Photodissociation in the B Band. J. Chem. Phys. 2011, 135, 021102. 34. Gitzinger, G.; Corrales, M. E.; Loriot, V.; de Nalda, R.; Bañares, L. A Femtosecond Velocity Map Imaging Study on B-Band Predissociation in CH3I. II. The 210 and 310 vibronic Vibronic Levels. J. Chem. Phys. 2012, 136, 074303. 35. Xu, H.; Pratt, S. T. A New Look at the Photodissociation of Methyl Iodide at 193 nm. J. Chem. Phys. 2013, 139, 214310. 36. González, M. G.; Rodríguez, J. D.; Rubio-Lago, L.; Bañares, L. Imaging the Sterodynamics of Methyl Iodide Photodissociation in the Second Absorption Band: Fragment Polarization and the Interplay between Direct and Predissociation. Phys. Chem. Chem. Phys. 2014, 16, 26330-26341. 37. Mulliken, R. S.; Teller, E. Interpretation of the Methyl Iodide Absorption Bands near λ2000. Phys. Rev. 1942, 61, 283-296. 38. Felps, S.; Hochmann, P.; Brint, P.; McGlynn, S. P. Molecular Rydberg Transitions. J. Mol. Spectrosc. 1976, 59, 355-379. 39. Scott, J. D.; Felps, W. S.; Findley, G. L.; and McGlynn, S. P. Molecular Rydberg Transitions. XII. Magnetic Circular Dichroism of Methyl Iodide. J. Chem. Phys. 1978, 68, 4678-4687. 40. Dagata, J. A.; Felps, W. S.; McGlynn, S. P. Spin Uncoupling in the 6s Rydberg States of Methyl Iodide. Rotational Subband Structure in One Photon Absorption. J. Chem. Phys. 1986, 85, 2483-2488. 41. Donaldson, D. J.; Vaida, V.; Naaman, R. Ultraviolet Absorption Spectroscopy of Dissociating Molecules: Effects of Cluster Formation on the Photodissociation of CH3I. J. 27



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Chem. Phys. 1987, 87, 2522-2530. 42. Donaldson, D. J.; Vaida, V.; Naaman, R. J. Ultraviolet Absorption Determination of Intramolecular Predissociation Dynamics in Methyl Iodide Dimers ((CH3I)2 and ((CD3I)2) J. Phys. Chem. 1988, 92, 1204-1208. 43. Eden, S.; Limão-Vieira, P.; Hoffmann, S. V.; Mason, N. VUV Spectroscopy of CH3Cl and CH3I. J. Chem. Phys. 2007, 331, 232-244. Digital data from Reference 43 is posted on the MPI-Mainz UV/VIS Spectral Atlas of Gaseous Molecules of Atmospheric Interest, by H. Keller-Rudek, G. K. Moortgat, R. Sander, Rüdiger Sörensen. (www.uv-vis-spectral-atlasmainz.org). 44. Locht, R.; Leyh, B.; Jochims, H. W.; Baumgärtel, H. Medium and High Resolution Vacuum UV Photoabsorption Spectroscopy of Methyl Iodide (CH3I) and its Deuterated Isotopomers CD3I and CH2DI. A Rydberg Analysis. Chem. Phys. 2009, 365, 109-128. 45. Parker, D. H.; Pandolfi, R.; Stannard, P. R.; and El-Sayed, M. A. Two-Photon MPI Spectroscopy of Alkyl Iodides. Chem. Phys. 1980, 45, 27-37. 46. Dobber, M. R.; Buma, W. J.; de Lange, C. A. Resonance Enhanced Multiphoton Ionization Photoelectron Spectroscopy on Nanosecond and Picosecond Time Scales of Rydberg States in Methyl Iodide. J. Chem. Phys. 1993, 99, 836-853. 47. Donovan, R. J.; Hennessy, J. T.; Lawley, K. P.; Ridley, T. A Critical Reassignment of the Rydberg States of Iodomethane Based on New Polarization Data. J. Chem. Phys. 2013, 138, 134308. 48. Herzberg, G. Molecular Spectra and Molecular Structure. Vol. III. (Van Nostrand Reinhold, New York, 1966). 49. van den Brom, A.; Lipciuc, M. L.; Janssen, M. H. M. A Correlation between I (2P3/2)/I(2P1/2) Branching and CH3 Rotation in Photolysis of Single Quantum State-Selected CH3I (JK = 11). Chem. Phys. Lett. 2003, 368, 324-330. 50. Wang P. G.; Ziegler, L. D. Rovibrational Raman Scattering of CH3I Vapor: Resonance with a Perpendicular Polarized Electronic Transition. J. Chem. Phys. 1989, 90, 4115-4124. 28


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51. Wang P. G.; Ziegler, L. D. Depolarization Ratios of Resonance Raman Scattering in the Gas Phase. J. Chem. Phys. 1989, 90, 4125-4143. 52. Wang, P. G.; Zhang, Y. P.; Ruggles, C. J.; Ziegler, The Spontaneous Resonance Raman Scattering of CH3I in a Supersonic Jet. L. D. J. Chem. Phys. 92, 2806-2817 (1990). 53. Campbell, D. J.; Ziegler, L. D. Resonance Hyper-Raman Scattering Polarization. A Measure of Methyl Iodide B State Subpicosecond Lifetimes. J. Chem. Phys. 1993, 98, 150-157. 54. P. G. Wang and L. D. Ziegler, Mode-Specific Subpicosecond Photodissociation Dynamics of the Methyl Iodide B State. J. Chem. Phys. 1991, 95, 288-296. 55. Syage, J. A. Predissociation Lifetimes of the B and C Rydberg States of CH3I. Chem. Phys. Lett. 1993, 212, 124-128. 56. Lao, K. Q.; Person, M. D.; Chou, T.; Butler, L. Emission Spectroscopy of the Predissociative Rydberg B State of CH3I and CD3I at 193.3 nm. J. J. Chem. Phys. 1988, 89, 3463-3469. 57. Aguirre, F.; Pratt S. T. Photoionization of Hot CH3 and CF3. J. Chem. Phys. 2005, 122, 234303. 58. Xu, H.; Jacovella, U.; Ruscic, B.; Pratt, S. T.; Lucchese, R. R. Near-Threshold Shape Resonance in the Photoionization of 2-Butyne. J. Chem. Phys. 2012, 136, 154303. 59. Xu, H.; Pratt, S. T. Photoionization Cross Section of the Propargyl Radical and Some General Ideas for Estimating Radical Cross Sections. J. Phys. Chem. A. 2013, 117, 93319342. 60. Eppink, A. T. J. B.; Parker, D. H. Velocity Map Imaging of Ions and Electrons Using Electrostatic Lenses: Application in Photoelectron and Photofragment Ion Imaging of Molecular Oxygen. Rev. Sci. Instrum. 1997, 68, 3477-3484. 61. Suits, A. G.; Continetti, R. E. Imaging in Chemical Dynamics (American Chemical Society, Washington, DC, 2001). 62. Garcia, G. A.; Nahon, L.; Powis, I. Two-Dimensional Charged Particle Image Inversion Using a Polar Basis Function Expansion. Rev. Sci. Instrum. 2004, 75, 4989-4996. 29



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63. C. N. Yang, On the Angular Distributions in Nuclear Reactions and Coincidence Measurements, Phys. Rev. 1948, 74, 764-772. 64. Zare, R. N. Angular Momentum: Understanding Spatial Aspects in Chemistry and Physics, (Wiley, New York, 1988). 65. Rakitis, T. P.; Zare, R. N. Photofragment Angular Momentum Distributions in the Molecular Frame: Determination and Interpretation, J. Chem. Phys. 1999, 110, 3341-3350. 66. Jonah, C. J. Effect of Rotation and Thermal Velocity on the Anisotropy in Photodissociation Spectroscopy, J. Chem. Phys. 1974, 55, 1915-1922. 67. Yang, S. C.; Bersohn, R. Theory of the Angular Distribution of Molecular Photofragments, J. Chem. Phys. 1974, 61, 4400-4407. 68. The numerical integration was performed by using a simple area integration routine as implemented in Kaleidagraph 4.5 (Synergy Software, Inc., 2013). 69. Note that the definition of β used in Reference 23 corresponds to one-half the value of the β value defined here.

30


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Table I. Vibrational Mode Dependence of Branching Fractions Vibronic Transitiona

λ(nm)

hν (eV)

Eavail(I 2P3/2) (eV)

Eavail(I 2P1/2) (eV)

φ( I 2P3/2)

φ( I 2P1/2)

0 00

201.175

6.163

3.797

2.854

0.01±0.01

0.99±0.01

[1] 610

199.893

6.203

3.837

2.894

0.03±0.02

0.97±0.02

310

199.235

6.223

3.857

2.914

0.13±0.02b

0.87±0.02

610

197.837

6.267

3.901

2.958

0.04±0.02

0.96±0.02

210

196.863

6.298

3.932

2.989

0.03±0.02

0.97±0.02

510

196.302

6.316

3.950

3.007

0.02±0.02

0.98±0.02

194.944

6.360

3.994

3.051

0.07±0.02

0.93±0.02

210 610

193.635

6.403

4.037

3.094

0.07±0.02

0.93±0.02

2 20

192.702

6.434

4.067

3.125

0.05±0.02

0.95±0.02

6 20 or 210310

a. The [1] 610 band is associated with the(2E3/2)6sa1 [1] 3E2 Rydberg state. All of the other transitions are associated with the (2E3/2)6sa1 [2] 3E1 Rydberg state. The assignments correspond to those of Felps et al. [Ref. 38], Eden et al. [Ref. 43], and Dobber et al. [Ref. 46]. b. The φ( I 2P3/2) branching fraction determined from the CH3 translational energy distribution is 0.11 ± 0.02 (see text for details).

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Table II. Angular Distribution Parameters for Different Vibronic Bands.a Bandb

Lifetime (ps)c

β2 (I P1/2)d 2

β2 e,f

(CH3)

β

β β2

4 e,f

(CH3)

4 2

e

(I P3/2)

(I 2P3/2)e

0 00

1.38

-0.33±0.01

-0.34±0.01

< 0.01

[1] 610

0.98

-0.42±0.01

-0.57±0.01

0.08±0.01

310

4.1

-0.46±0.01

-0.45±0.01

< 0.01

-0.03±0.01

-0.15±0.01

610

0.76

-0.36±0.01

-0.60±0.01

0.08±0.02

-0.60±0.01

-0.31±0.02

210

1.05

-0.23±0.01

-1.09±0.01

0.41±0.01

-0.80±0.01

-0.87±0.03

0.56±0.04

510 6 20 or 210310

0.20

-0.30±0.01

-1.00±0.05

0.34±0.06

-0.37±0.01

-0.07±0.01

210 610

0.91

-0.38±0.01

-1.06±0.05

0.34±0.07

-0.41±0.01

-0.06±0.01

2 20

0.76

-0.22±0.01

-1.19±0.02

0.52±0.03

-0.28±0.01

-0.11±0.01

a. The error bars for the β 2 and β 4 values only reflect the counting statistics, and do not include possible systematic errors, which may be substantially larger. b. The [1] 610 band is associated with the(2E3/2)6sa1 [1] 3E2 Rydberg state. All of the other transitions are associated with the (2E3/2)6sa1 [2] 3E1 Rydberg state. c. The lifetimes are from Reference 26. d. These β2 values are obtained from the fit to Equation 4. e. The β2 and β4 values are obtained from the fit to Equation 5 f. These β2 and β4 values are obtained from the part of the CH3 distribution correlated with the I 2P1/2 fragment. The CH3 signal for the I 2P3/2 component of the distribution is too weak to adequately determine the angular distribution parameters.

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FIGURE CAPTIONS Figure 1. (a) The potential energy curves of CH3I from the theoretical calculations of Alekseyev et al. [Reference 29]. The curves are slices through the potential energy surfaces along the C-I stretching coordinate, while remaining on the minimum energy path with respect to the CH3 umbrella vibration. (b) An expanded portion of selected potential curves relevant to the present study. Here the energies are given in eV for more direct comparison with Figure 2.

Figure 2. The

←

absorption spectrum of CH3I recorded by Eden et al.43 The assignments

are those of Reference 38, 43, and 46. The arrows indicate the energies at which images were recorded for the present study. See text for more details.

Figure 3. Reconstructed images and translational energy distributions arising from the 2 photodissociation of CH3I at 194.944 nm, 193.635 nm, and 192.702 nm, corresponding to the 6 0 1 1 2 or 2 0 30 band, 210 610 band, and the 2 0 band, respectively. The green traces show the signal

associated with I 2P3/2 scaled by the cross section ratio for I 2P3/2 : I 2P1/2 at 118.2 nm, i.e., 16.6.

Figure 4. Reconstructed images and translational energy distributions arising from the 1 photodissociation of CH3I at 197.837 nm, 196.863 nm and 196.302 nm, corresponding to the 6 0 ,

210 , and 510 bands, respectively. The green traces shows the signal associated with I 2P3/2 scaled by the cross section ratio for I 2P3/2 : I 2P1/2 at 118.2 nm, i.e., 16.6.

Figure 5. Reconstructed images and translational energy distributions arising from the photodissociation of CH3I at 201.175 nm, 199.893 nm, and 199.235 nm, corresponding to the 3

1 1 R1 origin band, the 3R2 6 0 band, and the 3R1 30 band, respectively. The green traces shows the

signal associated with I 2P3/2 scaled by the cross section ratio for I 2P3/2 : I 2P1/2 at 118.2 nm, i.e., 16.6.

33



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Alignment of D-state Rydberg molecules.

Atomic Fock state preparation using Rydberg blockade.

Sub-Poissonian statistics of Rydberg-interacting dark-state polaritons.

Storing single photons emitted by a quantum memory on a highly excited Rydberg state.

Evidence for strong van der Waals type Rydberg-Rydberg interaction in a thermal vapor.

Photofragmentation, state interaction, and energetics of Rydberg and ion-pair states: resonance enhanced multiphoton ionization of HI.

Single-photon switch based on Rydberg blockade.

Wireless network control of interacting Rydberg atoms.

Universal nonequilibrium properties of dissipative Rydberg gases.

Photoassociation of long-range nD Rydberg molecules.

Effective Field Theory for Rydberg Polaritons.

Dss1 is a 26S proteasome ubiquitin receptor.

Methylation of Sindbis virus "26S" messenger RNA.

Single-photon transistor mediated by interstate Rydberg interactions.

Designing frustrated quantum magnets with laser-dressed Rydberg atoms.

Selective production of Rydberg-stark states of positronium.

Strongly Correlated Growth of Rydberg Aggregates in a Vapor Cell.

Borromean three-body FRET in frozen Rydberg gases.

Spontaneous avalanche ionization of a strongly blockaded Rydberg gas.

Rydberg atoms in hollow-core photonic crystal fibres.

Quantum simulation of energy transport with embedded Rydberg aggregates.

Observing Rydberg atoms to survive intense laser fields.

Self-interaction corrected density functional calculations of molecular Rydberg states.

All-optical quantum information processing using Rydberg gates.