THEORY FOR LASER-INDUCED DICHROISM AND ITS EFFECT ON ABSORPTION, FLUORESCENCE, AND RAMAN-SPECTRA

The theory is presented for a two beam pump and probe laser experiment where a strong pumping beam induces a saturated anisotropy in the distribution of ground and excited state molecules in solution. The probing laser beam is (i) tuned to another transition of the molecule, or (ii) used to induce nonresonant ground state Raman scattering. Formulas are given for the laser induced anisotropic absorption for the general case, where the directions of transition moments of the molecule for the absorption of pump and probe light form an angle anywhere between 0-degrees and 90-degrees. Also, a comparison of the results to the existing theory for polarized fluorescence is made for weak intensity of the excitation source. Expressions are formulated for the polarization properties of the intensities of Raman bands modified by the presence of pumping light. The creation of the spatial anisotropy by the pump results in a Raman photoselection rule which permits one to distinguish asymmetric vibrational modes belonging to different representations of the same point group.