An experimental and ab initio reinvestigation of the Lyman-α photodissociation of H2S and D2S

The Lyman-α (λH=121.6 nm) photodissociation of both H2S and D2S has been reinvestigated using the experimental technique of H/D atom photofragment translational spectroscopy. Their total kinetic energy release profiles consist of two distinct components. The first, which is highly structured, is assigned to two body dissociation to H/D(2S)+SH/SD(A 2Σ+) fragments, with the latter formed in a range of rovibrational states. By assigning these various levels the dissociation energy of D2S (measured relative to the lowest rovibrational level of the products) is determined to be D0(D-SD)⩾31 874±22 cm−1. The second contribution, which is broad and relatively unstructured, is modeled in terms of two likely fragmentation pathways; secondary predissociation of SH/SD(A 2Σ+) partner fragments associated with the structured contour, and primary three-body dissociation to 2H/D(2S)+S(1D) atomic products. The presented data allow determination of the kinetic energy-dependent anisotropy parameter (β), which is positive over both profiles. This indicates a preferentially parallel distribution of H/D atom recoil velocities about the laser polarization axis. These data are presented in tandem with ab initio and classical trajectory calculations which seek to explain the lack of branching to ground state H/D+SH/SD(X 2ΠΩ) molecular products. The analogous channel is important in the Lyman-α dissociation of the lighter homologue, H2O.