Theory of the excitation of polyatomic molecules by intense infrared laser radiation

The molecular excitation process is represented by a system of coupled anharmonic oscillators, one of which is driven by a near-resonant applied electric field. The master equation for the density matrix of such a system is reduced to the form of a Boltzmann equation describing the absorption of energy by the driven mode and the transfer of energy to the rest of the molecule by intramolecular vibrational coupling. The solution of this Boltzmann equation is shown to lead to an energy absorption spectrum which, for sufficiently strong laser fields, is considerably broader than the anharmonic defect and shows maxima corresponding to multiphoton transitions. The magnitude which the vibrational coupling width must attain before dissociation of the molecule is possible may be estimated from the calculated energy absorption spectrum.