Non-adiabatic effects in the photodissociation of H₂S

The photodissociation of triatomic molecules is relevant to the atmospheric chemistry of terrestrial and astronomical systems, where H₂S photodissociation influences the non-equilibrium chemical networks of sulphur-rich environments. This work presents photodissociation cross sections for H₂S in its first electronic absorption band at temperatures 1 and 295 K as computed within a time-independent variational framework. The calculations are performed in the adiabatic and diabatic representations with modified ab initio surfaces available in the literature using an updated version of the EVEREST nuclear motion program. It is demonstrated that the inclusion of non-adiabatic effects, associated with conical intersections between the excited 1 ¹A₂ (1 ¹A″) and 1 ¹B₁ (2 ¹A″) electronic states, is essential to correctly model the low-energy wing of the spectrum. The new diabatic treatment thus leads to significantly improved agreement with measured data at 295 K, especially in the important low-energy threshold region, in comparison with the uncoupled adiabatic model. The methodology established herein provides a foundation for future temperature-dependent photodissociation calculations of H₂S and other triatomic molecules with coupled electronic states.