The technique of H(D) atom photofragment translational spectroscopy has been applied to the photodissociation of CH 4 (CD 4 ) at 121.6 nm. Contrary to the previous consensus view, we find simple C-H bond fission to be the dominant primary process following excitation at this wavelength. The resulting CH 3 fragments are formed with very high levels of internal excitation: Some (~25%) possess so much internal energy that they must undergo subsequent unimolecular decay. The present experiments do not provide a unique determination of the products of this secondary decay process, but statistical arguments presented herein suggest that they will be predominantly CH and H2 fragments. Similar considerations point to a significant role for the direct three body process yielding the same products H + H2 + CH. This overall pattern of energy disposal can be rationalized by assuming that most of the initially prepared CH 4 (A IT 2 ) molecules undergo rapid internal conversion (promoted by the Jahn-Teller distortion of this excited state) to high vibrational levels of the ground state prior to fragmentation. The realization that CH 4 photodissociation at 121.6 nm yields CH 3 (and CH) fragments, rather than methylene radicals, will necessitate some revision ofcurrent models of the hydrocarbon photochemistry prevailing in the atmospheres of the outer planets and some of their moons, notably Titan.2054

Some of the most detailed insight into the dynamics of molecular photofragmentation processes is provided by measurements of the translational energy distributions of the recoiling fragments. In this article we survey recent progress in the extraction of such fragment kinetic energy distributions from careful measurement and analysis of (a) their Doppler broadened spectral line shapes and/or (b) their time-of-flight spectra.

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.

No abstract