FEMTOSECOND OPTICAL KERR-EFFECT STUDIES OF LIQUID METHYL-IODIDE

The collective polarizability anisotropy dynamics of liquid methyl iod ide at room temperature and ambient pressure was studied by using opti cal heterodyne-detected Raman-induced Kerr effect spectroscopy (OHD-RI KES) with 45 fs laser pulses. The OHD-RIKES data are analyzed by using both the model-dependent approach, which assumes four distinct tempor al responses, and the model-independent Fourier transform approach, wh ich generates a spectral density. Near zero time, the OHD-RIKES transi ent is dominated by the instantaneous electronic response. The short-t ime nuclear response is characterized by two components. The first com ponent is interpreted as arising from an inhomogeneously broadened (fw hm approximate to 62 cm(-1)) underdamped intermolecular vibrational mo de with a mean frequency of similar to 60 cm(-1). The second component is an intermediate quasi-exponential response with a 1/e time constan t of similar to 200 fs. At longer times, the OHD-RIKES transient decay s exponentially with a lie time constant of 1.76 +/- 0.05 ps, which co rresponds to the collective reorientation time of CH3I. The spectral d ensity peaks at similar to 24 cm(-1) and has a fwhm of similar to 80 c m(-1). The spectral density can be well. fitted by an ohmic distributi on function with omega(c) approximate to 30 cm(-1). The spectral densi ty obtained from the OHD-RIKES data is consistent with previously meas ured depolarized Rayleigh scattering and low-frequency far infrared ab sorption spectra for liquid CH3I.