HOMOGENEOUS VIBRATIONAL DYNAMICS AND INHOMOGENEOUS BROADENING IN GLASS-FORMING LIQUIDS - INFRARED PHOTON-ECHO EXPERIMENTS FROM ROOM-TEMPERATURE TO 10 K

A study of the temperature dependence of the homogeneous linewidth and inhomogeneous broadening of a high-frequency vibrational transition o f a polyatomic molecule in three molecular glass-forming liquids is pr esented. Picosecond infrared photon echo and pump-probe experiments we re used to examine the dynamics that give rise to the vibrational line shape. The homogeneous vibrational linewidth of the asymmetric CO str etch of tungsten hexacarbonyl (similar to 1980 cm(-1)) was measured in 2-methylpentane, 2-methyltetrahydrofuran, and dibutylphthalate from 3 00 K, through the supercooled liquids and glass transitions, to 10 K. The temperature dependences of the homogeneous linewidths in the three glasses are all well described by a T-2 power law. The absorption lin ewidths for all glasses are seen to be massively inhomogeneously broad ened at low temperature. In the room temperature liquids, while the vi brational line in 2-methylpentane is homogeneously broadened, the line in dibutylphthalate is still extensively inhomogeneously broadened. T he contributions of vibrational pure dephasing, orientational diffusio n, and population lifetime to the homogeneous line shape are examined in detail in the 2-methylpentane solvent. The complete temperature dep endence of each of the contributions is determined. For this system, t he vibrational line varies from inhomogeneously broadened in the glass and low temperature liquid to homogeneously broadened in the room tem perature liquid. The homogeneous linewidth is dominated by the vibrati onal lifetime at low temperatures and by pure dephasing in the liquid. The orientational relaxation contribution to the line is significant at some temperatures but never dominant. Restricted orientational rela xation at temperatures below similar to 120 K causes the homogeneous l ine shape to deviate from Lorentzian, while at higher temperatures the line shape is Lorentzian. (C) 1995 American Institute of Physics.