CD3 Deformation Modes as Preferential Promoters of Methylamine-d3

Nov 23, 2015 - CD3 Deformation Modes as Preferential Promoters of Methylamine-d3 to the First Electronic State. Michael Epshtein, Alexander Portnov, a...
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CD3 Deformation Modes as Preferential Promoters of Methylamine‑d3 to the First Electronic State Michael Epshtein, Alexander Portnov, and Ilana Bar* Department of Physics, Ben-Gurion University of the Negev, Beer-Sheva 84105, Israel ABSTRACT: The room-temperature photoacoustic Raman and jetcooled H action spectra, measured in the region of the fundamental CD3 stretches and the almost isoenergetic overtones or combinations of CD3 deformations in the methylamine-d3 (CD3NH2) isotopologue, show different relative intensities of the vibrational bands. The observed difference and the vibrational assignment point to favored ultraviolet excitation due to larger Franck−Condon (FC) factors from the deformation modes, leading to more effective N−H bond cleavage in CD3NH2 predissociation. The comparable measured two-color reduced-Doppler ion images and total kinetic energy distributions resulting from the predissociation of molecules promoted from vibrationally excited and vibrationless ground states confirmed that the FC factors and not the ensuing dynamics are the main reason for the mode specificity in this molecule.

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INTRODUCTION The realization of mode and bond selectivity,1,2 which facilitate enhanced reactivity and control of product identity, has attracted considerable interest since the early days of lasers. Nevertheless, this issue still remains vague, driving further attempts toward the development of new approaches and studies on additional molecular systems, which are expected to lead to a better understanding of these phenomena. Indeed, efforts to achieve selectivity through coherent control3−5 as well as state-specific control6,7 performed via narrow-band resonant frequency excitation are subjects of intense research. In particular, in the latter approach, molecules are prepared in specific states and their interactions with photons,6,7 other reactants,6,8−10 or surfaces8,11−13 are studied. Although the preparation of specific vibrational modes of molecules can be relatively easily achieved, control of the reactivity and product branching ratio may not always succeed. This is mainly due to the fact that efficient energy randomization, i.e., intramolecular vibrational redistribution (IVR),14 can occur in polyatomic molecules on the time scale of picoseconds, which may eventually lead to breaking of the weakest bond in a molecule and consequently to the loss of bond selectivity. As pointed out above, one of the essential means for obtaining selectivity is vibrationally mediated photodissociation (VMP),1,2,6,7 which prepares specific vibrational states of molecules and then promotes them to excited electronic/ vibronic states, where they subsequently photodissociate or predissociate. This approach has been employed since the original theoretical predictions on photodissociation of a triatomic prototype, vibrationally excited monodeuterated water (HOD),15−17 in attempts to control the product identity by pre-excitation of fundamentals or overtones of O−H and O−D stretches.18−24 In this molecule it was possible to localize the energy in a specific bond, but nevertheless, bond selectivity could not always be observed. In fact, the additional key players for obtaining selectivity were enhanced Franck−Condon (FC) © 2015 American Chemical Society

factors (overlap integrals between the prepared intermediate vibrational states on the ground electronic state and those on the excited state) and close coupling of the initially prepared vibrational modes to the desired reaction coordinates. Later on, studies were performed on larger molecules, including for instance isocyanic acid (HNCO),25 acetylene (C2H2) and its deuterated isotopologue (C2HD),26−28 methylamine (CH3NH2),29 which includes a torsional degree of freedom, and pyrrole (C4H4NH).30 For the four-atom molecules, improved FC factors were observed, especially for excitation of combination bands including bending, which considerably affected the FC factor and eventually led in some cases to mode and bond selectivity. In the last two molecules, promotion of pre-excited eigenstates (consisting of C−H and N−H stretch fundamentals and overtones and combinations of deformations) to the excited electronic states by ∼243.1 nm photons resulted in methylamine predissociation and pyrrole photodissociation that eventually led to enhanced H photofragment release.29−31 The enhancement of the yield of H photofragments differed for the various initially prepared states but was most notable for pre-excited deformation and bending modes in methylamine and pyrrole, respectively. It was conjectured that the IVR in these relatively large molecules was incomplete on a nanosecond time scale, consequently bringing different FC factors into play and resulting in modedependent bond cleavage. Actually, the promotion of methylamine and its isotopologues from the ground electronic state, S0, to the first excited electronic state, S1, involves excitation in the vertical FC region of a nitrogen lone-pair electron into a bound 3s Rydberg state, Special Issue: Ronnie Kosloff Festschrift Received: October 21, 2015 Revised: November 19, 2015 Published: November 23, 2015 3049

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isoenergetic predissociation of vibrationless ground state and vibrationally excited molecules indicate that the process is affected by the initial vibrational excitation, which results in enhanced FC factors that guide the observed mode specificity in CD3NH2.

which becomes dissociative at longer N−H distances, eventually leading to dominant N−H bond rupture.32,33 Upon this electronic transition, a structural change from pyramidal NH2 to planar NH2 with respect to the C−N axis occurs, leading to activity in the CH3 rock (ν7′) and NH2 wag (ν9′) modes.34 Consequently, the exit from the bound part of the upper potential energy surface (PES) occurs by tunneling34−37 through a reaction barrier followed by passage through a conical intersection (CI) between the S0 and dissociative S1 electronic states33,38 (see Figure 1), leading to

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EXPERIMENTAL SECTION Three types of experiments were performed, including PAR spectroscopy of the methylamine-d3 molecules and action spectroscopy and TCRD ion imaging of the H photofragments released in predissociation of parent molecules. The PAR spectrum of CD3NH2 was obtained by the scheme shown in Figure 1a. The action spectrum was measured by promoting pre-excited vibrational states with UV photons to the first electronic state (Figure 1b), and the TCRD ion images were obtained either similarly (Figure 1b) or following UV excitation of ground vibrational state molecules (Figure 1c). The PAR spectrum was obtained from measurements performed in a PA cell using two-color excitation, while the action spectrum and the TCRD ion images were obtained in experiments carried out in a time-of-flight mass spectrometer (TOFMS) using four- and three-color photon excitation, respectively. The vibrational pre-excitation was driven by SRS in all of the experiments that required initial state preparation. Tunable pump (ωp) and Stokes (ωS) beams obtained from the signal and idler, respectively, of an optical parametric oscillator (OPO) pumped by the third harmonic of a seeded neodymium-doped yttrium aluminum garnet (Nd:YAG) laser were used. SRS was induced whenever the frequency difference ωp − ωS matched the frequency of a Raman-active transition. The spatially and temporally overlapping SRS beams (∼23 mJ/pulse with a full width at half-maximum of ∼5 ns) were focused by a 20 cm focal length (f.l.) lens into the PA cell, which included a small microphone and gas-phase parent molecules at a pressure of ∼20 Torr. This allowed the sound wave generated by nonradiative relaxation of the vibrationally excited molecules to be obtained. Essentially, the generated PA signal was detected by the microphone, amplified, and monitored with an oscilloscope, which allowed measurement of the height of the peak in the oscillatory PA signal by a gated boxcar integrator. This signal was passed to a computer, allowing the PAR spectrum of methylamine-d3 in the region of the CD3 stretches to be obtained. The wavelength calibration for the ωp and ωS beams was performed by measuring the PAR spectra of nitrogen and oxygen in the corresponding energetic region and comparing the positions of the lines in the measured spectra to those in the HITRAN database.40 For the experiments performed in the TOFMS, a mixture of ∼5% methylamine-d3 seeded in argon was expanded through a pulsed nozzle to produce a molecular beam. The SRS beams were focused into the interaction region of the TOFMS by a 20 cm f.l. lens. The vibrationally excited molecules were then predissociated, and the ensuing H photofragments were probed by (2 + 1) resonantly enhanced multiphoton ionization (REMPI) via the 2s 2S ← 1s 2S transition at 243.135 nm. Actually, TCRD probing41 was employed to measure the resultant H action spectrum and the ion images. The TCRD probing enabled effective elimination of the dissociationinduced Doppler width in the velocity distributions and consequently monitoring of all the released H photofragments. In these experiments, two counterpropagating laser beams (∼40 μJ/pulse for each of them) focused by 30 cm f.l. lenses were set at close wavelengths of 243.12 and 243.15 nm. These

Figure 1. Schematic energy diagrams and the photons used for the excitations, displaying the procedures of (a) stimulated Raman excitation, (b) predissociation initiated from vibrationally excited molecules, and (c) predissociation initiated from vibrationless groundstate molecules. The last two processes are shown together with a diagram on the right that exhibits the two-color reduced-Doppler probing of the H photofragments.

N−H bond cleavage. The passage through the CI can occur to the asymptote or by internal conversion to the ground electronic state, giving rise to H photofragments from the two different channels.39 Although we have previously shown that mode selectivity occurs by initially exciting C−H and N−H stretches and CH3 deformations in methylamine while accessing the ν7′ + 2ν9′ region on the upper electronic state, it remains to find whether this is a general phenomena in these prototype molecules. Therefore, in this work, we extend our study to the deuterated isotopologue of methylamine, i.e., methylamine-d3 (CD3NH2), which still includes the NH2 moiety. Excitation in the 2050− 2300 cm−1 region allows the preparation of fundamental CD3 stretch modes and overtones and combinations of CD3 deformations and eventually sampling of the region of the ν7′ + ν9′ band on the upper PES when ∼243.1 nm photons promote the molecules to an excited vibronic state. Here we examine whether behavior similar to that encountered in CH3NH2 can be observed even when a different region of the excited PES is accessed and furthermore whether the initial vibrational excitation can alter the ensuing dynamics. Therefore, we here use a comparison of the results obtained by room-temperature photoacoustic Raman (PAR) spectroscopy and jet-cooled H action spectroscopy, which indicate the efficacy of stimulated Raman scattering (SRS) and the H photofragment yield, respectively, to assess which of the initially prepared vibrational modes are more effective in inducing CD3NH2 predissociation. In addition, in another set of experiments, frequency-resolved two-color reduced-Doppler (TCRD) ion imaging was used to obtain the entire total kinetic energy distributions (TKEDs) of the departing H photofragments in each laser pulse. The comparable measured TCRD ion images and total TKEDs resulting from the almost 3050

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The Journal of Physical Chemistry A wavelengths were chosen to preclude Doppler probing by a single beam and to exactly match the REMPI transition by the two photons from the two different beams. One of the beams for TCRD was obtained from the doubled output of a tunable dye laser pumped by the third harmonic of an additional Nd:YAG laser, while the other was produced by frequency mixing of the doubled output of another dye laser pumped by the second harmonic of a Nd:YAG laser with the residual of the Nd:YAG fundamental. The dissociation of the vibrationally preexcited molecules was performed by photons from the TCRD beams with a 10 ns delay relative to the SRS beams. The resultant H ions were accelerated in the TOFMS by an ion-lens assembly toward a microchannel plate (MCP) detector coupled to a phosphor screen, allowing measurement of the TCRD images and the action spectrum. For these measurements, the molecular beam and the lasers were pulsed at 10 Hz, and mass selectivity was achieved by gating the MCP voltage for detection of H photofragments. The action spectrum was monitored by acquiring two-dimensional (2D) projections of the H ions on a charge-coupled device (CCD) camera while scanning the SRS beams across the Raman transitions and integrating the resulting ion signal. In addition, TCRD images resulting from predissociation of vibrationally excited molecules were obtained by parking the SRS beams on specific vibrational transitions. These SRS beams were passed through a beam shutter that was alternately opened and closed to subtract the contribution of the ∼243.1 nm predissociation signal from the total signal (including H photofragments from vibrationally excited and vibrationless ground state parent molecules). The measured 2D projections were transformed to three-dimensional (3D) images using an inverse Abel procedure while taking the Hankel transform of the Fourier transform of the projected intensity.41 The H atom velocities were calibrated by recording O atom velocity map images resulting from the ∼225 nm photolysis of O2. In another set of experiments, the TCRD images of H atoms resulting from one-photon photolysis of vibrationless ground state molecules (see Figure 1c) at an energy similar to that in the double-resonance experiments were measured. In this case, one of the lasers used for TCRD in the previous experiments provided the photolysis photons at ∼231.25 nm (∼25 μJ/ pulse), and the second TCRD beam was produced by doubling the OPO signal beam. Here the probe TCRD beams were delayed by ∼4 ns with respect to the photolysis beam, which was passed through the beam shutter for background subtraction.

Figure 2. Vibrational excitation spectra in the region of the CD3 symmetric stretch fundamental, ν3, of CD3NH2: (a) room-temperature photoacoustic Raman spectrum; (b) H action spectrum obtained by 243.135 nm predissociation of jet-cooled vibrationally excited molecules; (c) a portion of the H action spectrum obtained by onecolor UV excitation from the vibrationless ground state to the ν7′ + ν9′ band region.36 The intensity scales of the panels are different.

ground state molecules (the background in the spectrum). As can be seen from Figure 2b and Table 1, the observed features Table 1. Energies of the Observed Bands of the CD3 Stretch Fundamental Region in the Photoacoustic Raman and H Action Spectra, Together with the Band Assignments, Band Types, and Determined Enhancement Factors energy (cm−1) 2080 2114 2128 2142 2155 2205 2246 2285

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RESULTS Photoacoustic Raman and H Action Spectra. Figure 2 shows representative (a) room-temperature PAR and (b) jetcooled H action spectra in the 2070−2310 cm−1 region. Both spectra show multiple sharp peaks, corresponding to Q branches of different vibrational bands of A type and some broader features of B type. It is immediately seen that the features in the PAR and H action spectra appear at identical positions but that their intensity patterns differ considerably. In particular, a strongly dominant peak at 2080 cm−1 and several weak features at higher frequencies appear in the PAR spectrum, while more features seem to carry significant intensity in the action spectrum. The features in the action spectrum are a result of enhanced release of H photofragments for molecules promoted by ∼243.1 nm photons from initially vibrationally pre-excited molecules over that from vibrationless

band assignmenta ν3 ν5 + ν5 + ν7 + 2ν13 ν5 + ν11 2ν5

ν7 ν9 + ν15 ν14 + ν15 ν13

band type

enhancement factor

A A B A A B B A

1 8±2 3±1 0.6 ± 0.3 3±1 40 ± 10 undetermined 10 ± 2

a ν3, CD3 symmetric stretch; ν5, CD3 symmetric deformation; ν7, CN stretch; ν9, NH2 wag; ν11, CD3 degenerate stretch; ν13, CD3 degenerate deformation; ν14, CD3 rock; ν15, torsion. See refs 35 and 36.

are tentatively assigned (on the basis of data from refs 42 and 43) to fundamentals of CD3 stretches and to overtone and combination bands of CD3 deformations. Most notable is the observation that the enhancement factors, reflecting the ratio between the peak areas corresponding to the same feature in the action and PAR spectra, span a large range. Actually, when the enhancement factor of the CD3 symmetric stretch (ν3) vibration is set to unity, differences in the extent of enhancement for the other bands are observed (Figure 2 and Table 1). In particular, overtone and combination bands 3051

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The Journal of Physical Chemistry A involving the CD3 symmetric and degenerate deformation modes (ν5 and ν13, belonging to the A′ and A″ species of the Cs point group, respectively) are greatly enhanced in the H action spectrum. In addition, ν11, corresponding to the CD3 degenerate stretch mode (A″), also shows some enhancement, which might seem to contradict the conjecture that deformation modes are more effective in enhancement. However, since Fermi resonance can occur between levels of the same species,44 it is likely that the ν5 + ν13 zeroth-order bright state and the ν11 zeroth-order dark state, which is barely observed in the PAR spectrum, interact via anharmonic terms in the potential energy. Therefore, it is reasonable to assume that the observed states correspond to mixtures of the zeroth-order-approximation wave functions,45 implying that ν11 contains a small contribution of ν5 + ν13 that is probably responsible for the slight enhancement in the H action spectrum. In addition, Figure 2c includes a portion of the vibronic H action spectrum resulting from predissociation initiated from vibrationless ground state CD3NH2 molecules (from ref 36). It is to be emphasized that this spectrum should be different from the UV action spectrum that would be obtained in the case of vibrational pre-excitation. Nevertheless, as marked by the blue background, it shows the energetic region of the ν7′ + ν9′ vibronic state accessed when the molecules are initially excited to the vibrational states and then promoted by ∼243.1 nm photons. In addition, it shows the feature that was used to measure the TCRD image for the vibrationless ground state molecules (see below). Two-Color Reduced-Doppler Ion Images and Total Kinetic Energy Distributions. The raw TCRD ion images resulting from the isoenergetic predissociation of vibrationless ground state molecules and vibrationally excited molecules enable us to retrieve the TKEDs and to get some insight into both processes. Figure 3 shows representative raw 2D H photofragment TCRD ion images (unsymmetrized) resulting from predissociation of the region of the ν7′ + ν9′ vibronic state of CD3NH2 molecules, which was accessed by 231.25 nm excitation of the vibrationless ground state (Figure 3a) and by ∼243.1 nm excitation of ν3 (Figure 3d). Moreover, comparable raw TCRD images were obtained for predissociation of CD3NH2 molecules that were pre-excited to additional dominant vibrational states observed in the H action spectrum (Figure 2b). Also shown are the resultant central slices from the Abel-inverted images41 (Figure 3b,e, respectively) exhibiting the 3D distributions and the corresponding TKEDs (green circles in Figure 3c,f, respectively). These TKEDs were obtained by integrating the data in the 3D images over the angular coordinates while applying linear momentum conservation for the H and CD3NH photofragments. Therefore, they reflect the center of mass KEDs for the two different excitation processes, i.e., isoenergetic predissociation initiated from vibrationless ground state molecules and from vibrationally excited molecules. It can be seen that the raw 2D TCRD images (Figure 3a,d) and the 3D KED slices (Figure 3b,e) consist of very similar broad central features and sharp rings. This is even more evident from the corresponding TKEDs (Figure 3c,f), which were fitted by the red lines, which are the outcomes of the contributions of two deconvoluted asymmetric components corresponding to “fast” and “slow” H photofragments (dashed blue lines). The ratios of slow to fast H photofragments were found to be 0.23 ± 0.03 and 0.26 ± 0.03 for molecules

Figure 3. (a, d) Raw two-color reduced-Doppler ion images of H photofragments resulting from the 231.25 and ∼243.1 nm predissociation initiated from (a) vibrationless ground state and (d) vibrationally excited CD3NH2 molecules. The arrows in the bottom right corners denote the vertical polarization of the photolysis laser. (b, e) Corresponding central slices from the Abel-inverted images. (c, f) Corresponding measured total kinetic energy distributions (green circles) and fits (red lines), which represent the contributions of fast and slow atoms (blue dashed lines).

predissociated following promotion of vibrationally excited and vibrationless ground state molecules, respectively, to the excited state.

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DISCUSSION The above-mentioned results point to very different relative intensities of the vibrational bands in the PAR (Figure 2a) and H action (Figure 2b) spectra and to similar measured TCRD ion images and retrieved TKEDs with a bimodal distribution resulting from predissociation of the ν7′ + ν9′ vibronic state, which was accessed by excitation of vibrationless ground state molecules (Figure 3a,c) and vibrationally excited molecules (Figure 3d,f). This implies that the difference between the PAR and H action spectra could be attributed to their distinct origins. While the former reflects only the efficiency of vibrational excitation, the latter also depends on the efficacy of the electronic excitation, the FC factors for transitions out of the vibrationally pre-excited states,7 and the coupling to the N− H reaction coordinate that eventually leads to release of H photofragments. On the basis of the PAR and action spectra, it is difficult to identify the main reason for the encountered behavior. In our previous measurements on methylamine29,31 and pyrrole,30 we conjectured that the mode specificity was a result of different FC factors. Here it seems that some insight into the process can be obtained by referring to the measured TCRD ion images and TKEDs associated with H photofragment production and to the available rich information on methylamine photophysics. In fact, the similar TCRD ion images and TKEDs associated with release of H photofragments resulting from ∼243.1 nm predissociation of the dominant vibrational modes and from the isoenergetic one-photon predissociation of CD3NH2 (231.25 nm) allude to similar dynamics following the excitation 3052

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to the bound portion of the S1 PES. In both cases, the region of the ν7′ + ν9′ vibronic state is accessed,34,36 ensuring that the initial vibrational excitation does not play a principal role in the ensuing dynamics. Actually, the release of fast and slow H photofragments in predissociation initiated from vibrationally excited and vibrationless ground state CD3NH2 molecules, together with the similar ratios of slow and fast H atoms, indicates that following access to the ν7′ + ν9′ vibronic state the N−H bond cleavages occur in a similar manner. On the basis of the above description regarding predissociation in methylamine isotopologues, it is reasonable to consider that H tunneling occurs, followed by rolling down on the repulsive S1 PES, thus leading to passage through the CI (see Figure 1b,c) and eventually to fast and slow H photofragments. The former are obtained as a result of passage toward the asymptote and the latter from nonadiabatic internal conversion to a hot ground state39,46 that dissociates later following energy redistribution among the vibrational states. On the basis of these arguments, it is confirmed that the FC factors and not the ensuing dynamics are the main reason for the unique enhancement of each mode in CD 3 NH 2 predissociation. The enhancement factors (Table 1) point to the effectiveness of CD3 deformation modes in promoting CD3NH2 predissociation. This mode specificity in predissociation indicates that the initially excited modes preserve their character on a relatively long time scale, surviving IVR in a molecule with a torsional degree of freedom.

REFERENCES

(1) Schinke, R. Photodissociation Dynamics Spectroscopy and Fragmentation of Small Polyatomic Molecules; Cambridge University Press: Cambridge, U.K., 1993. (2) Rosenwaks, S. Vibrationally Mediated Photodissociation; Royal Society of Chemistry: Cambridge, U.K., 2009. (3) Tannor, D. J.; Kosloff, R.; Rice, S. A. Coherent Pulse Sequence Induced Control of Selectivity of Reactions - Exact QuantumMechanical Calculations. J. Chem. Phys. 1986, 85, 5805−5820. (4) Brumer, P.; Shapiro, M. Laser Control of Molecular Processes. Annu. Rev. Phys. Chem. 1992, 43, 257−282. (5) Gordon, R. J.; Rice, S. A. Active Control of the Dynamics of Atoms and Molecules. Annu. Rev. Phys. Chem. 1997, 48, 601−641. (6) Crim, F. F. Bond-selected Chemistry: Vibrational State Control of Photodissociation and Bimolecular Reaction. J. Phys. Chem. 1996, 100, 12725−12734. (7) Bar, I.; Rosenwaks, S. Controlling Bond Cleavage and Probing Intramolecular Dynamics via Photodissociation of Rovibrationally Excited Molecules. Int. Rev. Phys. Chem. 2001, 20, 711−749. (8) Crim, F. F. Chemical Dynamics of Vibrationally Excited Molecules: Controlling Reactions in Gases and on Surfaces. Proc. Natl. Acad. Sci. U. S. A. 2008, 105, 12654−12661. (9) Bechtel, H. A.; Kim, Z. H.; Camden, J. P.; Zare, R. N. Bond and Mode Selectivity in the Reaction of Atomic Chlorine with Vibrationally Excited CH2D2. J. Chem. Phys. 2004, 120, 791−799. (10) Citir, M.; Metz, R. B. Mode Selective Photodissociation Dynamics in V+(OCO). J. Chem. Phys. 2008, 128, 024307. (11) Bisson, R.; Sacchi, M.; Beck, R. D. Mode-specific Reactivity of CH4 on (110)-(1 × 2): The Concerted Role of Stretch and Bend Excitation. Phys. Rev. B: Condens. Matter Mater. Phys. 2010, 82, 121404. (12) Juurlink, L. B. F.; Smith, R. R.; Killelea, D. R.; Utz, A. L. Comparative Study of C-H Stretch and Bend Vibrations in Methane Activation on Ni(100) and Ni(111). Phys. Rev. Lett. 2005, 94, 208303. (13) Jiang, B.; Liu, R.; Li, J.; Xie, D.; Yang, M.; Guo, H. Mode Selectivity in Methane Dissociative Chemisorptions on Ni(111). Chem. Sci. 2013, 4, 3249−3254. (14) Nesbitt, D. J.; Field, R. W. Vibrational Energy Flow in Highly Excited Molecules: Role of Intramolecular Vibrational Redistribution. J. Phys. Chem. 1996, 100, 12735−12756. (15) Segev, E.; Shapiro, M. 3-Dimensional Quantum Dynamics of H2O and HOD Photo-Dissociation. J. Chem. Phys. 1982, 77, 5604− 5623. (16) Engel, V.; Schinke, R. Isotope Effects in the Fragmentation of Water - The Photodissociation of HOD in the 1st Absorption-band. J. Chem. Phys. 1988, 88, 6831−6837. (17) Imre, D. G.; Zhang, J. H. Dynamics and Selective Bond Breaking in Photodissociation. Chem. Phys. 1989, 139, 89−121. (18) Vander Wal, R. L.; Scott, J. L.; Crim, F. F. Selectively Breaking the O-H Bond in HOD. J. Chem. Phys. 1990, 92, 803−805. (19) Vander Wal, R. L.; Scott, J. L.; Crim, F. F.; Weide, K.; Schinke, R. An Experimental and Theoretical-Study of the Bond-selected Photodissociation of HOD. J. Chem. Phys. 1991, 94, 3548−3555. (20) Bar, I.; Cohen, Y.; David, D.; Rosenwaks, S.; Valentini, J. J. Direct Observation of Preferential Bond Fission by Excitation of a Vibrational Fundamental − Photodissociation of HOD (0,0,1). J. Chem. Phys. 1990, 93, 2146−2148. (21) Bar, I.; Cohen, Y.; David, D.; Arusi-Parpar, T.; Rosenwaks, S.; Valentini, J. J. Mode-selective Bond Fission - Comparison between the Photodissociation of HOD (0,0,1) and HOD (1,0,0). J. Chem. Phys. 1991, 95, 3341−3346. (22) Cohen, Y.; Bar, I.; Rosenwaks, S. Photodissociation of HOD (νOD = 3): Demonstration of Preferential O-D Bond Breaking. J. Chem. Phys. 1995, 102, 3612−3616. (23) Brouard, M.; Langford, S. R. The State-to-state Photodissociation Dynamics of HOD (Ã ). J. Chem. Phys. 1997, 106, 6354−6364. (24) Akagi, H.; Fukazawa, H.; Yokoyama, K.; Yokoyama, A. Selective OD Bond Dissociation of HOD: Photodissociation of Vibrationally Excited HOD in the 5νOD State. J. Chem. Phys. 2005, 123, 184305.

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CONCLUSIONS This study took advantage of the quantum-state-specific predissociation of CD3NH2 initiated from the region of the CD3 stretch fundamentals and of the isoenergetic predissociation initiated from vibrationless ground state molecules to assess the effect of vibrational pre-excitation on N−H bond rupture. Both CD3 stretches and CD3 deformations (overtones and combinations) enhanced the predissociation of CD3NH2, but the deformation eigenstates were more efficient. The comparable TCRD images and TKEDs associated with H photofragments released in the isoenergetic predissociations initiated from vibrationally excited and vibrationless ground state molecules emphasized that the ensuing dynamics following promotion to the region of the ν7′ + ν9′ vibronic state is similar. It is thus considered that the characteristics of the initially prepared deformation eigenstates are preserved on a relatively long time scale, allowing improved FC factors during the excitation to the bound well. Although the CD3NH2 molecule possesses a torsional degree of freedom, it still maintains its quantized modes and leads to mode specificity during predissociation.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The support of this research by the Israel Science Foundation (ISF) under Grant 1001/09 is gratefully acknowledged. 3053

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The Journal of Physical Chemistry A

(45) On the basis of first-order perturbation theory,37 the measured frequencies of the observed bands, and their roughly estimated amplitudes in the PAR spectrum, the values of the energies of the calculated zeroth-order states and the interaction matrix element, W, could be retrieved. The unperturbed energies of ν5 + ν13 and ν11 were found to be 2215 and 2236 cm−1 with W = 18 cm−1. Each of the zeroth-order states carries a consequent character of ∼88%. (46) Epshtein, M.; Portnov, A.; Bar, I. Evidence for Quantum Effects in the Predissociation of Methylamine Isotopologues. Phys. Chem. Chem. Phys. 2015, 17, 19607−19615.

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DOI: 10.1021/acs.jpca.5b10309 J. Phys. Chem. A 2016, 120, 3049−3054