Abstract

The technique of Raman echo's in a vibrational degree of freedom of the nitrogen molecule is shortly described. The essential of this experiment is a phase-matching for a frequency box CARS configuration for (i) the two pumping as rephasing laser frequencies, and (ii) the probe laser pulse and the coherent anti-Stokes scattering, in such a way that there is also a definite phase-matching for pumping and rephasing pulse as well as for the probe pulse and the anti-Stokes pulse. This prohibits a three frequency CARS generation influencing the population during the rephasing pulse and gives a sensitive alignment (by four frequency box CARS) for the echo experiment. The Raman echo has been applied to a binary mixture of the van der Waals liquids nitrogen and argon. The absence of an echo in pure nitrogen and pure argon indicates an inhomogeneous broadening of the transition due to the many nearest neighbour configurations surrounding the reference molecule which, due to the difference in intermolecular interactions, give different force fields. The results are compared with molecular dynamics calculations. The nonexponential decay of the echo shows that in contrast to the high frequency component of the fast rotation reorientational motion, also a low frequency component, due to diffusive processes changing slowly the composition and configuration in the first two solvatation layers, is present.

© 1986 Optical Society of America