Intra- and inter-molecular modes reflected in femtosecond time-resolved optical heterodyne-detected optical Kerr effect signal of molecules in liquids

Abstract

We present a theoretical work on the femtosecond time-resolved optical heterodyne-detected optical Kerr effect (OHD-OKE) measurement of molecules in liquids to clarify the role of intra- and inter-molecular low-frequency modes. An expression for the signal of the OHD-OKE measurement is derived on the basis of the double space representation. The theory is applied to the system of neat chloroform at room temperature. A Raman active vibrational mode with a frequency of 262cm −1 is considered, together with rotational degrees of freedom or Raman active inter-molecular vibrational modes. All the vibrational modes are assumed to be harmonic. For simplicity, a free and diffusive rotor model is adopted. A quantum mechanical treatment is also used to take into account rovibrational dephasings. The reorientational relaxation is treated in the Debye-Einstein model. Effects of the rovibrational dephasing appear in the OHD-OKE signal in a subpicosecond time regime, but intra-molecular quantum beats disappear in the early time region compared with the experimental signal. The effects of the reorientational relaxation appear in a picosecond time regime. Inclusion of inter-molecular vibrational modes qualitatively explains the OHD-OKE measurement, although calculations by the present model are only approximate.