Abstract

We have recently shown in a series of papers [1-7] that coupled equations methods of quantum collision theory can be conveniently used to investigate processes such as direct photodissociation [1-2], Resonance Raman Scattering in weak and strong fields [3-5], and higher order nonlinear spectroscopies [6-7] for diatomics involving several well isolated excited electronic states. This was done using the dressed molecule picture [8-12] of molecule-radiation interaction, wherein photon states are explicitly included into the theoretical description. All these methods are adequate only in the case of well isolated electronic states. There is an urgent need to derive a priori the most efficient representation for general electron-nuclear-radiation field systems such as occurs in strong field laser chemistry. In recent work we have examined this problem in an effort to incorporate as much as possible the electromagnetic field into the dynamics [13-14]. One might surmise that classical approaches should work sufficiently well at the high field intensities described here, and much work has been pursued in that direction. As we have pointed out previously, this involves treating both the molecule and the field classically. For electron-radiation interactions one would prefer a quantum formulation, since electronic states are, as a result of their large excitation energies, true quantum states. Furthermore, quantum mechanics leads to a linear theory of interactions whereas classical mechanics is a highly nonlinear theory [15]. Thus using a quantum formulation of matter- field interactions [16-17], we have been able to exploit methods of early (non-covariant) quantum electrodynamics (QED). In particular we have shown that the Bloch-Nordsieck (BN) representation [18-20] (which leads naturally to the concept of coherent states in momentum space was very convenient as a method of introducing strong field effects directly into the quantum dynamics of molecular systems. Coulomb gauge $\text{(}\overrightarrow{\text{A}}\text{.}\overrightarrow{\text{p}}\text{)}$ and Electric Field Gauge $\text{(}\overrightarrow{\text{E}}\text{.}\overrightarrow{\text{r}}\text{)}$ representations were shown to be poor zeroth order approximations for the dressed molecular eigenstates in the presence of strong fields, i.e., the adiabatic coupled equations for the latter two gauges define the appropriate adiabatic state as unperturbed (zero field) molecular states [13-14].

© 1986 Optical Society of America