Translational spectroscopy of H(D) atom fragments arising from the photodissociation of H2S(D2S): a redetermination of D 00(S–H)

Abstract

The technique of H/D atom photofragment translational spectroscopy has been used to further investigate the collision-free photodissociation of H₂S and D₂S molecules both in the near ultraviolet (at 218.2 and 221.6 nm) and in the vacuum ultraviolet (at 121.6 nm). Measurements of the H/D atom photofragment angular distributions confirms that the near UV dissociation occurs promptly, following a perpendicular photo-excitation. More than 99% of the resulting SH/SD fragments are formed in their ground vibronic level, with a ca. 3 : 2 preference in favour of the lower (²Π_3/2) spin–orbit component. Product rotation accounts for ca. 1% of the available energy in the case of H₂S photolysis at these near UV wavelengths (ca. 2% in the case of D₂S dissociation). The groundstate SH/SD photofragments can also be photolysed at these near UV excitation wavelengths. Simulations of the kinetic energy distribution of the resulting H/D atomic fragments show that the secondary photolysis also involves a perpendicular transition, and that the partner S atoms are formed in all three ³P_J spin–orbit states. The product energy disposal following 121.6 nm photolysis of D₂S closely parallels that deduced in an earlier study of H₂S photodissociation at this same wavelength (Schnieder et al., J. Chem. Phys., 1990, 92, 7027). The D-atom kinetic energy spectrum shows clear evidence for the formation of rovibrationally excited SD(A ²Σ⁺) fragments amongst the primary products, and also suggests an important role for the three-body dissociation process leading to D + D + S(¹D) atoms.

Given D⁰₀(HS–H)= 31 480 ± 40 cm^–1, the present results provide a refined value for the S–D bond strength in the D₂S molecule; D⁰₀(DS–D)= 32 030 ± 50 cm^–1; for the SH and SD radical bond dissociation energies; D⁰₀(S–H)= 29 300 ± 100 cm^–1 and D⁰₀(S–D)= 29 700 ± 100 cm^–1, and an improved expression for the potential-energy function for the A ²Σ⁺ state of the mercapto radical.