Observation of intensity dependent, non-exponential vibrational relaxation in liquid bromoform

The intensity dependence of the vibrational relaxation in liquid bromoform after excitation of the CH stretching vibration was investigated using a picosecond infrared double resonance spectrometer. Varying the excitation energy density in the range from 2% to 50% of the saturation energy density a 20% decrease of the measured decay time with increasing pump intensity is observed. Based on the assumption of binary interactions, this dependence can be explained even quantitatively by the increased relaxation probability due to bottlenecked population on low-lying vibrational modes of CHBr3.