Vibrational dynamics of liquids and solids investigated by picosecond light pulses

Abstract

With well-defined coherent light pulses of several ${10}^{\ensuremath{-}12}$ sec duration we are in a position to investigate a variety of ultrafast vibrational processes in liquids and solids. Several new experimental techniques have been devised to study directly the dynamics of different vibrational modes and molecules in the electronic ground state. A first light pulse excites the vibrational system via stimulated Raman scattering or by resonant infrared absorption. A second interrogating pulse allows one to determine the instantaneous state of the excited system. Using a coherent probing technique one can measure the dephasing time of homogeneously broadened vibrational transitions and a collective beating of multiple isotope levels. In addition, one can investigate inhomogeneously broadened vibrational modes and observe the dephasing time of a small molecular subgroup. Different information is obtained when the coherent anti-Stokes Raman scattering of the probe pulse is measured. The population lifetime of known vibrational modes can be investigated and evidence for inter- and intra-molecular interactions is obtained. In a third probing technique, the vibrationally excited jolecules are promoted to the first electronic state by a second pulse and the fluorescence is measured. In this way it is possible to see the very rapid change of population of the primary excited vibrational mode. The article gives a detailed theoretical treatment of different excitation and probing processes. Several experimental techniques successfully applied in the authors investigations are outlined and a variety of results is presented and discussed. New information, not available from other experimental methods, is obtained.