10min:

A. D. WEBB, R. N. DIXON AND M. N. R. ASHFOLD, School of Chemistry, University of Bristol, Bristol, BS8 1TS, U.K..

Ion imaging methods have been used to study the dynamics of H₂/D₂ elimination from H₂S⁺/D₂S⁺ cations following photo-excitation to the A²A₁ state in the wavelength range 300< <360 nm. Ground (X²B₁) state parent ions were formed by 2+1 REMPI of H₂S/D₂S via the v=0 level of the ¹A₂(...2b₁¹4pb₂¹) Rydberg state. This Rydberg state predissociates sufficiently slowly that the REMPI spectrum shows resolved rovibronic structure, thereby allowing full quantum state selectivity at this stage of the cation preparation process. Analysis of the S⁺ ion images following one photon excitation of the resulting H₂S⁺/D₂S⁺ cations reveals that these fragments are formed in their ground (⁴S) state, and that the H₂/D₂ co-fragments are formed 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-H₂/D₂ products can be traced to the state selectivity introduced in the REMPI preparation step. Two distinct fragmentation pathways for H₂S⁺/D₂S⁺(A) cations are identified. One involves non-adiabatic (Renner-Teller) coupling to the X state at near linear configurations and subsequent (spin-orbit induced) coupling to the repulsive ⁴A₂ potential energy surface (PES) at smaller bond angles. This process operates throughout the photolysis wavelength range investigated and yields rotationally 'cool' and vibrationally 'cold' H₂ products. The second shows a long wavelength threshold 335 nm, and gradually becomes dominant as the photolysis wavelength is reduced. This mechanism involves vibronically facilitated non-adiabatic transfer from the A to the B²B₂ state, followed by spin-orbit induced transfer to the ⁴A₂ PES; the resulting H₂ products carry higher levels of rotational and vibrational excitation.