Ultrafast optical spectroscopy using laser pulses as short as 6 femtoseconds provides a novel probe for molecular nuclear motions ( l , 2). Since the duration of such pulses is comparable to or shorter than typical molecular vibrational periods ( 1 0 fsec is the period of a 3300 cm -1 vibration), these pulses make it possible to probe molecular vibrations and elementary photophysical and photochemical events in real time. Femtosecond dynamics and relaxation studies have been carried out in solutions (39a,b), neat liquids ( 1 01 3), conjugated polymers ( 1 41 5), proteins and biological systems ( 1 6-1 7), crystals (I 8a-c) , surfaces ( 1 9a,b), semi­ conductors (20-21 ), molecular aggregates (22-23), the hydrated electron (24a-c), isolated molecules in supersonic beams (25-26), and the gas phase (27a,b). Femtosecond nonlinear optical measurements have numerous unique advantages. Consider, for example, the simplest ultrafast spectro­ scopic technique: pump-probe spectroscopy (4-6, 25a-c). Tn this technique the system is subjected to two short pulses separated by a time delay r. The first pulse (the pump pulse) has a frequency OJ 1 and the absorption of

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