Coupled Equations Approach To Intense Field Molecular Multiphoton Processes

Abstract

Coupled equations approach to intense field molecular multiphoton processesAndre D. BANDRAUK and Nadia GELINASDepartement de chimie, Faculte des sciences, Universite de SherbrookeSherbrooke, Que., Canada J1K 2R1AbstractMolecules in intense fields (I ? 109 W /cm2) cannot be treated by perturbative methods.We have developed nonperturbative methods to calculate multiphoton cross sections in mole-cules based on the coupled equations of quantum collision theory. This enables us toinclude bound and continuum states, and to bridge the weak field (classical spectroscopy)and strong field (laser induced phenomena) cases. In particular, in the case of diatomicmolecules, one can show that strong fields will induce new bound states which can drasti-cally influence photodissociation cross sections. Examples will be presented of calcula-tions of two photon and three photon effects in intense fields where perturbative approa-ches no longer apply.IntroductionMolecules in intense electromagnetic fields are much more complex systems than atoms,since one must now consider the nuclear degrees of freedom. Thus in addition to electronicexcitations or levels, one must also include rotational and vibrational degrees of freedomin any meaningful description of molecules. This of course implies that molecules areintrinsically multilevel systems so that two level descriptions such as used in the dressedatom picture are an oversimplification1,2. We have previously investigated direct molecu-lar photodissociation and photon scattering as a tractable molecular problem where one cancalculate with reasonable accuracy the effect of a strong laser field on such a process3 -5.This was done for the diatomic molecule A4 using ab- initio electronic energypotentials and the corresponding transition moments. It was found that intense fields mo-dify considerably the photophysical processes. Thus at intensities of about 1010 W /cm2 andwith a transition moment u = 2 atomic units (5 Debyes), one obtains an electronic Rabi fre-quency wR = pE/E -