Vibrational energy distribution in molecules excited by infrared radiation in the collisionless regime

An investigation is made of the excitation kinetics of polyatomic molecules in the quasicontinuum. An approach is proposed to allow for a gradual transfer of molecules from lower levels to the quasicontinuum without making a detailed study of their population kinetics. A solution of the diffusion equation in the quasicontinuum is used to derive analytic expressions for the vibrational energy distribution function of the molecules under the conditions of collisionless photodissociation experiments. Expressions are also derived for the dissociation yield and the average energy absorbed by the molecules as a function of the laser radiation energy flux. The results of the calculations show good agreement with the experimental data for CF₃J and SF₆ molecules.