Rotational and nuclear-spin level dependent photodissociation dynamics of H2S

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.

Photodissociation branching ratios of 12C16O from 110 500 to 113 045 cm−1: first observation of the C(1S) channel

Carbon monoxide (CO) is one of the most abundant molecular species in comets. Its photodissociation by the solar radiation in the vacuum ultraviolet (VUV) region produces excited atomic fragments C(1D), C(1S), and O(1D), which radiate at characteristic wavelengths when they decay to lower states. The fractional rate constants for generating these fragments from CO photodissociation under the entire range of the solar radiation field are key input values in modelling the observed atomic emission intensities from comets. In this study, the branching ratios of the four lowest dissociation channels C(3P)+O(3P), C(1D)+O(3P), C(3P)+O(1D), and C(1S)+O(3P) of the 12C16O photodissociation are measured in the VUV energy range between the threshold of producing the C(1S)+O(3P) channel (~110 500 cm−1) and the ionisation energy (IE) of 12C16O (~113 045 cm−1). We measured these ratios using the VUV time-slice velocity-map ion imaging apparatus. We observe a number of high Rydberg states in the aforementioned energy range, with most of them mainly producing ground C(3P) and O(3P) atomic fragments, and only a few of them producing a significant amount of excited C(1D) or O(1D) fragments. We also observe the excited C(1S) fragment from CO photodissociation and measured its branching ratio for the first time. Based on the photodissociation branching ratios measured in the current and previous studies, we are able to estimate the relative percentages of the excited atomic fragments C(1D), C(1S), and O(1D) from the solar photolysis of 12C16O below its IE. We discuss the implications for the photochemical modelling of the CO-dominated comet C/2016 R2 (Pan-STARRS).

How big is the substituent dependence of the solar photolysis rate of Criegee intermediates?

Substituent dependence of the UV spectra of Criegee intermediates were calculated using multireference methods.