Imaging studies of the photodissociation of H2S+ cations.: I.: Illustrations of the role of nuclear spin

Ion imaging methods have been used to study the dynamics of H-2(D-2) molecular elimination from H2S+(D2S+) cations following photoexcitation to the (A) over tilde (2)A(1) state in the wavelength range 300 < 360 nm. Ground ((X) over tilde B-2(1)) state parent ions were formed by multiphoton ionization of a jet cooled H2S(D2S) sample, resonance enhanced at the two photon energy by the v=0 level of the (1)A(2)(center dot center dot center dot 2b(1)(1)4pb(2)(1)) Rydberg state. This Rydberg excited state predissociates sufficiently slowly that the 2+1 resonance enhanced multiphoton ionization (REMPI) spectrum shows resolved rovibronic structure, thereby allowing full quantum state selectivity at this intermediate stage of the cation preparation process. Analysis of the S+ ion images following one photon excitation of the resulting H2S+(D2S+) cations shows that these fragments are formed in their ground S-4 state, and that the H-2(D-2) cofragments are formed predominantly (if not exclusively) in rotational states with either odd or even J rotational quantum number-depending on the chosen REMPI preparation wavelength. This striking specificity for forming ortho- or para-H-2(D-2) products can be traced back to the state selectivity introduced in the REMPI preparation step. In the case of H2S, therefore, the nuclear spin symmetry of the two equivalent H nuclei in ortho-H2S (and H2S+) carries through into ortho-H-2 products, and para-H2S molecules map into para-H-2 fragments, surviving photoionization of the Rydberg state, photolysis of the resulting parent cation, and two subsequent radiationless transitions during the evolution from the photoexcited ((A) over tilde (2)A(1) state) cation through to S(S-4)+H-2 products. We identify two distinct fragmentation pathways. One, which we term route I, involves nonadiabatic (Renner-Teller) coupling to the (X) over tilde state at near linear configurations and subsequent (spin-orbit induced) coupling to the repulsive (4)A(2) potential energy surface (PES) at smaller bond angles. This process operates throughout the photolysis wavelength range investigated and yields rotationally cool and vibrationally cold H-2 products. The second (route II) shows a long wavelength threshold lambda similar to 335 nm, and gradually becomes dominant as the photolysis wavelength is reduced. Route II dissociation involves vibronically facilitated nonadiabatic transfer from the (A) over tilde to the (B) over tilde B-2(2) state, followed by spin-orbit induced transfer to the (4)A(2) PES; the route II fragmentation dynamics results in H-2 products carrying higher levels of rotational and vibrational excitation. (c) 2007 American Institute of Physics.