Nonadiabatic effects in the photodissociation of H2S in the first absorption band: An ab initio study

Abstract

The photodissociation of H2S through excitation in the first absorption band (λ≊195 nm) is investigated by means of extensive ab initio calculations. Employing the MRD-CI method we calculate the potential energy surfaces for the lowest two electronic states of 1A″ symmetry varying both HS bond distances as well as the HSH bending angle. (In the C2v point group these states have electronic symmetry 1B1 and 1A2, respectively.) The lower adiabatic potential energy surface is dissociative when one H atom is pulled away whereas the upper one is binding. For the equilibrium angle of 92° in the electronic ground state they have two conical intersections, one occurring near the Franck–Condon point. Because of the very small energy separation between these two states nonadiabatic coupling induced by the kinetic energy operator in the nuclear degrees of freedom are substantial and must be incorporated in order to describe the absorption and subsequent dissociation process in a realistic way. In the present work we treat the coupling between the two electronic states in a diabatic representation extracting the coordinate-dependent mixing angle from the CI coefficients of the electronic wave functions. The nuclear motion is treated in three dimensions in an exact quantum mechanical approach by propagation of a two-component time-dependent wave packet. The calculated absorption spectra for H2S and D2S satisfactorily agree with the measured spectra. In particular, the calculations reproduce the diffuse structures with energy spacing of about 1200 and 850 cm−1 for H2S and D2S, respectively. Furthermore, the calculated rotational- and vibrational-state distributions of the HS and DS fragments reproduce recent measurements in a convincing way. The photodissociation of H2S is a prototype for very fast electronic predissociation. The photon preferentially excites the binding (diabatic) state. This state, however, is quickly depleted by strong coupling to the dissociative (diabatic) state with the complex finally breaking up into products H and HS. The electronic quenching takes place on the time scale of one internal vibrational period only. Our calculations unambiguously confirm that the diffuse structures superimposed to the broad background are caused by symmetric stretch motion—in the binding state—and not by activity in the bending mode as originally assumed.

HN2(2A‘) Electronic Manifold. II. Ab Initio Based Double-Sheeted DMBE Potential Energy Surface via a Global Diabatization Angle

“…Their ab initio calculations 61 covered a broad area of configuration space for the coupled 1 1 A′′/2 1 A′′ states, ground state (1 1 A′) as well as TDM surfaces, followed by construction of a sequence of interpolated PESs for the three diabats, adiabats, and TDM surfaces. In fact, their calculations allowed a satisfactory description of the first absorption band of H 2 S. However, as noted in ref 48, the resulting mixing angles do not show a correct asymptotic behavior, with the diabats not merging the adiabats at the atom-diatom dissociation limits. Using a similar diabatization scheme, Dobbyn and Knowles 38 have reported the same difficulty in a study of the 1 A′ states of the water molecule.…”

“…In an early attempt to establish a purely first-principles understanding of the photodissociation of H 2 S, Heumann et al 48 suggested a procedure for constructing mixing angles by exploring at each geometry the CI coefficients of the reference configurations that have the desired diabatic character at the locus of C 2V symmetry. Their ab initio calculations 61 covered a broad area of configuration space for the coupled 1 1 A′′/2 1 A′′ states, ground state (1 1 A′) as well as TDM surfaces, followed by construction of a sequence of interpolated PESs for the three diabats, adiabats, and TDM surfaces.…”

“…In turn, Simah et al 49 reported a new firstprinciples study of to the coupled 1 1 A′′/2 1 A′′ states of H 2 S, where they attempt to correct for failures of the previous study for the same system. 48 By aiming at a substantial improvement on the quality of the ab initio description of the crossing seams and avoided crossings of higher excited states, they have employed a larger basis set with polarized functions and more excited states (one 1 A′ state and seven 1 A′′ states on the CASSCF calculations, followed by one 1 A′ state and three 1 A′′ states in the subsequent MRCI calculations). A large number of geometries has been computed with the new diabatization scheme.…”

“…As a result, the topology of the diabats obtained from the above direct diabatization schemes [23][24][25][26][27][28]48,49 at the asymptotes represent a serious drawback for MBE 6,8 modeling. We remark that the authors have themselves revealed skepticism on the hypothesis of obtaining diabats that simultaneously account for the crossing seams, show a vanishing diabatic coupling element < φ d |∇ R |φ d > (producing smoothly varying electronic wave functions over the whole configuration space), and merge the adiabats at the asymptotes.…”

“…9,[29][30][31][32][33][34][35][47][48][49][50][51][52][53][54][55] For convenience, we divide them in two categories. The first gathers approaches that yield directly the diabatic states, [23][24][25][26][27][28][29][30][31][32][33][34][35]36 either from ab initio energies or diabatic electronic wave functions, thus without an intermediate step.…”