Femtosecond Optical Kerr Effect Studies of Liquid Methyl Iodide

The collective polarizability anisotropy dynamics of liquid methyl iodide at room temperature and ambient pressure was studied by using optical heterodyne-detected Raman-induced Kerr effect spectroscopy (OHD-RIKES) with 45 fs laser pulses. The OHD-RIKES data are analyzed by using both the model-dependent approach, which assumes four distinct temporal responses, and the model-independent Fourier transform approach, which generates a spectral density. Near zero time, the OHD-RIKES transient is dominated by the instantaneous electronic response. The short-time nuclear response is characterized by two components. The first component is interpreted as arising from an inhomogeneously broadened (fwhm ≈ 62 cm-1) underdamped intermolecular vibrational mode with a mean frequency of ∼60 cm-1. The second component is an intermediate quasi-exponential response with a 1/e time constant of ∼200 fs. At longer times, the OHD-RIKES transient decays exponentially with a 1/e time constant of 1.76 ± 0.05 ps, which corresponds to the collective reorientation time of CH3I. The spectral density peaks at ∼24 cm-1 and has a fwhm of ∼80 cm-1. The spectral density can be well fitted by an ohmic distribution function with ωc ≈ 30 cm-1. The spectral density obtained from the OHD-RIKES data is consistent with previously measured depolarized Rayleigh scattering and low-frequency far infrared absorption spectra for liquid CH3I.