Mechanism of collisionless dissociation of polyatomic molecules

Abstract

A detailed investigation of the effect of collisionless dissociation of polyatomic molecules was carried out in a cycle of experiments [4], in which collisionless dissociation of polyatomic molecules was revealed to have a distinguishing feature of exceeding importance for laser photochemistry and laser isotope separation, viz., isotopic selectivity of the dissociation process. The experimentally observed features of collisionless dissociation were theoretically explained in [5-6]. The explanation was based on the subdivision of the energy spectrum of the vibrational states into two qualitatively different regions. The first part of the spectrum includes the lower vibrational levels down to the energies of 4-5 laser photons. As shown in [5-6], the remaining vibrational levels, located in regions of higher energies and contained in the second part of the spectrum, are so closely spaced that they form a quasicontinuum of vibrational states. This division of the spectrum into two parts is generally speaking arbitrary, since the quasicontinuum does not have an abrupt boundary on the lowenergy side; nevertheless, this division reflects correctly the specifics of the spectrum of polyatomic molecules. As a result, in contrast to the one-dimensional nonlinear oscillator model, satisfactory qualitative agreement was obtained between the theoretical and the experimental results. The process of excitation of polyatomic molecules was correspondingly subdivided in [5-6], in accord with the subdivision of the spectrum, into two stages. The first stage is resonant excitation of several low-lying levels of a mode active in the IR region, up to states lying in the quasicontinuum. Use was made in this case of the system of vibrational levels of a one-dimensional nonlinear oscillator. This made it possible to explain the resonant character of the energy acquisition during the first stage, and consequently the isotopic selectivity of the collisionless dissociation of polyatomic molecules~ During the second stage, the molecules landing in the quasicontinuum region were resonantly excited, in view of the high level density, in the region of the quasicontinuum, up to dissociation states or even ionization states [7]. It is precisely the second stage which strongly distinguishes the process of excitation of polyatomic molecules from the excitation of diatomic molecules, and permits a qualitatively correct identification of the distinguishing features of collisionless dissociation of polyatomic molecules.