Theory of Collisionless Molecular Multiphoton Processes

Abstract

We derive reduced equations of motion (REM) which describe the dynamics of polyatomic molecules in the presence of strong infrared lasers. The derivation is made starting from the complete molecular Liouville equation and making use of the Zwanzig-Mori projection operator formalism. The complete molecular information relevant for the dynamics of molecular multiphoton processes (MMP) is expressed in terms of a hierarchy of intramolecular dipole correlation functions. We show how by invoking simple statistical assumptions (the random phase approximation and separation of time scales) this information is considerably reduced to essentially four quantities per transition: An integrated Rabi frequency\(\overline \Omega \) nm, a dephasing rate Γnm, a detuning \(\overline \omega \) nm and ratios of statistical weights of the levels dn/dm. Depending on our choice of reduction scheme we may obtain either generalized Bloch equations or simple rate equations. The interrelation between the two and their limits of validity are precisely specified. The present formulation enables us to describe the dynamics of MMP all the way from “region I” via the quasicontinuum and up to the dissociation. Finally we present novel spectroscopic results done in supersonic beams which provide us directly with the intramolecular dephasing rates which are the key dynamical quantities in MMP.