Optical control of molecular dynamics in a liquid

We report the results of a study of the influence of solvent fluctuations on the efficiency of selective population transfer from an initial state to a designated target state of a solute molecule. Our model of the influence of liquid fluctuations on the states of the solute assumes that dephasing is the dominant relaxation process, and utilizes an analog of the Kubo stochastic theory of line shape. The solvent fluctuations are represented as a Gaussian random process that independently modulates each of the energy levels of the solute molecule. For typical liquid densities the maximum amplitude of these fluctuations is taken to be of the order of 150 cm(-1), and the correlation time of the fluctuations is taken to be of the order of a few hundred femtoseconds, but we have also explored the effects of varying the fluctuation frequency and correlation time. It is shown that STIRAP (stimulated Raman adiabatic passage) generated population transfer to a designated target state of the solute remains efficient when the frequency of the solvent fluctuations is large or small relative to the inverse of the widths of the pump and Stokes pulses. It is further shown that extended STIRAP generated selective transfer to one of a pair of degenerate states of the solute remains efficient under the same conditions. These results suggest, subject to the accuracy of the representation of the influence of the solvent on the solute, that it should be possible, using coherent superpositions of states generated with picosecond excitation, to control population transfer, hence reactivity, for a class of reactions carried out in the liquid phase. (C) 2002 American Institute of Physics.