Quantum dynamics of molecular multiphoton excitation in intense laser and static electric fields: Floquet theory, quasienergy spectra, and application to the HF molecule

This is the published version, also available here: http://dx.doi.org/10.1063/1.444219.The multiphoton excitationdynamics of vibration‐rotation states in diatomic molecules in intense laser and static electric fields is investigated. The Floquet matrix method is used to calculate the quasienergy and multiphoton absorptionspectra of the HF molecule as functions of field strengths and frequency. Nonlinear effects such as power broadening, dynamic Stark shift, Autler–Townes multiplet splitting, hole burning, and S‐hump behaviors, etc., are observed and discussed in terms of quasienergy diagrams. Many of the salient features in the spectral line shapes may be qualitatively understood in terms of an analytical three‐level model. The addition of a dc electric field removes the restriction of the rotational dipole selection rule and causes significant intermixing of the bare molecular vibrator states. Due to the greater number of strongly coupled nearby states in the dc field, nonlinear effects such as those mentioned above appear at a much lower ac field strength than they would in the absence of the dc field. The introduction of an external dc field, therefore, strongly enhances the multiphoton excitation probabilities and results in a much richer spectrum