Collisionless dissociation of SF 6 in an intense IR field

Abstract

The model for multiphoton dissociation of polyatomic molecules which has been previously discussed by a number of authors is carefully investigated. It involves the coherent excitation of the molecule within the lower discrete energy levels, “leakage” into the quasicontinuum of levels, the incoherent excitation within the quasicontinuum and the dissociation itself. A closed set of equations for the populations is derived assuming quasistationary populations of the lower levels. An approximate solution to these equations is given in the post-threshold region where they can be reduced to a simpler system of two equations for the relative concentrations of activated and nonactivated molecules. In this approximation the problem becomes mathematically equivalent to the classical problem of thermal dissociation with the only difference that the rates of activation and deactivation are functions of the field intensity, I. A relatively slow (power-like) rise of dissociation yeild, W, with increasing I in the post-threshold region is shown to occur under someconditions. The reults of the numerical solution to the initial set of equations for SF 6 are reported and compared with the predictions of the approximate theory and with experiment. The theory presented explains well the dependence of W upon I in the post-threshold region provided that the field frequency does not satisfy the mutliphoton-resonance condition, agreement between the calculated and observed absolute values of W being quite good as well in spite of the very low populations of the excited vibrational states. There is, however, disagreement between theory and experiment concerning the dependence of W on the pulse duration τ at a fixed energy fluence Φ = Iτ. The vibrational heating of the molecules is also calculated compared with experiment.