Time-domain vibrational spectroscopy of disordered media

Abstract

Abstract Results of femtosecond-microsecond time-domain spectroscopy experiments on vibrational modes of MHz-THz frequencies in disordered media are described. In simple molecular liquids, weakly oscillatory librational motion can be observed on the femtosecond time scale. Librational frequencies and inhomogeneous dephasing rates yield configuration-averaged intermolecular forces and the extent of inhomogeneity in the liquid. The temperature and pressure dependences of librational dynamics provide information about spectral diffusion rates which are associated with rearrangement of local liquid-state structure. The results support an “inhomogeneous cage” model of simple liquids in which local cage structure persists for at least several molecular collision times. Structural relaxation dynamics have also been examined in detail in glass-forming liquids. Extensive information about the complex, temperature-dependent distributions of relaxation times in viscoelastic liquids has been obtained through time-domain observations of MHz-GHz acoustic waves and of the Mountain mode associated with structural relaxation. Recently, it has become possible to observe shear as well as longitudinal acoustic waves, and to examine the onset of shear wave propagation at various frequencies. Finally, a multiple-pulse vibrational amplification technique exploiting recent developments in femtosecond pulse-shaping is described.