Femtosecond Time-Resolved CARS Spectroscopy on Binary Gas-Phase Mixtures:  A Theoretical and Experimental Study of the Benzene/Toluene System

Femtosecond time-resolved CARS measurements on benzene, toluene, and binary mixtures of these molecules have been performed in the gas phase. Whereas a two-pulse excitation process prepares benzene in a single vibrational state, a vibrational wave packet is excited in toluene, which gives rise to oscillatory patterns in the CARS signal. For the mixed system one finds beats between modes coherently excited in both molecules. The results are analyzed using perturbative quantum calculations. It is shown that the rotational degree of freedom is responsible for the decay of the CARS signal in all cases. Furthermore, it is demonstrated that the details of the excitation process have to be incorporated in the calculation to obtain the correct amplitudes of the oscillating signals.