Ultraviolet photolysis of H 2 S and its implications for SH radical production in the interstellar medium

Hydrogen sulfide radicals in the ground state, SH(X), and hydrogen disulfide molecules, H 2 S, are both detected in the interstellar medium, but the returned SH(X)/H 2 S abundance ratios imply a depletion of the former relative to that predicted by current models (which assume that photon absorption by H 2 S at energies below the ionization limit results in H + SH photoproducts). Here we report that translational spectroscopy measurements of the H atoms and S( 1 D) atoms formed by photolysis of jet-cooled H 2 S molecules at many wavelengths in the range 122 ≤ λ ≤155 nm offer a rationale for this apparent depletion; the quantum yield for forming SH(X) products, Γ, decreases from unity (at the longest excitation wavelengths) to zero at short wavelengths. Convoluting the wavelength dependences of Γ, the H 2 S parent absorption and the interstellar radiation field implies that only ~26% of photoexcitation events result in SH(X) products. The findings suggest a need to revise the relevant astrochemical models.