Non-thermal nuclear hyperfine populations in the products of a photodissociation reaction

Coherent anti-Stokes Raman scattering (CARS) spectra of iodine atoms produced thermally in iodoform, CHI3, vapour and photochemically in I2 vapour, with well-resolved hyperfine structure are reported. Conditions in the I2 vapour studies were adjusted to create, effectively, either collision-free or thermal regimes. Thermal I atom spectra compare well with predictions based on the hyperfine level Raman scattering cross-sections calculated by Drühl [Phys. Rev. A 26 (1982) 863]; a set of experimental relative Raman scattering cross-sections is presented. In contrast, collision-free I atom spectra exhibit features explicable in terms of non-thermal hyperfine level populations in the nascent atoms.