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Using Anisotropy Measurements from A‑Band Photodissociation to Interrogate the Excited States of H2Se Xiaodong Zhang, Michael Johnson, and Brent Koplitz* Department of Chemistry, Tulane University, New Orleans, Louisiana 70118, United States ABSTRACT: The A-band photodissociation of H2Se has been studied by measuring Hatom velocity-aligned Doppler spectroscopy (VADS) spectra at five wavelengths from 210 to 266 nm. These spectra have been subsequently simulated by assigning cross-sections and anisotropy β values to the two SeH spin−orbit exit channels. While the SeH(2Π3/2) exit channel has a β value close to −1 throughout the studied wavelength range, the spin−orbit excited SeH(2Π1/2) exit channel’s β value switches from near −1 to near +0.5 when the photolysis wavelength increases from 210 to 266 nm. These results have been examined in the light of available ab initio calculations. Throughout the studied wavelengths, the contribution from excitation to the 11B1 state predominates and provides the source of the −1 β value. In order to account for the +0.5 β value, it is necessary to assume that the 21A1 state as well as the 4A′ and 5A′ states (both originating from a 3B1 state) also contribute at short wavelengths. More interestingly, at the longer wavelength end (266 nm), contribution of a +0.5 β value from the 3A′ state (originating from a 3A2 state) exceeds the contribution of the −1 β value for the SeH(2Π1/2) channel.

I. INTRODUCTION Identifying exactly which excited electronic states are participating in the photodissociation of a polyatomic molecule is central to understanding the entire event.1 While it is possible to make electronic-state assignments by analogy with similar systems that have already been studied, it often turns out that no matter how similar the systems appear to be, each still needs to be scrutinized on an individual basis. In the present study, we consider the photodissociation of the group 6A hydrides (H2O, H2S, H2Se, and H2Te) in their first diffusive absorption bands (the A-bands) with the focus of the work being the H2Se molecule. The first two members of the group 6A hydrides have been examined extensively.2,3 In the case of the H2O molecule, it was initially believed that the fragmentation of H2O proceeds via a single electronic state with the electronic symmetry being 1B1 in the C2v configuration.1 Subsequently, experiments by Plusquellic et al.,4 who measured the H/D branching ratio in the 193 nm photodissociation of HOD, as well as a corresponding theoretical study by Schröder et al.5 led the latter authors to conclude that at long photodissociation wavelengths such as 193 nm, the lowest triplet level (a 3B1 state) plays a significant role. Essentially, the authors assert that the surprisingly small measured values for the H/D branching ratio and the H2O/ D2O cross-section ratio cannot be explained if only excitation to the first excited singlet state is considered. For the case of H2S, both experimental evidence6−9 and ab initio calculations10−13 have suggested that the photodissociation of H2S in its first diffusive band involves two coupled excited singlet states of H2S which, in the limit of C2v symmetry, transform as 1B1 and 1A2 states. Under C2v symmetry, only the transition to the 1B1 state is electric-dipole allowed. However, the dynamics should be described in a lower symmetry (i.e., the © 2013 American Chemical Society

Cs symmetry) in order to treat the whole process consistently since when the molecule starts to break apart the two bonds are not equal. Also, the dynamics is more conveniently described by diabatic potential surfaces. For this treatment, the original two adiabatic surfaces (which can still be labeled as 1B1 and 1A2 according to their transformation properties in the Franck− Condon region) are mixed to form two new diabatic surfaces. One of the new diabatic surfaces is predicted to be bound and the other repulsive with respect to the products H + SH(X). Heumann et al.12,13 suggest that the dissociation mechanism at photolysis energies corresponding to the absorption maximum is one of the first recognized examples of a very fast electronic predissociation where initial population of the bound excited singlet state is rapidly depleted by transfer onto the repulsive singlet surface. In contrast with the first two members, the photodissociation of the next two members of the 6A group hydrides, H2Se and H2Te, in their first diffusive bands have been studied less extensively.14−18 In analogy with their lighter homologues, the diffuse bands of H2Se and H2Te are assigned to np → (n + 1)s Rydberg excitations possibly overlaid by weak valence shell promotions.19 In a previous paper,15 we used a variation on velocity-aligned Doppler spectroscopy (VADS) and reported on rotational and vibrational state distributions for the SeH radical subsequent to the photodissociation of H2Se in the wavelength range of the A-band. It was found that the SeH(X2Π3/2, v = 1) population has a maximum around 230− Special Issue: Curt Wittig Festschrift Received: March 31, 2013 Revised: September 14, 2013 Published: September 16, 2013 11963

dx.doi.org/10.1021/jp403196k | J. Phys. Chem. A 2013, 117, 11963−11969

The Journal of Physical Chemistry A

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220 nm. In the current article, we also use this modified VADS approach to probe the angular dependence of the H2Se photolysis system in the same wavelength range (266−210 nm). It is demonstrated that from such measurements it is possible (in conjunction with theoretical calculations)17,20−22 to infer which excited states are involved in the photodissociation of H2Se in its first diffusive band. Finally, it should be noted that understanding the fate of photolyzed group VI compounds under conditions ranging from ices to metal deposition environments remains a challenge,23−25 and the current work lends insight into understanding the photolysis of such species.

II. EXPERIMENTAL SECTION The details of the experimental setup have been described previously.15,26−28 Here, we give a brief description pertinent to the present study. The photolysis of expansion-cooled H2Se is initiated with the focused (focal length = 30 cm) and frequency-doubled output from a broadband optical parametric oscillator (OPO) pumped by the third harmonic (355 nm) of a Nd:YAG laser (Coherent Infinity). The fundamental OPO output is typically ∼20−30 mJ/pulse over the 420−532 nm range. The frequency-doubled light has a pulse energy of ∼1−2 mJ, and its frequency bandwidth is estimated to be 10−20 cm−1. H atoms generated by H2Se photodissociation are probed at a distance 0.45 m from the focal point of photolysis. These H atoms are detected with probe laser radiation via 1 + 1′ (121.6 + 365 nm) resonance ionization through Lyman-α. To generate the probe light, the output of an Ar+-pumped ring laser (Coherent 899-29; Ti:sapphire; 730 nm) is pulse amplified in three sequential dye cuvettes. After frequency doubling, the resulting 365 nm radiation is pulse amplified and frequency tripled in Kr. Typical 365 nm pulse energies were 12−15 mJ. In these experiments, the probe light propagates perpendicular to the photolysis k-vector and against the traveling direction of the H-atom fragment (the so-called T-configuration).15,28 Delay-time-synchronized VADS experiments are performed. In this type of experiment, the probe laser frequency and the photolysis/probe delay time are adjusted synchronously so that H atoms with the desired Doppler shift are interrogated at the correct time in the ionization region. To probe any anisotropy in the photolysis event, the E-vector of the photolysis light is set either perpendicular or parallel to the detection direction using a double Fresnel rhomb (Optics for Research). The resulting spectra are normalized by the pulse energy of the photolysis laser to reveal the variation of the relative signal intensity against the photolysis wavelength. The UV absorption spectrum of H2Se was measured on a Cary 100 UV−visible spectrometer using a 10 cm gas cell with MgF2 windows.

Figure 1. VADS spectra for H2Se photolysis at five selected wavelengths. At each wavelength, the E-vector of the photolysis light is set either 90° or 0° to the detection direction. Each pair of spectra is scaled so that the maximum signal is one unit. Dots are experimental data, while the lines are spectral simulations.

H 2Se(X) + hv → H(2 S) + SeH (X2Π3/2, v = 0, 1, 2, ..., Ν = 0, 1, 2, ...)

(1)

H 2Se(X) + hv → H(2 S) + SeH (X2Π1/2, v = 0, 1, 2, ..., Ν = 0, 1, 2, ...)

(2)

These channels are marked by two combs in the 220 nm spectra to show the proper spin−orbit and vibrational levels of the corresponding SeH fragment. The angular dependence of the photolysis event is observed for each pair of spectra at each wavelength with one spectrum taken with the photolysis Evector at 90° to the detection direction and the other taken with the E-vector at 0° to the detection direction. If the signal vanishes when this angle changes from 90° to 0°, the corresponding photolysis channel is initiated with a perpendicular electronic transition in H2Se, i.e., the transition dipole moment is perpendicular to the recoil direction of the H atom. In contrast, the fragmentation may be produced via a transition that is nonperpendicular between the E-vector and the H-atom recoil direction. A simple check of the spectra shown in Figure 1 reveals that channel 1 is predominantly caused by a perpendicular transition at all wavelengths. However, while channel 2 is also predominantly caused by a perpendicular transition at short wavelengths (e.g., 220 nm), at long wavelengths (e.g., 266 nm) channel (2) is produced via a transition dipole moment that lies at a much smaller angle to the H-atom recoil direction.

III. RESULTS A. Experimental Spectra. Figure 1 shows five pairs of Hatom VADS spectra resulting from H2Se photolysis initiated at five selected wavelengths spanning the 266−210 nm range with the E-vector of the photolysis light at either a 90° or 0° angle with respect to the detection direction. For better comparison, the spectra are normalized by the appropriate multiplication factor. All spectra show features due to the following two photolysis channels: 11964



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For illustration purposes, we present Figure 2. This figure contains enlarged regions of the data shown in Figure 1,

Table 1. Cross-sections and β values for photolysis channels 1 and 2 of H2Se λ (nm)

σ(1)

σ(2)

β(1)

β(2)

σabs

210 220 235 250 266

1.99 1.00 0.66 0.27 0.05

0.40 0.17 0.10 0.04 0.01

−0.92 −0.94 −0.92 −0.93 −0.91

−0.88 −0.92 −0.77 −0.41 +0.41

0.14 0.18 0.09 0.04 0.01

Figure 3. β values for the two photolysis channels of H2Se. Figure 2. Enlarged portions of the 210 and 266 nm data shown in Figure 1. The change in anisotropy for the SeH(2Π1/2) channel (smaller peak) is evident.

When moving from longer to shorter photolysis wavelengths, the β value of channel 1 that leads asymptotically to the ground state of SeH(X2Π3/2) keeps relatively constant at a value of −0.92. In contrast, the β value for channel 2 that leads to the spin−orbit excited state of SeH(X2Π1/2) drops dramatically from 0.41 to −0.92 when the wavelength decreases from 266 to 220 nm. While there is a slight upswing in the β value on going from 220 to 210 nm, this final trend may or may not be statistically significant. Note also that Table 1 includes the absorption cross-sections (in arbitrary units) read from the UV absorption spectrum that can be found in Figure 4.

specifically portions from the 210 and the 266 nm photolysis studies. Here, the effects of changing the direction of the photolysis E-vector are pronounced. B. Spectral Simulation Results. To gain quantitative insight into the photolysis process, spectral simulations have been carried out. In accord with Schinke,1 the photolysis of H2Se can be described by the cross-sections of H2Se that lead to the exit channels. These cross-sections (in some arbitrary unit) are assigned to the exit channels with the spin−orbit, vibrational, and rotational states of SeH all being distinguished. As part of the results of this simulation, the rotational and vibrational populations of the SeH fragment have already been reported in a previous paper.15 The angular dependence of the photolysis process is modeled using the β anisotropy parameter (see Section C discussion). Unlike the cross-sections, an anisotropy parameter is assigned to each channel in a way that does distinguish the spin−orbit states of SeH but does not distinguish its vibrational and rotational states. Figures 1 and 2 show that simulations done in this manner are quite satisfactory. Consequently, for the purpose of exploring the angular behavior we only consider the two exit channels shown by eqs 1 and 2 and quantify the spin−orbit states of SeH but do not distinguish the vibrational and rotational excitations of this fragment. The cross-sections and β values for channels 1 and 2 have been assigned in the simulation and are listed in Table 1. Here, the cross-section for SeH is actually the sum of the crosssections leading to all vibrational and rotational levels of SeH for each spin−orbit state. The β values are also plotted in Figure 3 where the features of the angular dependence of the photolysis channels seen in the experimental spectra are revealed more strikingly.

Figure 4. Total absorption cross-section for H2Se from 190 to 340 nm.

C. Assigning the Excited States of H2Se to the Photolysis Channels. 1. Symmetry Analysis. According to Franck−Condon considerations, electronic transitions are more likely when the positions of the nuclei change little during the excitation that accompanies the absorption of the UV photon. For H2Se, the resulting excited state should retain the same C2v symmetry that the ground state possesses prior to absorbing the 11965



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2. Ab Initio Results on the Excited Electronic States of the Heavier Hydrides. Rauk and Collins calculated singlet excited states of H2Se using the experimental geometry configuration for the ground state.20 Their results are summarized in Table 3.

photon. Under C2v symmetry, the excited state can be A1, A2, B1, or B2, with the bond angle of the ground state H2Se being 90.9°.29 To describe the type of state properly, the axes are taken as x (out-of-plane, b1), y (in-plane, b2), and z (in-plane C2 axis, a1). Also, given the relatively large excess energy (greater than 8600 cm−1), it is assumed that the trajectories of the fragments will be in the same direction as that of the vibrational motion of the bond being broken.30 In other words, axial recoil is assumed with the H atom leaving approximately along the H−Se bond direction. Under axial recoil, the β value for photolysis may be described as β = 2P2(cos θ) = 3 cos2(θ) − 1

Table 3. Singlet Excited States of H2Se (ab Initio Calculation from ref 20)

(3)

where θ is the angle between the transition dipole moment and the direction of the H-atom trajectory. For each type of electronic excited state of H2Se under C2v symmetry, the orientation of the transition dipole moment, the angle between this dipole moment and the H-atom recoil direction, and the β value for fragmentation through the corresponding excited state are analyzed in Table 2. It should be mentioned that in the C2v

A1 (A′ in Cs) A2 (A″ in Cs) B1 (A″ in Cs) B2 (A′ in Cs)

direction of transition dipole moment, μ

angle between μ and H trajectory, θ

β value

z

45.45°

0.48

x

90°

−1

x

90°

−1

y

44.55°

0.52

description

excitation energy (eV)

11A2 11B1 21A2 21B1 21A1 31B1 31A1 11B2

4px → (σSeH*)− 4px → 5s, (σSeH*)+ 4px → 5py 4px → 5pz 4px → 5px 4px → (σSeH*)+, 5s 9a1 → 5s 9a1 → (σSeH*)−

5.61 5.69 7.11 7.16 7.31 8.42 8.97 9.52

These excited states can be roughly divided into three groups. The first group includes the 11A2 and 11B1 states and has the lowest excitation energies (5.61 and 5.69 eV). The next group has an excitation energy higher by ∼1.5 eV and includes the 21A2, 21B1, and 21A1 states. The remaining five states have excitation energies higher than the first group by at least 2.8 eV and form the third group. For H2Te and H2Po, Sumathi and Balasubramanian performed calculations on the excited electronic states.21 They included both singlet and triplet states in their calculations. However, these calculated state energies were only determined for the Franck−Condon region. More recently, Alekseyev, Liebermann, and Wittig performed higher level calculations for H2Te.17 Their results include asymmetric stretch potential profiles (see Figure 5) that are more relevant for analyzing photolysis data. The authors also calculated transition dipole moments connecting ground and excited states. 3. UV Absorption Spectrum of H2Se. An early, complete UV absorption spectrum of H2Se was reported by Price et al.31 They found a region of diffuse absorption with a maximum at

Table 2. Photolysis Properties of the Excited States of H2Se excited state

state

limit a transition from the A1 ground state to an A2 excited state is electric-dipole forbidden. However, if we consider the asymmetric vibrational mode of H2Se, there are times when the two bonds are not equal and the molecular symmetry is lowered from C2v to Cs. The A2 species in C2v symmetry becomes the A″ species in Cs symmetry. Under Cs symmetry, a transition from the ground state A′ to an excited A″ state is electric-dipole allowed with the transition dipole moment being perpendicular to the molecular plane. This situation gives a θ angle of 90° and a β value of −1. Table 2 indicates that there are two possible sources for a given β value. Photolysis through an excited state of type B1 or A2 (both A″ under Cs) yields a β value of −1, while photolysis through an excited state of type A1 or B2 (both A′ under Cs) yields a β value of ∼+0.5. (Note that the value is not exactly 0.5 because the H−Se−H bond angle is 90.9° not 90°.) Understandably, mixing these source states gives an observed β value between −1 and +0.5. By combining this analysis with the experimentally measured β values shown in Figure 3, we may draw the following conclusions. Photolysis channel 1, which leads to the spin−orbit ground state of SeH(X 2Π3/2), mainly involves a route(s) with a β value of −1 with a small contribution from a route(s) having a β value of +0.5. In contrast, channel 2, which leads to the spin−orbit excited state of SeH(X 2Π1/2), involves a mixture of two types of routes: one with a β value of +0.5 and the other with a β value of −1. Interestingly, at longer wavelengths the route(s) with a β value of +0.5 predominates, but the predominant route(s) switches to a β value of −1 when the wavelength becomes shorter.

Figure 5. H2Te potential curves adapted with permission from ref 17. Copyright 2004 American Institute of Physics. The H2Te bond angle and one TeH bond length are held at their ground equilibrium values. Note that the energies for states in H2Se will be different. 11966



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can provide 0.5 β value. Note that in the Franck−Condon region the 3B1 states are located lower in energy than the 4A″ state (which originates from the 11B1 state), while the 3A2 states are located even lower in energy. So, the contribution of the 0.5 β value is expected to be more apparent at longer wavelengths than 219 nm. This expectation matches the experimental observation that the SeH(X2Π1/2) channel is dominated by routes having a 0.5 β value at the longer wavelength end.

about 197 nm and strong discrete bands between 169 and 125.5 nm. A more recent result was reported by Mayhew and Connerade.32 They observed 12 unassignable bands in the 169−153 nm region right below the first diffuse absorption region (the A-band). Several Rydberg series in the 153−120 nm region were also seen. Although the spectrum was measured in the entire 260−120 nm range, there was no report on the diffuse A-band in their paper. In the measurement provided in Figure 4, the A-band spectrum is shown in the 190−350 nm range with a maximum at 219 nm. This result appears to be more consistent with the series of maxima for the A-bands of H2O (165 nm),33 H2S (195 nm),34 and H2Te (250 nm)16 rather than the 197 nm maximum reported by Price et al.31 4. Possible Excited States of H2Se Involved in Photolysis. The maximum absorbance at 219 nm of the A-band for H2Se corresponds to a 5.66 eV photon energy. Thus, the first group of singlet excited states calculated by Rauk and Collins20 matches the maximum absorbance of the A-band of H2Se rather closely and includes the 11A2 and 11B1 states. Since the 11A2 state is forbidden in the C2v limit of the Franck−Condon region, we estimate that the A-band mainly involves absorption to the 11B1 state. Because the 11A2 state is in the same energy range as the 11B1 state (as in the case of H2S A-band photodissociation),3,10−13 the 11A2 state may also play a similar role in the photodissociation of H2Se. However, using this group of excited states alone cannot account for the experimentally observed β values because fragmentation through either 11A2 or 11B1 (or both) only gives a β value of −1. In order to account for the observed β values, some source of a β value of +0.5 is also required, which means either an A1 state or a B2 state (both are A′ in Cs) must be involved. In the calculation for the singlet excited states of H2Se (see Table 3),20 there is one 1A1 state in the second group, and two 1 A1 and two 1B2 states in the third group. According to the calculated excitation energies, the absorption maxima of the second group of excited states should appear at 170 to 174 nm, and those of the third group of excited states should appear at 121 to 147 nm. It is assumed that the third group is too far away in energy to contribute in a meaningful way to the A-band (whose maximum lies at 219 nm). The second group is also much higher in energy than the A-band, so these states cannot make a significant contribution to the A-band either, but it is still possible for them to provide a small contribution. For example, one can imagine a very weak tail of the second group of peaks extending into the A-band. So, one possible source for a β value of 0.5 is the 21A1 state, but if this pathway contributes, its expected contribution to the photodissociation of H2Se in the A-band is very small. Apart from the singlet states, one should also consider possible participation by the triplet states. In the case of H2O, Schröder et al.5 found a 3B1 state that plays a significant role in photodissociation at wavelengths such as 193 nm. Here, the 3B1 contribution was found to be only 0.4% of that of 1B1, but this small amount is enough to account for the experimentally observed H/D branching ratio for the photolysis of HOD at 193 nm.4 In the case of H2Se, we are looking for a possible source of a 0.5 β value, so attention must be given to triplet states that transform as either A1 or B2 in the C2v limit or as A′ under Cs symmetry. In Figure 5 (adapted from ref 17), the 1A″, 2A′, and 3A′ states arise from a 3A2 state, and of these three, the 2A′ and 3A′ states can provide a 0.5 β value. The 3A″, 4A′, and 5A′ states arise from a 3B1 state, and here the 4A′ and 5A′ states

IV. DISCUSSION The photolysis of H2Se in its A-band is similar to H2S in that the process mainly involves two coupled potential energy surfaces, specifically the 11B1 and 11A2 states. However, H2Se also differs from H2S because a third (or maybe even more) potential energy surface should play an important role, especially on the long wavelength side of the absorption band. This involvement is manifested by the fact that the β value for channel 2 approximates 0.5 at 266 nm. In this section, we assess the participation of various states in the photolysis of H2Se. The bulk of the following discussion is in the strong spin−orbit coupling limit, but in actuality, H2Se is intermediate between H2Te and H2S. Note that the spin−orbit splitting values for OH, SH, SeH, and TeH are 139, 377, 1763, and 3810 cm−1, respectively.35 A. Magnitudes of Transition Dipole Moments to Various Excited States. Alekseyev, Liebermann, and Wittig calculated transition dipole moments connecting the ground state to various excited states of H2Te as labeled in Figure 5.17 The excited states can be roughly divided into five groups: (1) the 4A″ state having the largest transition dipole moment, (2) the 4A′ and 5A′ states having a transition dipole moment about 1 order of magnitude smaller, (3) the 2A′ and 3A′ states having a transition dipole moment an additional three times smaller, (4) the 1A″ state having a transition dipole moment three times smaller than (3), and (5) the 2A″ and 3A″ states having zero transition dipole moments. Consequently, excitation to 4A″ should dominate the A-band, while excitation to 4A′ and 5A′, to 2A′ and 3A′, and to 1A″ may have some contribution. B. Routes for a −1 β Value. Four A″ states (4A″, 3A″, 2A″, and 1A″) may provide a −1 β value with the 4A″ state expected to be dominant. Assuming that H2Se has curves similar to those shown in Figure 5, the 1A″ state is repulsive and directly leads to the SeH(X2Π3/2) exit channel. Excitation to 1A″ will result in direct dissociation into the spin−orbit ground HSe exit channel, and this path can account for a β value of −1 at the longer wavelength end (266 nm) for that channel. There is some difficulty with interpreting the fate of an H2Se molecule excited into the 4A″ state (the same as the 11B1 state in the C2v limit). As pointed out above, this excitation constitutes the main contribution to the A-band absorption. The photolysis cross-sections shown in Table 1 indicate that channel 1 (the SeH(X2Π3/2) channel) is the predominant exit channel throughout the studied wavelength range, and this channel has a β value close to −1 for all the wavelengths. Most of the H2Se molecules that are promoted to the 4A″ state will eventually exit via the SeH(X2Π3/2) channel. A similar result is also observed in H2Te photolysis.18 Figure 5 suggests that 4A″ is a bound state with an additional well formed on the dissociation path due to an avoided crossing with the 3A″ state. The 3A″ curve has a well near the Franck−Condon region, but it dissociates into the SeH(X2Π1/2) channel after passing a potential barrier. There are two possible ways for an initial 4A″ 11967



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value from 1A″ can also take place, but note that 1A″ has a transition dipole moment three times smaller than 2A′ and 3A′.17 Note also that dissociation flux originating from 4A″ (of −1 β value) and flowing along 2A″ toward the SeH(X2Π3/2) channel has not completely diminished. This point is manifested by the fact that at 266 nm the β value of SeH(X2Π3/2) channel is still close to −1. If such flux has diminished totally, the only −1 flux will be from 1A″, which should be smaller than the +0.5 flux from 2A′. Since all of these fluxes flow to the SeH(X 2Π3/2) channel, the β value for this channel should be close to +0.5 instead of close to −1. However, the −1 β value flux originating from 4A″ has indeed diminished to such an extent that the rising +0.5 β value flux from 3A′ (not very much due to small nature of the transition dipole moment) can actually surpass the part of the −1 β value flux that has crossed over from 2A″ to 3A′ (plus some −1 β value flux directly leaking from 4A″ to 3A″ at the avoided crossing). The end result is that the β value of the SeH(X2Π1/2) channel has swung from near −1 at 210 nm to near +0.5 at 266 nm.

population to deplete into the 3A″ state: (1) through overlapping of higher vibrational levels of the 3A″ state with lower vibrational levels of 4A″ in the Franck−Condon region or (2) through coupling at the crossing on the repulsive dissociation path. Possibility (1) must dominate over (2) otherwise a significant amount of dissociation flux will lead to the SeH(X2Π1/2) exit channel, a scenario directly contradicting the experimental observation. The population of 3A″ will itself be depleted through coupling of 3A″ with 2A″, the latter being the 11A2 state in the C2v limit.17 It directly dissociates via the SeH(X2Π3/2) exit channel. Further theoretical and/or experimental studies are needed to clarify how excitation to the 4A″ state (the same as the 11B1 state in C2v symmetry) predominantly leads to a HSe spin− orbit ground exit channel. Photolysis experiments involving D2Se may provide additional data for clarifying the strength of the coupling between dissociating states. Also, more ab initio calculations are required to discover how close the curves shown in Figure 5 actually represent the positions of the states of H2Se. According to Rauk and Collins’ calculation in the Franck−Condon region20 (see Table 3), the 11A2 state (same as the 2A″ state) has an excitation energy only 0.08 eV lower than the 11B1 state (same as the 4A″ state). This result indicates that for H2Se these two states might directly couple with each other, then there would be less difficulty in explaining how initial excitation to the 4A″ state predominantly leads to the HSe spin−orbit ground exit channel. C. Routes for a +0.5 β Value. Four A′ states (2A′, 3A′, 4A′, and 5A′) can provide a +0.5 β value. All have excitation energies lower than the 4A″ state (the 11B1 state in the Franck−Condon region). As mentioned previously, the 21A1 state might be another source for the +0.5 β value, and it has an excitation energy higher than the 11B1 state. At the lower wavelength end (near 210 nm), the 21A1 state can act as a possible source for a +0.5 β value. However, how the population of this higher excited state depletes to lower excited states, eventually exiting via SeH(X2Π1/2) and/or SeH(X2Π3/2) channels, is unknown. In this lower wavelength region, the 4A′ and 5A′ states may also be sources for a +0.5 β value. After passing a potential barrier, these two potential curves both lead to the SeH(X2Π1/2) channel. However, in this wavelength region the SeH(X2Π1/2) channel has a β value near −1, which means the predominant contribution to this channel should be routes with a −1 β value instead. There are two possibilities for contributing a −1 β value to the SeH(X2Π1/2) channel: (1) through the coupling of 4A″ with 3A″ at the avoided crossing on the dissociation path and (2) immediate coupling of 4A″ to 3A″ near the Franck−Condon region with subsequent coupling to 2A″, a crossing to 3A′, and exit via the SeH(X2Π1/2) channel. At this time, we cannot assess which of these two possibilities predominates; probably both contribute. In the previous section we have accounted for the near −1 β value for the SeH(X2Π3/2) channel. As the β value for this channel is not exactly −1, some contribution from a +0.5 β value source is required. For this to occur, it is necessary to assume there is some coupling between 4A′ and 2A″ as well as between 5A′ and 2A″. Thus, some population of 4A′ and 5A′ may leak to 2A″ and exit through the SeH(X2Π3/2) channel. When wavelengths become much longer (near 266 nm), the source of a −1 β value from 4A″ as well as the source of a +0.5 β value from 4A′ and 5A′ are likely all to be diminishing. Here, contributions of +0.5 β values from 2A′ and 3A′ should become relatively important. To a lesser extent, contribution of a −1 β

V. CONCLUSIONS For H2Se photolysis in its A-band, measured β values for the photolysis channels of SeH(X2Π3/2) and SeH(X2Π1/2) show distinctive trends. The β value for the SeH(X2Π1/2) channel switches from near −1 at 210 nm to near +0.5 at 266 nm. However, the SeH(X2Π3/2) channel retains a β value near −1 over the entire region. A spin−orbit coupled picture lends qualitative insight into the photodissociation mechanism, but a quantitative explanation awaits more detailed calculations for the excited electronic states of H2Se.

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AUTHOR INFORMATION

Corresponding Author

*(B.K.) Phone: 504-865-5573. Fax: 504-865-5596. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Louisiana Board of Regents. We thank C. Wittig for inspiration. We also thank a reviewer for helpful comments.

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REFERENCES

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