Infrared multiphoton decomposition and energy-dependent absorption cross section of chloroethane

The results are presented of an investigation of the multiple infrared photon decomposition of chloroethane with radiation of wavelength ∼ 10 μm, which is absorbed by the C-C stretching mode of the molecule. The study employed a focussed beam of maximum fluence 190 J/cm². The fluence dependence of the fractional decomposition with ṽ = 966 cm^-1 radiation under nearly collision-free conditions (0.035 torr) was quantitatively interpreted by solution of the appropriate master equation (taking postpulse collisions into account) to obtain the energy-dependent absorption cross section, σ(E). It was found that decomposition at high fluence (arising from energies in the range 19000-28000 cm^-1) and energy absorption at low fluence (mean absorbed energy ∼ 150-1000 cm^-1) could be simultaneously fitted with σ(E)/cm² = 2.5 × 10^-19 exp[-3.45(E/hv)^0.14]. Comparison of this dependence with that found from data obtained with ṽ ∼ 2700 cm^-1 suggests that the oscillator strength is strongly dependent on the mode being excited, even when the energy is thoroughly randomized within the molecule.