Photodissociation Dynamics of Benzaldehyde-d5 at 205 nm: The H

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Photodissociation Dynamics of Benzaldehyde-d at 205 nm: The H Atom Production Channel

Sung Man Park, Sang Hyuck Yoon, Chan Ho Kwon, and Hong Lae Kim J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b10134 • Publication Date (Web): 14 Dec 2017 Downloaded from http://pubs.acs.org on December 19, 2017

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The Journal of Physical Chemistry

Photodissociation Dynamics of Benzaldehyde-d5 at 205 nm: The H Atom Production Channel

Sung Man Park, Sang Hyuck Yoon, Chan Ho Kwon, Hong Lae Kim*

Department of Chemistry, College of Natural Sciences and Institute for Molecular Science and Fusion Technology, Kangwon National University, Chuncheon 24341, South Korea

Abstract

Photodissociation dynamics of benzaldehyde-d5 (C6D5CHO) at 205 nm was investigated by measuring laser-induced fluorescence spectra of fragment H atoms. From the Doppler-broadened spectra, center-of-mass translational energy release into the C6D5CO + H channel was obtained as 68.8 ± 5.8 kJ/mol with the absolute quantum yield, 0.17 ± 0.03. The observed translational energy was successfully estimated from two-dimensional potential energy surfaces along the C-H dissociation coordinate and the CCO bent angle and the out-of-plane H angle, respectively calculated at the B3LYP/cc-pVDZ level. The dissociation of H should take place along the triplet surface via intersystem crossing from S1 after internal conversion from the initially excited S3 state and on the triplet surface, the dissociation proceeds along the CCO bent-linear-bent configuration with H being planar to non-planar pathway.

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Introduction

Benzaldehyde, the smallest aromatic aldehyde, generates one of the precursors of aromatic peroxy radicals that are important intermediates of atmospheric pollutants, from reactions with ozone, NO2, and OH in atmosphere, which can also be produced by solar radiation.1 The electronic structures of benzaldehyde in the gas phase were extensively studied by measuring its UV absorption spectra.2-4 In addition to the nπ* transition in aliphatic aldehydes, the higher energy ππ* transitions upon UV absorption in benzaldehyde would induce various kinds of non-radiative relaxation processes such as internal conversion (IC) and intersystem crossing (ISC). The weakest, well-structured band observed at 290 ~ 379 nm in the UV spectrum is assigned as the S0 → S1 (nπ*) transition whose origin is at 371 nm. The absorption bands with increasing intensities in the spectra arise from the transitions to the S2 and the S3 states whose origins were identified at 284 and 242 nm, respectively, taking advantage of theoretical calculations.5,6 Phosphorescence was measured from the T1 state after excitation to the S2 and S3 states, from which it was concluded that fast relaxation from the initially excited singlet states to the S1 state followed by ISC to the T1 state should dominate over the S1 → S0 internal conversion.7-9 The recent ultrafast electron diffraction measurement suggested that the fast IC (250 fs) to the S1 state from the initially excited S2 state at 266.7 nm should take place followed by ISC to T1, which is responsible for the phosphorescence and dissociation to the triplet C6H6 and CO.10

The photodissociation dynamics of aliphatic aldehydes such as acetaldehyde has thoroughly been investigated.11-14 For acetaldehyde, the CH3 + CHO channel was studied in detail by measuring rotationally resolved spectra of the CHO fragments employing laser-induced fluorescence.11,13 Observed energy partitioning among the fragments and the dynamics of dissociation were successfully modeled by the impulsive dissociation with an exit channel barrier. In addition, the roaming mechanism was suggested from the dissociative production of H and CH3 upon UV absorption followed by production of CO and CH4.12 The H atom production channel was also studied by measuring translational energies of the H atom fragments.14 The dissociation of H from 2


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acetaldehyde should take place along the triplet surface with the exit channel barrier. Contrary to numerous studies on the aliphatic aldehydes, the photodissociation dynamics of aromatic aldehydes have rarely been investigated except for benzaldehyde,10,

15-17

wondering whether the similar

dissociation mechanisms to those for the aliphatic aldehydes could be applied. The photodissociation of benzaldehyde mainly proceeds via the following primary processes.

C6H5CHO → C6H6 + CO

(1)

C6H5 + CHO

(2)

C6H5CO + H

(3)

The photodissociation dynamics of benzaldehyde was extensively studied at 193, 248, and 266 nm by ion imaging of the mass-selected photofragments.18 Potential energy surfaces were calculated for the ground singlet and the triplet states at the CCSD (T)/6-311+G (3df, 2p) level of theory for different product channels with corresponding transition states. From photofragment yield measurements, the channel branching ratios were measured, where at lower photon energies, the molecular product channel (1) was dominant but as the photon energy increased, the radical product channel (2) became more important. However, since the C6H5CO fragments were not directly detected, the channel (3) could only be estimated from the secondary dissociation of the other channels. The observed relative yields were compared to the statistical theory calculations, from which it was concluded that at the wavelengths studied, (1) and (2) should be the major channels and that the triplet state produced via the intersystem crossing from the initially prepared excited singlet states should mainly contribute to the product formation.

Most of the polyatomic photofragments in the ion-imaging mass-spectrometry can usually be ionized by absorption of a photon at any convenient wavelength in the vacuum ultraviolet (VUV) region but for the ionization of H, the resonance-enhanced multiphoton ionization should be employed. The observe ion-imaging spectra provide velocity and angular distributions of the photofragments, from 3



which the dissociation dynamics can be investigated. Alternatively, laser-induced fluorescence detection of the photofragments can be applied to study the photodissociation dynamics if the resolution is high enough to deduce the dynamical information such as momentum distribution from the observed Doppler profile in the spectra. The spectra of H atoms can be measured by VUV fluorescence and/or resonance-enhanced ionization from the 2p state by VUV absorption from the 1s state at 121.6 nm. Instead, in our laboratory, the Doppler-broadened H atom spectra were measured by the fluorescence from the 3s → 2p transition at 656.46 nm after the two-photon excitation at 205.14 nm to the 3s, 3d state,19-21 where at this wavelength, almost all the polyatomic molecules holding H release the H atoms as the photofragments.14,22,23 The H atom dissociation channel (3), the minor channel among possible primary dissociation channels from benzaldehyde upon UV absorption has been investigated in the present study by measuring the Doppler broadened spectra of H atom fragments, from which average translational energy release into the products was obtained. The aldehyde-H was discriminated against the phenyl-D by isotopic substitution and the quantum yield for the aldehyde-H dissociation from benzaldehyde was measured as well. Quantum chemical density functional theory calculations for construction of the potential energy surfaces were also performed to elucidate the detailed dissociation mechanism.

Experiments

Experiments were described in detail in the previous report.23 In brief, the experiments were performed in a conventional, low pressure flow-cell with the gaseous sample slowly flowing through the cell whose pressures were controlled by needle valves. The liquid benzaldehyde-d5 (C6D5CHO) was purchased from Aldrich (>98% purity) and used without further purification. The spectra of H atom fragments were measured by laser-induced fluorescence employing the 1s → 3s, 3d transition by two-photon absorption at 205.14 nm and the fluorescence from the 3s → 2p transition at 656.46 nm (Balmer-α line). The 205.14 nm light was generated in a BBO crystal by mixing of the fundamental 4


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with the doubled output of a dye laser (Continuum ND6000) which was pumped by the second harmonic of an Nd:YAG laser (Continuum Surelite III). Within the same light pulse, the parent molecule absorbs a photon of energy high enough to produce H atoms, which results in the one-color experiment. Thus, with our linearly polarized laser light, the polarization direction of the dissociating light is perpendicular to the probe direction. The fluorescence was detected by a photomultiplier tube (Hamamatsu R-928) at a right angle through a bandpass filter centered at 656 nm and the measured signal was recorded by a digital storage oscilloscope.

The spectra of H atoms were measured in a linear regime of the log-log plot of the fluorescence signal vs. the laser power through a properly chosen lens. The lenses with various focal lengths (f = 10 ~ 50 cm) were attempted to adjust the size of the laser beam and to keep all the fast-moving fragment H atoms in the viewing zone. When in fact, a lens with the very short focal length (f = 15 cm) was placed, the H atoms moving in the perpendicular direction with respect to the probe direction were found to escape from the viewing zone resulting in wider profiles in the spectrum. Thus, the lens was chosen (f = 40 cm) to provide no change in the observed Doppler profile in the spectrum. With this lens, the log-log plot of the fluorescence signal in terms of the laser power was measured up to 100 µJ/pulse and the spectrum was measured at 30 µJ/pulse where the linear slope of the plot was near 3, which implied the overall three-photon processes, the one-photon absorption by the parent molecule and subsequent two-photon absorption by the fragment H atoms. The linewidth of the laser light was measured from the gaseous I2 spectra near 615 nm, which was 0.07 cm-1 at FWHM. This laser line profile was deconvoluted from the measured Doppler profiles to deduce the translational energies of the H atoms.

Results

A spectrum of H atoms produced from photodissociation of benzaldehyde-d5 is presented in Figure 1a. 5



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The Doppler profile should be symmetrical centered at 97,494 cm-1 arising from one-dimensional velocity distribution in the spectrum. The observed profile is a result of averaging five spectra separately measured to increase S/N ratio in the spectrum but is not still perfectly symmetrical due to the small signal since the H atom dissociation channel is minor. The observed spectrum is also an overlap of the spectra between fine-structure distributions arising from spin-orbit couplings in H (~0.04 cm-1) but not resolved under our spectral resolution. Nevertheless, the observed onedimensional profile could well be fitted to a single Gaussian function implying that the velocity distribution of the H atoms would be isotropic. In addition, when there exist the H atoms produced in another channel, the profile would be a convolution of the profiles with different widths but in the present experiment, the fit with one Gaussian function implies no other significant contribution in the observed profile. Then, the average relative translational energy of the two photofragments is obtained from the second moment of the excitation spectrum using the following formula,

= (mRH/mR)3mH c2/2

(4)

where mRH, mH, and mR are the mass of the parent molecule, of the H atom, and of the sibling fragment of the H atom, respectively. In this expression, ν0 and ∆ν are the center frequency (97,494 cm-1) and the width of the observed spectrum, respectively and c is the speed of light. The measured center-of-mass translational energy release in the C6D5CO + H channel was then obtained as 68.8 ± 5.8 kJ/mol where the error bar was determined from the lower and upper bound of the Gaussian fit with dotted lines in Figure 1a. A laser-power dependence of the fluorescence signal from the H atoms was measured and the linear slope of 2.96 in the log-log plot of the signal intensity vs laser power ensures the overall three-photon processes that is, the one-photon absorption by benzaldehyde for photodissociation and the two-photon absorption by the fragment H atoms for florescence detection.

In the case of competing pathways from an initially prepared state upon the electronic transition, branching ratios between these pathways are represented as the quantum yields defined as the ratio of 6


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the number of molecules undergoing a designated process to the number of photons absorbed at a given wavelength. Thus, the quantum yield corresponds to the probability of a specific reaction taking place among all possible reaction channels. In the present study, the absolute quantum yield for the C6D5CO + H channel was measured. Avoiding difficulties in measuring the absolute two-photon absorption cross section of H at 205 nm, the quantum yield was measured relative to that of H2S producing the H atoms at 205 nm.23 The photodissociation of H2S in UV was thoroughly investigated.24-26 where the dissociation is impulsive from the perpendicular transition resulting in a distinct Doppler profile in the H atom spectra. The measured Doppler profile in the spectra from H2S provides the total amount of the H atom produced, whose quantum yield should be unity (Figure 1b). Thus, the absolute quantum yield of the H atom channels from benzaldehyde can be measured relative to that of H2S. From separate absorption measurements, the quantum yield for the C6D5CO + H channel at 205 nm was measured to be 0.17 ± 0.03. With the present experimental set-up, the quantum yield for H production from CH2Cl2 was measured for comparison, which is 0.0017 ± 0.0004, in good agreement with the previously reported at 193 nm (0.002 ± 0.001).27

Discussion

Energy partitioning of the available energy among products is governed by the dissociation dynamics that depends upon the shape of potential energy surfaces (PES) leading to the individual product channels. Based upon the observed translational energy release, a detailed dissociation mechanism could then be deduced with an aid of the PES calculations. According to spin-correlation arguments, the two spin-doublet ground state products, C6D5CO (2A') and H (2S), should be correlated to one singlet and triplet states of benzaldehyde. Assuming the dissociation on the ground singlet state after internal conversion, the translational energy release for the C6D5CO + H channel was calculated employing the prior model, albeit simple but one of the statistical models describing slow dissociation on the vibrationally hot ground electronic state.28 In this model, the available energy among the 7



products is assumed to be distributed all degrees of freedom of the products with equal probability. Thus, the translational energy distribution of the H atom products can be obtained by counting the number of vibrational states in the sibling polyatomic products at the energy, E = Eavl – Et, where the vibrational frequencies are calculated based upon the optimized structure obtained in the density functional theory (DFT) calculations at the B3LYP/cc-pVDZ level employing the GAUSSIAN 09 program package.29 The calculated average translational energy release, 20.3 kJ/mol was much less than the experiment (68.8 kJ/mol), which suggests that the H atom dissociation should not occur on the ground singlet state but mainly on a triplet state. As mentioned in the introduction, ISC to the triplet states mostly responsible for the dissociation upon UV excitation and the observed phosphorescence with near unit quantum yield thus suggest that even though the ground singlet state is correlated to the H atom production channel, the amount of H atom produced on the ground state, which would contribute to the Doppler profile in the observed spectrum as a slow speed component, should be negligible. Another possible production of H would be the production of CHO upon UV absorption followed by subsequent dissociation of H from CHO, where the barrier for the H production from HCO is 89.5 kJ/mol and the reaction enthalpy is 52.1 kJ/mol, respectively.30 In the study on the photodissociation dynamics of C6H5CHO by Lee et. al.,18 the C6H5 + CHO channel was thoroughly investigated by measuring the velocity map-imaging of both the C6H5 and CHO fragments and in addition, IR emission from the CHO fragments. They measured a bimodal momentum distribution for the CHO fragments with the slow component of about 3%, which originates from the direct dissociation on the ground singlet surface. In this case, the internal energy of the CHO fragments might be large enough to produce H from the secondary dissociation of CHO but the amounts should be too small for H to be detected in the present experiment compared to those that are directly produced. The source of the major, fast CHO fragments should be the dissociation on the triplet surface with the average translational energy of 30 kJ/mol. From their IR emission measurements, however, the internal energy in the CHO fragments was determined to be 20 kJ/mol as the upper limit. Thus, assuming the similar energy distribution in CHO in the present 205 nmdissociation (smaller available energy than that in the previous 193 nm-dissociation), the H 8


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production from the secondary dissociation of CHO should be unlikely judging from the observed translational energy of H and the barrier for the H atom production from CHO. The alpha-cleavage from carbonyl compounds such as acetone and acetaldehyde takes place impulsively in few hundred femtoseconds,31 then within the 7 ns laser pulse in the present experiment, the CHO could dissociate after absorption of one more photon at 205 nm. In this case, however, the available energy would be more than 700 kJ/mol. Thus, the production of H from the secondary dissociation upon two-photon absorption would also be unlikely because the observed translational energy would be too small compared to the available energy assuming the two-photon absorption and the linear slope of near 3 instead of 4 was observed in the measured power dependence.

As studied in the UV absorption spectra, the absorption at 205 nm could lead benzaldehyde to the S4 state but the calculated oscillator strength for the S0 → S4 transition is zero,18 from which it can be suggested that the transition at this wavelength should lead the molecule to the vibrationally excited S3 state. The fast internal conversion from the S3 (ππ*) state to the S1 (nπ*) state would take place through the highly excited vibrational states in lower electronic states as mentioned by Lim.8 The intersystem crossing then brings the molecule to the triplet state followed by dissociation. The previously calculated potential energies at the CCSD (T)/6-311+G (3df, 2p) level exhibited a reverse barrier, 34.7 kJ/mol along the triplet surface on the H atom production channel.18 The quantum chemical density functional theory (DFT) calculations at the B3LYP/cc-pVDZ level were performed in the present study to obtain energies of the molecular species with optimized geometries for the triplet dissociation channel. The potential energy curves along the C-H bond length were obtained employing the DFT calculations for the ground singlet and triplet states, and the time dependent DFT calculations for the excited electronic states, respectively and presented in Figure 2 with molecular geometries on the transition state and at the product asymptotes. After the zero-point energy correction, the energy differences of the optimized molecular species along the reaction path turned out to be very similar to the previously reported and in good agreement with the experimental reaction enthalpy, 363.6 kJ/mol.18 In the figure, the potential energy curves along the T1 surface show exit 9



channel barriers but two different dissociation pathways. One is planar H dissociation with linear CCO products sustaining the transition-state geometry and the other is non-planar dissociation with bent CCO products different from the transition-state where the energy for the bent CCO products is lower by 25.0 kJ/mol than that for the linear CCO products. The forward barrier to the linear products is higher than that to the bent products due to a large angle difference in the structures between the transition state and the equilibrium but the lower energy in the bent products results in the similar reverse barrier energies (17.0 kJ/mol for the linear product and 18.0 kJ/mol for the bent product, respectively) between the two product channels. The average translational energy release in the H atom dissociation channel could then be calculated from the barrier-impulsive (BIM) model. The BIM model in general could be applied to estimate the energy partitioning from the reaction when there exists a potential well and an exit channel barrier along the reaction coordinate.32 In this model, it is assumed that the parent molecule would spend a few vibrational periods in the potential well and then dissociate along the repulsive part of the potential energy surface beyond the barrier. Then, the amount of energy associated with the reverse barrier would be transformed into product translation and rest of the available energy would statistically be distributed among all degrees of freedom of the products. The statistical energy partitioning into the product translation was obtained from the prior model mentioned above while the energy partitioning associated with the reverse barrier was obtained from the impulsive model.28 In the impulsive model, no vibrational excitation in the polyatomic products is assumed since the dissociation is fast and the translational energy in the products is calculated invoking conservation of energy and momentum. The calculated average translational energy releases for the dissociation to the linear CCO product and the dissociation to the bent CCO product were 35.1 kJ/mol and 36.4 kJ/mol, respectively, both of which are still substantially smaller than the experiment.

In order to elucidate the dissociation mechanism in detail, multi-dimensional potential energy surfaces (PES) for the H atom dissociation channel were constructed. Recently, importance of examining the multi-dimensional PES with at least one more coordinate directly coupled to the reaction coordinate 10


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was demonstrated comparing with the one-dimensional potential energy curves to investigate the detailed dissociation dynamics of the molecules upon photon absorption.23 Especially, when the molecule experiences a curve crossing between the electronic states induced by nuclear motion at the conical intersection, examination of the potential energies along the multiple nuclear coordinates including the reaction coordinate is required to understand the underlying dynamics in detail.33,34 In the case of photodissociation of H2O2 leading to the H atom production, on the two-dimensional PES, the surface crossing to the repulsive part of a triplet surface from S1 in the Franck-Condon region was identified and the experimentally observed fast, impulsive dissociation of H along the triplet surface was well explained.35 Although the potential energies should be calculated as a function of all nuclear coordinates, which is a very formidable task for polyatomic molecules, a few intersections of the multi-dimensional surface relevant to a specific dissociation channel can reliably be selected to investigate the detailed dissociation mechanism. In this sense, a nuclear coordinate directly coupled to the dissociation coordinate would be a plausible choice. Namely, an overall shape of PES, especially in the case of non-adiabatic couplings between the two electronic states, should be examined but PES along the two with the one at least directly coupled to the dissociation coordinate can be investigated because PES along all degrees of freedom of the nuclear motion could hardly be described. The unique feature such as the surface crossing that could not be observed in the one-dimensional potential energy curve along the reaction coordinate can be discovered in the two-dimensional surfaces.

The equilibrium structures and potential energies along the reaction coordinate leading to the C6D5CO + H channel on the S0 and T1 states were calculated at the B3LYP/cc-pVDZ level of density functional theory and the time-dependent density functional theory for the excited singlet states. The equilibrium structures of benzaldehyde on the S0, S1, S2, S3, and T1 states are depicted, respectively in Figure 3 where the CCO angles are bent revealing the sp2 character on the carbonyl C atom. As the other coordinate for the two-dimensional PES along the dissociation coordinate, the CCO angle along with RC-H was chosen and the calculated PES is presented in Figure 4a. The potential energies were 11



calculated with geometry optimization except for the chosen parameters, such that RC-H and the CCO angle were changed step by step from the equilibrium values, respectively. The PES’s for the S1 and the T1 states appear similar showing a well and an exit channel barrier that would originate from the structural differences between the asymptotic products and the transition state. The singlet surfaces were calculated only beyond the barrier because at least one of the correlated products should be produced in the excited electronic state. In the two-dimensional PES, the singlet-triplet surface crossing can be noticed in the barrier region with elongated H but the CCO angle is almost linear, not bent as in equilibrium. As the other nuclear coordinate for the two-dimensional PES, the dihedral angle of H, the out-of-plane angle of H against the CCO plane was chosen and the similar calculations were performed. The overall shape of the constructed PES in Figure 4b appears similar to the PES obtained with the CCO angle as the other coordinate but shows that the in-plane H with linear CCO on the barrier moves to the out-of-plane position as the dissociation proceeds. These two intersections of the multi-dimensional PES examined above should be the plausible choices to investigate the detailed H atom dissociation dynamics because the rest of the molecular motion would not be directly coupled to the dissociation coordinate. It can thus be suggested that upon UV absorption, the parent molecule at the equilibrium position with the bent CCO configuration relaxes to S1 by the internal conversion from the initially prepared singlet state (S3) and then moves to the crossing region at the transition state where CCO is almost linear with H being planar followed by further dissociation of H along the triplet surface toward the product asymptote where CCO is bent and the H atom is at the out-of-plane position. In this case, the energy partitioning among the products can be modeled by the barrier impulsive model along the CCO bent-linear-bent configuration with the H being planar to nonplanar pathway with the reverse barrier energy of 42.0 kJ/mol. The calculated translational energy release for the C6D5CO + H channel based upon the two-dimensional PES is 58.0 kJ/mol, which is closer to the experiment (68.8 kJ/mol).

Conclusions 12


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The detailed dissociation dynamics of H from benzaldehyde-d5 has been investigated upon electronic excitation at 205 nm by measuring the laser-induced fluorescence spectra of the fragment H atoms. In addition, the absolute quantum yield for the C6D5CO + H channel was measured, which is 0.17 ± 0.03. In order to elucidate the detailed dissociation dynamics, the two-dimensional potential energy surfaces that is, some intersections along the full-dimensional potential energy surface, including at least one more coordinate directly coupled to the dissociation coordinate should be examined. The 205 nm photon absorption leads the molecule to the S3 state followed by the internal conversion to the S1 state and the dissociation of H from benzaldehyde should take place along the triplet surface via the surface crossing from S1 near the barrier where the CCO is almost linear. On the triplet surface, the dissociation of H takes place along the out-of-plane direction from the equilibrium planar geometry leading to the asymptotic product where the CCO is bent. The observed translational energy release was successfully estimated by the barrier impulsive model based upon the calculated two-dimensional potential energy surfaces.

Acknowledgments

This work was supported by National Research Foundation in Korea (2017R1A4A1015405 and 2017R1A2B2007993) and in part, 2015 research grant from Kangwon National University (520150433).

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10802-10808. (13) Gejo, T.; Takayanagi, M.; Kono, T.; Hanazaki, I. Photodissociation Dynamics of Acetaldehyde. Vibrational Energy Distribution in the Photofragment HCO. Chem. Phys. Lett. 1994, 218, 343-348. (14) Kang, T. Y.; Kang, S. W.; Kim, H. L. Photodissociation of Acetaldehyde at 205 nm: The H Atom Channels. Chem. Phys. Lett. 2007, 434, 6-10. (15) Bruehlmann, U.; Monella, M.; Russegger, P.; Huber, J. R. The Triplet State Decay (T1(nπ*)→S0) of Benzaldehydes in the Dilute Gas Phase. Chem. Phys. 1983, 81, 439-447. (16) Yang, J. J.; Gobeli, D. A.; El-Sayed, M. A. Change in the Mechanism of Laser Multiphoton Ionization-Dissociation in Benzaldehyde by Changing the Laser Pulse Width. J. Phys. Chem. 1985, 89, 3426-3429. (17) Silva, C. R.; Reilly, J. P. Laser Ionization Measurements of the Photodissociation Kinetics of JetCooled Benzaldehyde. J. Phys. Chem. A 1997, 101, 7934-7942. (18) Bagchi, A.; Huang, Y. H.; Xu, Z. F.; Raghunath, P.; Lee, Y. T.; Ni, C. K.; Lin, M. C.; Lee, Y. P. Photodissociation Dynamics of Benzaldehyde (C6H5CHO) at 266, 248, and 193 nm. Chem. Asian J. 2011, 6, 2961-2976. (19) Goldsmith, J. E. M. Photochemical Effects in 205 nm, Two Photon Excited Fluorescence Detection of Atomic Hydrogen in Flames. Opt. Lett. 1986, 11, 416-418. (20) Goldsmith, J. E. M. Two Photon Excited Stimulated Emission from Atomic Hydrogen in Flames. J. Opt. Soc. Am. B 1989, 6, 1979-1985. (21) Quandt, R.; Wang, X.; Min, Z.; Kim, H. L.; Bersohn, R. One-Color Molecular Photodissociation and Detection of Hydrogen Atoms. J. Phys. Chem. A 1998, 102, 6063-6067. (22) Kang, T. Y.; Kim, H. L. Photodissociation of Formamide at 205 nm: The H Atom Channels. Chem. Phys. Lett. 2006, 431, 24-27. (23) Park, S. M.; Kwon, C. H.; Kim, H. L. Dynamics of H Atom Production from Photodissociation of Acetic Acid-d1. J. Phys. Chem. A 2015, 119, 9474-9480. (24) Johnston, G. W.; Katz, B.; Tsukiyama, K.; Bersohn, R. Isotopic Variants of the H + H2 Reaction. 1. Total Reaction Cross Sections of the H + D2 and H + HD Reactions as a Function of Relative 15



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Figure Captions

Figure 1 Laser-induced fluorescence spectra of H atoms produced from photodissociation of (a) benzaldehyde-d5 at 205 nm and the Gaussian fit with dotted lines as the upper and lower bound of the fit and (b) H2S at 205 nm (and from benzaldehyde-d5 in (a)) for quantum yield measurements.

Figure 2 One-dimensional potential energy curves for dissociation of benzaldehyde with molecular geometries along the C-H dissociation coordinate obtained by density functional theory calculations for S0 and T1, and time dependent density functional theory calculations for the S1, S2, and S3 states, respectively at the B3LYP/cc-pVDZ level.

Figure 3 Optimized structures of benzaldehyde on the S0, S1, S2, S3, and T1 states obtained from density functional theory and time-dependent density functional theory calculations at the B3LYP/ccpVDZ level.

Figure 4 Intersection of the potential energy surface for the C6D5CO + H channel for the S0, T1, S1, S2, and S3 states of benzaldehyde along the C-H dissociation coordinate and (a) the CCO angle (b) the out-of-plane H angle

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Figure 1

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Figure 2

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Figure 3

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Figure 4

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The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46

C6D5CHO → C6D5CO + H S3 S2

T1

S1

S0 ACS Paragon Plus Environment

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Photodissociation Dynamics of Benzaldehyde-d5 at 205 nm: The H

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