Homogeneous vibrational dynamics and inhomogeneous broadening in glass-forming liquids: Infrared photon echo experiments from room temperature to 10 K

Abstract

A study of the temperature dependence of the homogeneous linewidth and inhomogeneous broadening of a high-frequency vibrational transition of a polyatomic molecule in three molecular glass-forming liquids is presented. Picosecond infrared photon echo and pump–probe experiments were used to examine the dynamics that give rise to the vibrational line shape. The homogeneous vibrational linewidth of the asymmetric CO stretch of tungsten hexacarbonyl (∼1980 cm−1) was measured in 2-methylpentane, 2-methyltetrahydrofuran, and dibutylphthalate from 300 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 T2 power law. The absorption linewidths for all glasses are seen to be massively inhomogeneously broadened at low temperature. In the room temperature liquids, while the vibrational line in 2-methylpentane is homogeneously broadened, the line in dibutylphthalate is still extensively inhomogeneously broadened. The contributions of vibrational pure dephasing, orientational diffusion, and population lifetime to the homogeneous line shape are examined in detail in the 2-methylpentane solvent. The complete temperature dependence of each of the contributions is determined. For this system, the vibrational line varies from inhomogeneously broadened in the glass and low temperature liquid to homogeneously broadened in the room temperature liquid. The homogeneous linewidth is dominated by the vibrational 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 relaxation at temperatures below ∼120 K causes the homogeneous line shape to deviate from Lorentzian, while at higher temperatures the line shape is Lorentzian.