Vibrational spectral diffusion and population dynamics in a glass-forming liquid: Variable bandwidth picosecond infrared spectroscopy

The temperature-dependent vibrational population dynamics and spectral diffusion of the CO stretching mode of tungsten hexacarbonyl in 2-methylpentane are observed from the room temperature liquid to the low temperature glass using picosecond infrared transient grating and pump–probe experiments. These experiments were performed between 10 and 300 K on the triply degenerate T1u asymmetric CO stretching mode at 1984 cm−1 using pulses with bandwidths narrower and wider than the absorption bandwidth of the transition. The rate of vibrational population relaxation (100≤T1<150 ps) is observed to decrease with increasing temperature. The orientational dynamics for this transition are observed on a faster time scale than the population relaxation. Although the liquid viscosity changes over 14 orders of magnitude, the orientational relaxation rate slows by less than one order of magnitude over the full temperature range. By comparing polarization-dependent experiments performed with both narrow and broad bandwidth transform-limited pulses, it is possible to measure temperature-dependent spectral diffusion in both the liquid and the glass. The spectral diffusion and the orientational relaxation are shown to be intimately related. It is proposed that both arise from the time evolution of the superposition of the three degenerate states created by the excitation pulse.