It is now routinely possible to produce optical pulses whose duration is short compared to the time required for nearly any motion of atoms or molecules, i.e. collective vibrations in condensed materials, molecular rotations, and many molecular vibrations. This makes time-resolved observation of these motions possible in principle. The other requirement is a mechanism through which ultrashort pulses can initiate and monitor collective or molecular motion. We have reviewed the mechanisms discovered and exploited to date, primarily impulsive stimulated scattering and impulsive absorption, for initiation of phase-coherent motion in electronic ground and excited states, respectively. The conceptual and theoretical underpinnings for the excitation mechanisms and the different ways in which phase-coherent motion can be monitored were presented. Spectroscopic applications of impulsive stimulated scattering and impulsive absorption were reviewed. The initial efforts in each case were aimed at demonstration and understanding of the excitation mechanisms. More recently, a substantial body of results which were not obtainable through other spectroscopic means has been amassed. Through impulsive stimulated scattering, information about the dynamics of structural phase transitions, molecular liquids, and vibrational relaxation of chemical reaction products has been extracted. Impulsive absorption has provided information about electron-phonon interactions in a variety of crystalline solids and has made possible the entire area of “femtochemistry” which now extends throughout gas, liquid, and solid phases and includes photobiological materials. Finally, multiple-pulse femtosecond spectroscopy at the impulsive limit was reviewed. This is a relatively new area but one whose accessibility to broad use and whose prospects are growing rapidly. Just as the availability of ultrashort pulses led to dramatic advances in our capabilities for observation of elementary collective and molecular motions, the availability of tailored ultrafast waveforms may be expected to herald similar advances in our level of control over these motions and over the chemical and structural changes resulting from them.
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