Polarization spectroscopy as a probe of Raman optical activity

Abstract

Polarization-sensitive coherent antistokes Raman spectroscopy and the Raman-induced Kerr effect are considered as possible techniques for investigating Raman optical activity. Generalized nonlinear optical susceptibilities including both magnetic dipole and electric quadrupole interactions are derived semiclassically. These terms can be distinguished from electric dipole terms through their particular symmetry, and several polarization configurations are discussed which seem appropriate to the study of Raman optical activity. These might constitute a convenient alternative to the conventional technique of circular differential Raman scattering for measuring this property.

Nonlinear optical spectroscopy of chiral molecules

“…Nonlinear optical analogs of Raman optical activity (ROA) have also been considered and their theory has been discussed. 93,94 ROA measures a small difference in the vibrational Raman spectrum for left-and rightcircularly polarized incident light. 95,96 Although Raman optical activity is now routinely used to study chiral (bio)molecules in aqueous solution, 95 it is an incoherent spectroscopy and the signals are thus weak.…”

“…Nonlinear Raman spectroscopies could be of value in the study of vibrational optical activity, as they make it possible to probe Raman resonances through coherent rather than incoherent scattering. 94 The signal strength in nonlinear Raman spectroscopies, such as coherent anti-Stokes Raman scattering (CARS) and Raman-induced Kerr effect spectroscopy (RIKES), is therefore typically at least 3 orders of magnitude larger than in spontaneous Raman scattering. 97 Nonlinear Raman spectroscopy should make it possible to eliminate interference from fluorescence, and to obtain previously unavailable temporal information about a molecule's vibrations.…”

“…Nonlinear Raman optical activity is described by nonlocal third-order susceptibilities of the form eeem (N 1 À N 2 + N 1 ). 93,94 In CARS, the laser at N 2 is tuned, and when the difference in angular frequencies N 1 À N 2 is proportional to a vibrational energy h -N vib , a resonantly enhanced signal is observed at N 1 + N vib . To our knowledge, there is only one report of a successful nonlinear Raman optical activity measurement from a chiral liquid to date.…”

Polarization CARS spectroscopy, chiral molecules and magnetic field effects

“…Therefore linear polarization is used in the measurement of CARS-ROA. The theory of CARS-ROA has been worked out recently [37,38] and a detection scheme has been proposed [38]. ROA is predicted to induce a small ellipticity in the CARS beam for strong Raman modes.…”

“…The other two independent tensor elements Xzlll and Xll12 in which the polarizations of the CARS and Stokes beams are perpendicular to each other are practically identical. They represent magnetic dipole and electric quadrupole interactions which should give rise to the natural ROA effect in CARS [38). The depolarization ratio due to the conventional CARS signal XI 2 2 1 /Xl corresponds to the depolarization ratio in spontaneous Raman,…”