The pure rotational spectra of SrSH (X̃ 2A′) and SrS (X 1Σ+): Further studies in alkaline-earth bonding

Abstract

The pure rotational spectrum of the SrSH radical in its ground electronic (X̃ 2A′) and vibrational states has been measured using millimeter/submillimeter-wave direct absorption techniques. This work is the first observation of SrSH with rotational resolution. The spectrum of its deuterium isotopomer SrSD and SrS (X 1Σ+) has been recorded as well. These species were created by the reaction of strontium vapor and H2S, in the presence of a dc discharge. SrS was also made with CS2. For SrSH and SrSD, eight rotational transitions were recorded, respectively, for which asymmetry components up to Ka=8 were measured; fine structure was also resolved in each component. Thirteen transitions of SrS in each of its v=0, 1, and 2 states have additionally been observed. These data have been analyzed and spectroscopic parameters determined for all three species, including spin-rotation terms for the strontium hydrosulfides. From an r0 structure calculation, the bond angle in SrSH was determined to be 91.48(3)°, very close to that of H2S and CaSH. This geometry indicates that SrSH is a covalently bonded molecule, as opposed to linear (and ionic) SrOH. The Sr–S bond length in SrSH was also found to be greater than that of SrS (rSr—S=2.705 Å versus 2.441 Å), indicating a change in bond order. In addition, the spin-rotation interaction in SrSH and SrSD includes a small contribution from the off-diagonal term, (εab+εba)/2, resulting from the crossing of energy levels with ΔJ=0, ΔKa=±1. Second-order spin-orbit coupling appears to make a significant contribution to the spin-rotation splitting, as well, which must arise from mixing of the à 2A′ and B̃ 2A″ excited states.