Photodissociation of RNCS and RSCN (R=H, CH₃, C ₂H₅): Evidence for an excited state isomerization and energy deposition in the NCS product

The 248 and 193 nm photodissociation of the series of molecules RNCS and RSCN (R = H, CH₃, C₂ H₅) has been investigated. Laser-induced fluorescence of the radical NCS was used to probe the energy deposition in the R + NCS channel of this dissociation, however excitation to many vibrational states of NCS caused spectral congestion which prohibited direct measurement of populations of individual levels. A single-photon dissociation channel leading to RS and CN was observed for both isomers of all of the precursor molecules. While this was expected for the thiocyanate species, no direct channel to these products is available for the isothiocyanates. This observation is discussed in terms of an excited state isomerization of these molecules. Measured ratios of CN to NCS production for all compounds provide additional support for this mechanism and suggest a significant barrier to the process. Comparison of nascent laser-induced fluorescence (LIF) spectra of NCS with spectra obtained following different amounts of collisional relaxation suggests that all three vibrational modes were equally excited with a vibrational temperature of roughly 4500 K. The rotational temperature was estimated to be less than 1000 K. A spin-orbit population inversion was observed for all vibrational levels with approximately 75% of the population in the upper spin-orbit component of the vibrationless and low lying bending excited levels. No simple model is found to explain the photodissociation dynamics or the isomerization.