A comparative study of the collective polarizability anisotropy dynamics of benzene, 1,3,5-trifluorobenzene, and hexafluorobenzene was carried out by using optical heterodyne-detected Raman-induced Kerr effect spectroscopy (OHD-RIKES) with 45 fs laser pulses. The OHD-RIKES data were analyzed by using a modeldependent approach and a Fourier transform approach, which yields a spectral density for the liquid. From an analysis of the long-time tails, collective reorientation times of 2.45 ( 0.06, 9.05 ( 0.10, and 13.5 ( 0.1 ps were obtained respectively for C 6 H 6 , 1,3,5-C 6 F 3 H 3 , and C 6 F 6 . The spectral densities are narrower for 1,3,5-C 6 F 3 H 3 and C 6 F 6 than for C 6 H 6 by roughly a factor of 2. Information about the intermolecular vibrational modes of the liquid is contained in the reduced spectral density which is obtained by subtracting the tailmatched diffusive reorientational response from the OHD-RIKES data. In the case of C 6 H 6 and C 6 F 6 , the intermolecular spectral density can be decomposed into at least two broad overlapping bands. In contrast, the intermolecular spectral density for 1,3,5-C 6 F 3 H 3 is characterized by a single band. These spectra are rationalized in terms of the librational dynamics of perpendicular dimers in liquid C 6 H 6 and C 6 F 6 and parallel dimers in liquid 1,3,5-C 6 F 3 H 3 .

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We measure the diffuse reflection spectrum of solid samples such as explosives (TNT, RDX, PETN), fertilizers (ammonium nitrate, urea), and paints (automotive and military grade) at a stand-off distance of 5 m using a mid-infrared supercontinuum light source with 3.9 W average output power. The output spectrum extends from 750-4300 nm, and it is generated by nonlinear spectral broadening in a 9 m long fluoride fiber pumped by high peak power pulses from a dual-stage erbium-ytterbium fiber amplifier operating at 1543 nm. The samples are distinguished using unique spectral signatures that are attributed to the molecular vibrations of the constituents. Signal-to-noise ratio (SNR) calculations demonstrate the feasibility of increasing the stand-off distance from 5 to ~150 m, with a corresponding drop in SNR from 28 to 10 dB.

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 CH 3 I. 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 ω c ≈ 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 CH 3 I.