Polarization-Sensitive CARS of Excited-State Rhodamine 6G: Induced Anisotropy Effects on Depolarization Ratios

Abstract

Resonance polarization-sensitive coherent anti-Stokes Raman scattering (PS CARS) spectra of the electronic ground state and excited singlet S₁ state of rhodamine 6G in ethanol were obtained with the use of the pump-probe technique with nanosecond time resolution. Variation of the polarization orientation of the pump laser beam showed differences in the excited-state spectra due to optically induced anisotropy. The pure electronic susceptibility of ground-state rhodamine 6G was shown to be small in comparison with nonresonant susceptibility of the solvent, and was neglected in further analyses. The pure electronic susceptibility of excited rhodamine 6G was examined by coherent ellipsometry. The complex third-order susceptibility was analyzed by means of a nonlinear least-squares fit program that provides detailed information on the Raman vibration parameters, including depolarization ratios and phases. In the isotropic case the measured depolarization ratios are close to 1/3, whereas in the anisotropic case, ground-state depolarization ratios are 0.5–0.65 and in the excited state 0.17–0.22. Estimated depolarization ratio changes in ground-state and excited-state rhodamine 6G are in agreement with theoretically predicted values in the case of induced anisotropy under the assumption of parallel dipole moments of the CARS process. The effects of possible changed molecular structure or symmetry and changed enhancement of different electronic transitions cannot be determined without making some assumptions about one of these effects. The obtained phase differences reflect different enhancements and vibronic coupling for ground-state and excited-state vibrations. The ground-state and excited-state hyperpolarizabilities, , of rhodamine 6G were estimated to be 3.8·10⁻³⁵ esu and 27.4·10⁻³⁵ esu, respectively.