The effect of a strong external field on the electronic dephasing of a solute that is strongly coupled to a solvent

A recent theory of strong field spectroscopy (SFS) [R. I. Cukier and M. Mor illo, Phys. Rev. B 57, 6972 (1998), M. Morillo and R. I. Cukier, J. Chem. P hys. (110, 7966 (1999)] is generalized to apply to strong solute-solvent co upling. In SFS, a strong external field is used to connect, with the transi tion dipole, two electronic states of a solute immersed in a medium. In con trast to weak fields, (z) over bar(t), the average population difference of the solute electronic states is changing significantly. For resonant, stro ng fields, (z) over bar(t) and the average absorbed power, (P) over bar(t), exhibit oscillatory decays in time that reflect the changing (z) over bar( t) and the dissipation arising from the coupling to the medium. When the so lute-solvent coupling is relatively weak, the time evolution of the solvent only depends on the initial solute state (autonomous behavior). In this wo rk, appropriate to strong coupling, we derive an equation of motion for the solvent dynamics that depends on the solute's instantaneous state (nonauto nomous behavior). The consequences to (z) over bar(t) and (P) over bar(t) a re explored. We find that instead of equalizing the solute populations at l ong times, now the population is inverted relative to its initial state. We also find that the degree of long-time population inversion can be control led by turning off the external field before the system has fully relaxed. (C) 1999 American Institute of Physics. [S0021-9606(99)50936-7].