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 reactive processes in condensed phases are directly probed, following an optical excitation. A voltage-biased semiconductor wafer, excited with a 70 femtosecond laser pulse, is employed as a source for intense terahertz pulses, which are used as the probe arm of a pump-probe experiment. Difficulties in using terahertz probe pulses are discussed. We then demonstrate the method with two examples. In the first, a terahertz signal is shown to follow the solvent response to charge transfer in betaine. In the second example, relaxation of sapphire lattice phonons, excited impulsively by a 405 nm pulse, is observed.