Resonances in molecular photoionization. IV. Theory of one-color and two-color near-threshold photoionization of molecules

A relatively comprehensive theoretical description of one-color and two-color photoionization of molecules by strong laser fields is developed. The molecular system is modeled by a number of discrete electronic configurations and a number of electronic ionization continua in a diabatic representation, allowing for intramolecular coupling of the discrete states and the continua. The vibrational degrees of freedom are included in compact operator notation without invoking the Born-Oppenheimer approximation. The relevant radiative dipole couplings are treated nonperturbatively on an equal footing with the intramolecular couplings. The important Coulomb threshold effects (accumulating Rydberg series at the electronic-vibrational ionization thresholds) are included via a threshold expansion of the complex level-shift operators representing intramolecular and radiative couplings. The weak-field, long-time ionization rate (golden-rule formula) is rederived from the general theory both in the one-color and the two-color case. In the two-color case, strong-field effects caused by either one of the two lasers are briefly discussed. We derive a simplified multichannel-quantum-defect model from the general two-color formalism which nicely reproduces a number of characteristic features of vibronic autoionization recently observed in several polyatomic molecules.