LADDER CLIMBING AND MULTIPHOTON DISSOCIATION OF POLYATOMIC-MOLECULES EXCITED WITH SHORT PULSES - BASIC THEORY AND APPLICATIONS TO HCO

We present a numerically efficient algorithm for multiphoton dissociat ion of molecules with light pulses in the infrared frequency regime. T he shape of the external electric held can be arbitrary. The method is essentially based on discretizing the continuum; dissociation is simu lated by a coordinate-dependent cutoff function. A substantial reducti on of the computer time required for solving the large set of first-or der differential equations in time (N approximate to 2000) can be achi eved if the interaction with the field can be separated according to ( W) over cap(t)=E(t)<(mu)over cap>(eff), where <(mu)over cap>(eff) is a n effective coordinate-dependent dipole moment function. We applied th is method to the excitation and dissociation of the triatomic molecule HCO including all three vibrational degrees of freedom. Comparison wi th calculations based on the propagation of a wave packet on a three-d imensional grid yields good agreement, provided the rate of excitation is not too large. The accuracy of classical trajectory calculations i s also tested.