FEMTOSECOND FAR-INFRARED PUMP-PROBE SPECTROSCOPY - A NEW TOOL FOR STUDYING LOW-FREQUENCY VIBRATIONAL DYNAMICS IN MOLECULAR CONDENSED PHASES

Low-frequency (terahertz) motions that play an important part in react ive processes in condensed phases are directly probed, following an op tical excitation. A voltage-biased semiconductor wafer, excited with a 70 femtosecond laser pulse, is employed as a source for intense terah ertz pulses, which are used as the probe arm of a pump-probe experimen t. Difficulties in using terahertz probe pulses are discussed. We then demonstrate the method with two examples. In the first, a terahertz s ignal is shown to follow the solvent response to charge transfer in be taine. In the second example, relaxation of sapphire lattice phonons, excited impulsively by a 405 nm pulse, is observed. (C) 1997 Published by Elsevier Science B.V.