Kinetics of low-intensity infrared laser photodissociation. The thermal model and application of the Tolman theorem

Abstract

Infrared multiphoton dissociation by low-power cw lasers under collision-free conditions is analyzed from the point of view of thermal kinetic theory. It is pointed out first that the laser has the same effect on the molecular population as irradiation by an equivalent black-body source, assuming only that the dominant IR emissions of the molecule at steady state are at wavelengths not far from the irradiating IR laser wavelength. Then the dissociation reaction kinetics are considered for this thermal population under the assumption that the reaction perturbs only the high-energy tail of the Boltzmann distribution. Analysis from this point of view leads to a slightly modified version of the Tolman theorem giving the activation energy as a function of temperature. A random walk simulation of the kinetics for dissociation of a model molecular system was carried out and the validity of the thermal kinetic approach was verified by the excellent agreement between the activation energies from the Tolman-theorem analysis and those from the random-walk simulations. The extent of dissociation is strongly dependent on intensity (at constant fluence) under these conditions.