This paper is aimed at establishing a statistical theory of rotational and vibrational excitation of polyatomic molecules by an intense IR laser. Starting from the Wigner function of quantum statistical mechanics, we treat the rotational motion in the classical approximation; the vibrational modes are classified into active ones which are coupled directly with the laser and the background modes which are not coupled with the laser. The reduced Wigner function, i.e., the Wigner function integrated over all background coordinates should satisfy an integrohyphen;differential equation. We introduce the idea of lsquo;lsquo;viscous dampingrsquo;rsquo; to handle the interaction between the active modes and the background. The damping coefficient can be calculated with the aid of the wellhyphen;known Schwartzndash;Slawskyndash;Herzfeld theory. The resulting equation is solved by the method of moment equations. There is only one adjustable parameter in our scheme; it is introduced due to the lack of precise knowledge about the molecular potential. The theory developed in this paper explains satisfactorily the recent absorption experiments of SF6irradiated by a short pulse CO2laser, which are in sharp contradiction with the prevailing quasihyphen;continuum theory. We also refined the density of energy levels which is responsible for the muliphoton excitation of polyatomic molecules.
展开▼
