A simple model study of reactive collisions in an intense nonresonant laser field

A simple model study of laser induced transitions between electronic surfaces in reactive molecular collisions has been undertaken. The investigation is characterized by laser and nonadiabatic couplings which are turned on during the course of a collision. Transition probabilities are determined within an exact quantum-mechanical framework, for switching between the model one-dimensional potential curves as a function of various system parameters. Such parameters include the photon energy, the reactant collision energy, and the coordinate separation between the positions of potential barrier maxima. The processes studied involve not only laser switching but, also, cooperative laser and nonadiabatic effects. A number of features of the results are emphasized.

We consider the problem of photon angular momentum approximation for molecular collisions in an intense nonresonant laser field. A couple of orientation averaging schemes are proposed for dealing with the absence of rotational invariance in the full (inclusive of photon angular momentum) dynamical equations. Application of the schemes is made for reactive laser switching between a pair of one-dimensional potential curves which are free to rotate in space. The preferred scheme of the two depends upon there being only a single initial or final state of interest but both are very effective over the range of intensities examined. A third approximation scheme which essentially blots out photon angular momentum effects is less satisfactory.

Low energy reactive transition probabilities for a model multichannel collision problem, are determined within a so-called quasiadiabatic (QA) representation of the system electronic energy. The procedure involves setting up a set of coupled nonreactive surfaces (the QA representation) and then perturbatively mixing coupled-channel wave functions on the QA surfaces. It is applied to a hard-sphere-type model of the collinear A+BC reaction and for a relatively high system mass (5.0×104 a.u.). Optimization of the representation (which we have previously argued should temper maximization of the QA reactivity with a drive for balance between its diabatic and nonadiabatic components) yields results which are in very good agreement with exact ones (errors <10%) over a wide range of collision energies. At the same time, as the collision energy approaches the classical reactive threshold, we see evidence of QA failure; we trace this to difficulties with our particular optimization procedure when the diabatic contribution becomes dominant. ‘‘Conventional’’ perturbative results are generated for the same model problem and found to be poor in general (errors ≂40%–50%). It is demonstrated that the ineffectiveness of the conventional approach may be ascribed to the system’s high mass.