A re-examination of the use of rate equations to account for fluence dependence, intramolecular relaxation, and unimolecular decay in laser driven polyatomic molecules.

Abstract

A general set of rate equations including intramolecular relaxation and laser‐induced excitation in a polyatomic molecule is proposed. These equations differ from relaxation equations transcribed from nmr theory and applied recently to polyatomic molecules in that they explicitly account for the dynamics of heat bath feedback on the pump mode excitation. Feedback of heat bath excitation to the pump mode is shown to result in decomposition which depends exclusively on laser fluence only when T₁ relaxation is rapid, but which depends more generally on peak laser intensity. Analytic expressions for stimulated absorption and emission coefficients, consistent with heat bath feedback when T₁ relaxation is rapid, are derived and incorporated in a degenerate rate equation. Previous arguments advanced to justify the calculation of decomposition rates from RRKM theory with a thermal distribution of absorbed energy are shown to fail precisely because they require absorption and emission coefficients which are not consistent with the dynamics of heat bath feedback.