Laser-induced molecular orientation effects in vibrational resonance-Raman spectroscopy

In most studies of Raman scattering in fluids, the electric field associated with the laser light cannot effect any significant degree of molecular orientation since the induced dipole moments are small. Under resonance conditions however, the induced moment can be increased a thousand-fold, and at laser irradiances of 1012−1015 W m−2 (strong-field resonance Raman), the coupling with the electric field of the laser beam can lead to significant orientational effects. In addition, sizeable orientational effects should be observable in colloids and polymers at much lower laser irradiances of only 105−107 W m−2. In these systems it is necessary to calculate the effect of the induced bulk anisotropy on the Raman spectra. In this paper, the appropriate Boltzmann-weighted ensemble average is calculated using a non-trigonometric procedure based on irreducible Cartesian tensor methods, and it is shown how for particular beam configurations the induced anisotropy relaxes certain symmetry restrictions normally imposed on a system of randomly oriented molecules. The results obtained are general in that no restriction is placed on molecular symmetry. In some cases the effect significantly modifies the depolarization ratios and contributes to the intensity enhancement. In addition, it is demonstrated how the same calculational procedure can be used to derive results for the changes in refractive index associated with the optical Kerr effect.