The technique of H/D atom photofragment translational spectroscopy has been used to further investigate the collision-free photodissociation of H,S and D,S 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 (2113/2) 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 0,s 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, s p i w r b i t states. The product energy disposal following 121.6 nm photolysis of D2S closely parallels that deduced in an earlier study of H,S photodissociation at this same wavelength (Schnieder eta!., J. Chern. Phys., 1990, 92, 7027). The D-atom kinetic energy spectrum shows clear evidence for the formation of rovibrationally excited SD(A 'C+) fragments amongst the primary products, and also suggests an important role for the three-body dissociation process leading to D + D + S('D) atoms. 40 cm-', the present results provide a refined value for the S-D bond strength in the D,S molecule; Dg(DS-D) = 32030 & 50 cm-' ; for the SH and SO radical bond dissociation energies: Dg(S-H) = 29300 & 100 cm-' and Dg(S-0) = 29700 & 100 cm-', and an improved expression for the potential-energy function for the A 'C+ state of the mercapto radical.

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The technique of H Rydberg atom photofragment translational spectroscopy has been applied to a high resolution study of the primary photochemistry of methanethiol (CH 3 SH) following excitation at a wide range of wavelengths in the near ultraviolet. In accord with previous studies of this molecule, excitation within its first (1 1 A" -X lA') absorption continuum is shown to result in S-H bond fission. Spectral analysis yields a refined value for the bond dissociation energy: D8(CH 3 S-H)=30 250± 100 cm-I . The resulting CH 3 S(X) fragments are deduced to carry only modest vibrational excitation, distributed specifically in the V3 (C-S stretching) mode and in one other mode having a wave number of ~ 1 040 cm -1. We associate this latter mode with bending of the CH 3 moiety in the plane containing the C and S nuclei and the lobe of the unpaired electron which was originally involved in the S-H bond. Decreasing the excitation wavelength (while remaining within the first absorption continuum) results in an increase in both the vibrational and rotational excitation of the CH 3 S(X) fragments, but a decrease in the relative yield of the upper eE 1I 0 spin-orbit component. Excitation at still shorter excitation wavelengths accesses the second (2 IA" -X I A ') absorption band of CH 3 SH. The CH 3 S fragments reSUlting from S-H bond fission at these excitation wavelengths are observed to carry very much higher levels of vibrational excitation in the above two modes. The observation of H atoms attributable to secondary photolysis of SH(X) fragments indicates increased competition from the alternative C-S bond fission channel at these shorter excitation wavelengths. Additional peaks in the H atom time-of-flight spectrum, most clearly evident following excitation at wavelengths in the range 213-220 nm, are interpretable in terms of secondary photolysis of the primary CH 3 S(X) fragments yielding thiofonnaldehyde (H 2 CS), primarily in its A 1 A z excited electronic state. Symmetry arguments provide an explanation for this specific electronic branching in the near ultraviolet photolysis of CH 3 S fragments.

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