Abstract
The detailed features of molecular photochemistry are key to understanding chemical processes enabled by non-adiabatic transitions between potential energy surfaces. But even in a small molecule like hydrogen sulphide (H2S), the influence of non-adiabatic transitions is not yet well understood. Here we report high resolution translational spectroscopy measurements of the H and S(1D) photoproducts formed following excitation of H2S to selected quantum levels of a Rydberg state with 1B1 electronic symmetry at wavelengths λ ~ 139.1 nm, revealing rich photofragmentation dynamics. Analysis reveals formation of SH(X), SH(A), S(3P) and H2 co-fragments, and in the diatomic products, inverted internal state population distributions. These nuclear dynamics are rationalised in terms of vibronic and rotational dependent predissociations, with relative probabilities depending on the parent quantum level. The study suggests likely formation routes for the S atoms attributed to solar photolysis of H2S in the coma of comets like C/1995 O1 and C/2014 Q2.
Highlights
The detailed features of molecular photochemistry are key to understanding chemical processes enabled by non-adiabatic transitions between potential energy surfaces
The deduced dynamics can all be reconciled within the framework illustrated in Fig. 5, but a complete interpretation will require much better knowledge represents the energetic molecule formed by absorbing one 139.1 nm photon
The scope of the present data will inspire state-of-the-art computational studies of H2S, enabling it to acquire status as a reference molecule within the photophysics and chemical reaction dynamics communities—comparable to that enjoyed by its lighter analogue H2O51
Summary
The detailed features of molecular photochemistry are key to understanding chemical processes enabled by non-adiabatic transitions between potential energy surfaces. Analysis reveals formation of SH(X), SH(A), S (3P) and H2 co-fragments, and in the diatomic products, inverted internal state population distributions. These nuclear dynamics are rationalised in terms of vibronic and rotational dependent predissociations, with relative probabilities depending on the parent quantum level. But the abundances of known sulphur-containing molecules in the interstellar medium (ISM) sum to much less than this value[3]. Given the high hydrogen abundances and the mobility of hydrogen in the ice matrix, sulphur atoms incident on interstellar ice mantles are expected to favour formation of H2S, the chemical- and photo-induced desorption of which is considered the main source of gas-phase H2S molecules[10,11]. The electronic spectrum of H2S displays weak continuous absorption at wavelengths λ < 260 nm and stronger absorption features at λ < 155 nm associated with excitations to Rydberg states[21,22,23], as illustrated in Fig. 1 and shown in more detail in
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