Picosecond laser study of the quasicontinuum of C2F5Cl

Abstract

The quasicontinuum of the vibrationally hot (∼10 CO2 laser photons) C2F5Cl molecule was investigated using a two-laser pulse arrangement. The first laser pulse preheated the molecules into the QC (quasicontinuum) while the second picosecond CO2 laser was employed to probe the absorption characteristics of the excited molecules. The second laser could be tuned either in frequency or in pulse duration. Several experiments were performed. In the first experiment, the small signal absorption spectrum of C2F5Cl was measured and compared with the dissociation spectrum obtained by Borsella et al.1 The same ν6+ν10 hot band was observed with a considerably narrower linewidth. In the second experiment, the saturation properties of the hot band were investigated. It was found that the absorption line could be characterized as inhomogeneously broadened with no evidence for any broadening due to intramolecular damping. In a third series of experiments, the absorption of C2F5Cl at low as well as high laser energy fluences was measured as a function of the laser pulse duration between 22 and 120 ps. It was found that (i) the absorption can be characterized as predominantly intensity dependent and (ii) the small signal absorption cross section increased considerably as the pulse duration was decreased. These observations were discussed using the multitier energy level classification scheme in the QC. It was concluded that within the laser pulse duration, individual states within the first tier which interacted with the laser were not coupled to each other. The change in the absorption cross section as a function of the pulse duration is consistent with the picture that under laser irradiation, a rapid equilibrium was achieved with the final state populations remaining relatively constant in time. The final populations were dependent almost entirely on the laser intensity. Increasing the laser pulse duration had little effect on the final state distribution and the total energy absorbed by the molecule.