OBSERVATION OF LOW-FREQUENCY RAMAN MODES IN SEVERAL HALOGENATED METHANES BY THE OPTICAL KERR EFFECT

The time-resolved optical Kerr effect was used to probe the ultrafast relaxation of four halogenated methanes, CHCl3, CBrCl3, CHBr3 and CFBr3. A 65 fs coherent laser pulse from a synchronously pumped antiresonant ring dye laser with Fourier components ranging over 300 cm-1 is separated into a pump and a probe beam, aligned for heterodyne detection of the Kerr signal. The present paper focuses on the intramolecular vibrational resonant component of the relaxation curve, corresponding to low-frequency (> 300 cm-1) Raman modes of the halogenated methanes that are coherently excited by the spectrally broad laser pulse. It is demonstrated that some symmetric modes, as well as asymmetric modes, are detectable by the optical Kerr effect, the probability of this depending on the depolarization ratio of the mode, and hence its anisotropy.