INFRARED VIBRATIONAL PHOTON-ECHO EXPERIMENTS IN LIQUIDS AND GLASSES

The vibrational dynamics of polyatomic solutes in polyatomic liquid an d glassy solvents are examined using picosecond infrared photon echo e xperiments and pump-probe experiments from room temperature to 10 K. T he photon echo experiments measure T-2, the homogeneous dephasing time (homogeneous line shape), while the pump-probe experiments measure th e vibrational lifetime, T-1, and the orientational relaxation dynamics . By combining these measurements, a complete analysis of vibrational dynamics is obtained in the liquid, in the supercooled liquid, through the glass transition, and in the glass. Experiments were conducted on the asymmetric CO stretching mode of tungsten hexacarbonyl (similar t o 1980 cm(-1)) in 2-methylpentane (2-MP), 2-methyltetrahydrofuran, dib utyl phthalate (DSP), carbon tetrachloride, and chloroform. The experi ments were conducted using the picosecond IR pulses from a superconduc ting-accelerator-pumped free electron laser. The absorption line width s for all glasses are massively inhomogeneously broadened at low tempe rature. In the room temperature liquids, while the vibrational line in 2-MP is homogeneously broadened, the line in DBP is still extensively inhomogeneously broadened. The temperature dependences of the homogen eous line widths in the three glasses area T-2 power law. The contribu tions of vibrational pure dephasing, orientational relaxation, and pop ulation lifetime to the homogeneous line shape are examined in detail in the 2-MP solvent. The complete temperature dependence of each of th e contributions is determined. In addition; the temperature dependence of T-1 is observed to be ''inverted'' in most of the solvents; i.e., the lifetime becomes longer as the temperature is increased. Analysis shows that this is caused by temperature dependence of the anharmonic coupling matrix elements.