Multiphoton molecular dissociation in intense laser fields

In this paper we advance a model for multiphoton photofragmentation of an "isolated," collision-free, polyatomic molecule on the ground state potential surface. The molecular energy levels are separated into three regions. In the low energy range the level structure is sparse and only dynamic Stark shifts will be exhibited. In the intermediate energy range the density of bound vibrational states is high and the level structure can be described in terms of mixed (zero-order) states. We argue that in this energy range inlrastate anharmonic scrambling may be of central importance in the excitation process, but that intrastate vibrational relaxation and energy redistribution is not encountered for medium-sized molecules. In the high energy region dissociative channels open up and reactive intramolecular decay is handled in terms of resonance theory. The time evolution of a multilevel system, whose highest energy levels are metastable, and which is driven by an intense laser field, is handled by the effective Hamiltonian formalism. Explicit expressions are derived for the photofragmentation yields and their dependence on the molecular parameters and on the field parameters. Specific applications to two distinct model systems are presented. First, we treat the quasidiatomic model, which disregards level scrambling in the intermediate energy range, whereupon near-resonant radiative coupling prevails between the states of a truncated anharmonic oscillator. Second, we have considered the two-ladder model where in the low energy range near-resonant radiative coupling occurs within an anharmonic ladder, while in the intermediate energy range resonant radiative coupling between mixed states prevails. We present numerical simulations of the photofragmentation yields and their dependence on the molecular parameters, such as the diagonal anharmonicity, the molecular dissociation energy, the predissociative widths, and the isotopic shift. We have also explored the dependence of the photofragmentation yields on the pulse parameters, such as the off-resonance energy, the field intensity, and ihe pulse duration. The quasidiatomic model seems to overestimate the power onset for photodissociation and the power required for the onset of saturation effects, while the two-ladder model is quite adequate to account for the gross features of coherent multiphoton molecular photofragmentation.