Master Equations for Photochemistry with Intense Infrared Light

Abstract

AbstractFour master equations are formulated for unimolecular reactions induced by collisions, by blackbody radiation, by intense, incoherent, quasimonochromatic infrared radiation and by coherent, monochromatic infrared radiation. The relationship between these equations is discussed and the fundamental differences which are due to the different structures of the rate coefficient matrix are pointed out. The properties of the solutions of these equations are investigated in some detail for both time dependent and time independent rate coefficient matrices. A systematic method for determining all the characteristic parameters (eigenvalues, characteristic populations and characteristic times) of the master equations from experimental data or numerical calculations is proposed. A model for the parametrization of the rate coefficient matrix in the master equation for unimolecular reactions induced by monochromatic infrared radiation (URIMIR) is put forward, which uses only easily measurable molecular parameters. The implications of this model for spontaneous infrared emission of highly excited polyatomic molecules are considered. The limitations of the use of intensity proportional rate coefficients (phenomenological cross sections) for optical excitation in URIMIR are pointed out. The determination of the steady state characteristics in URIMIR from experimental data is discussed. In particular, a simple, new method for obtaining the steady state rate constant from measurements of the product yield as a function of laser energy fluence is presented and illustrated with the first evaluation of rate constants for the IR‐photodissociation of several molecules from published experimental data of other authors. The intensity dependence of the rate constant in URIMIR is predicted to be nonlinear and nontrivial in the limit of both low and high radiation intensities. The reducibility of the rate coefficient matrix for URIMIR is considered and the consequences for the interpretation of experimental data are illustrated.