Observation of a bottleneck in the vibrational relaxation of liquid bromoform Original Research Article

We present a study on the vibrational dynamics of the C–H stretch vibration of bromoform (CHBr3) in the pure liquid using picosecond mid-infrared pump–probe spectroscopy. Delay scans and transient spectra confirmed earlier results that relaxation occurs via an intermediate level, which causes a transient red shift of the C–H stretch frequency. We determined this red shift to be 17 cm−1 and identified the intermediate level with the singly excited C–H bend mode. We observed that relaxation from the intermediate level is exceptionally slow and non-exponential. These observations suggest the presence of a “bottleneck” in the relaxation. The occupation of a long-living “bottleneck” level may also be responsible for the observation of a long-living absorption around 3022 cm−1.