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. Morillo, Phys. Rev. B 57, 6972 (1998), M. Morillo and R. I. Cukier, J. Chem. Phys. (110, 7966 (1999)] is generalized to apply to strong solute-solvent coupling. In SFS, a strong external field is used to connect, with the transition dipole, two electronic states of a solute immersed in a medium. In contrast to weak fields, (z) over bar(t), the average population difference of the solute electronic states is changing significantly. For resonant, strong 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 solute-solvent coupling is relatively weak, the time evolution of the solvent only depends on the initial solute state (autonomous behavior). In this work, appropriate to strong coupling, we derive an equation of motion for the solvent dynamics that depends on the solute's instantaneous state (nonautonomous behavior). The consequences to (z) over bar(t) and (P) over bar(t) are explored. We find that instead of equalizing the solute populations at long times, now the population is inverted relative to its initial state. We also find that the degree of long-time population inversion can be controlled by turning off the external field before the system has fully relaxed. (C) 1999 American Institute of Physics. [S0021-9606(99)50936-7].