General method for separation of the nuclear and electronic contributions to femtosecond four-wave mixing transients

Abstract

We describe a novel, time-domain, Fourier-transform technique that separates uniquely the nuclear and (nonresonant) electronic contributions to x, that is generally applicable to most femtosecond four-wave mixing experimentson transparent media when optical heterodyn detection techniques are used. This separation is obtained with no curve-fitting or pulse-shape approximations, and reveals directly the impulse response function for nuclear motion with no assumed models being necessary. The general method, which we refer to as Fourier-transform four-wave mixing (FT 4WM) spectroscopy, can be applied to two-beam (stimulated Raman gain, optical Kerr effect) or three-beam (transient grating) femtosecond experiments for any polarization configuration and thus, determine directly the relative magnitude and dynamics of the nonresonant electronic and nuclear contributions for each of the different nonvanishing x matrix elements. Results will be presented for a variety of molecular liquids, illustrating the general utility of the FT 4WM method for investigation of the complex dynamics of molecular motion in condensed-phase media.