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.

Collision induced intersystem crossing of O(2p 1D) to O(2p 3Pj) j=0,1,2 was studied by vacuum-ultraviolet laser induced fluorescence. The nascent j-branching ratios of O(3Pj) produced from collision of O(1D) with Xe, Kr, N2, CO2, and CF3H at the collision energy of 15.6, 14.7, 11.4, 13.0, and 14.4 kcal/mol are [O(3P0)]/[O(3P1)]/[O(3P2)]=(0.28±0.04)/(0.58±0.05)/1, (0.33±0.05)/(0.45±0.07)/1, (0.23±0.05)/(0.35±0.05)/1, (0.14±0.05)/(0.41±0.05)/1, and (0.20±0.05)/(0.45±0.05)/1, respectively. The j-branching ratios are discussed in terms of potential curve crossing between singlet and triplet surfaces and nonadiabatic nature of the potential surfaces. Doppler profiles of the product O(3Pj) atoms were measured as a function of time delay between pump and probe laser pulses. By calculating the kinetic energy from the Doppler profiles, electronic energy transfer efficiencies to the internal energy of N2, CO, CO2, and CF3H are obtained to be (30±7), (31±7), (49±3), and (52±5)%, respectively.

No abstract

No abstract