Band shapes in CARS: Background effects and overlapping resonances

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Abstract
Quantitative intensity measurements are reported for the CARS spectra of the symmetric stretch (ν3) mode in liquid CH3I, the totally symmetric stretch (ν1) in liquid CCl4 and the Q branch for the vibration mode of gaseous N2 at a series of densities. Calculations of the observed band shapes were carried out for both polarized and depolarized bands of CH3I, for the band in CCl4 and for the N2 Q branch at the three lowest densities. Good agreement was obtained after the calculated band shapes were adjusted by varying the values chosen for the nonresonant susceptibility, and for the relaxation times of the vibrational normal coordinates and the reorientation (for the depolarized band). Relaxation times for the liquids and collision broadening factors for the gases are listed; in most cases, the values obtained are more accurate than the corresponding numbers from spontaneous Raman spectra. The problems involved in calculating CARS intensities in the presence of nonresonant susceptibility and overlapping resonances are discussed.