ONE-COLOR AND 2-COLOR POLARIZATION EFFECTS INDUCED IN EMISSION AND ABSORPTION-SPECTRA OF MONOMERIC AND DIMERIC CHLOROPHYLL A IN SOLUTION

The results of polarization measurements are reported for a chlorophyll α dimer in solution. The experiments were conducted with a single laser beam (depolarization measurements) and two laser beams. In the latter case, the wavelength of one of the lasers is tuned to an absorption band of one of the constituent molecules of the dimer, that of the other is tuned to the absorption band of the other molecule. One of these beams produces effective population saturation of an excited state of one of the molecules of the dimer which has an appropriate transition moment parallel to the electric vector of the pump beam. The other, but weaker,laser beam probes either the resonance Raman or the absorption spectrum of the other molecule of the loosely bonded dimer. As a direct result of the slow rotational relaxation time of the dimer in comparison to the duration of the laser pulse, an anisotropy is created in the absorption as well as the resonance Raman spectrum probed with the weaker laser beam. The anisotropy measurements yield information on the direction of the principal axes of polarizability and transition moments of the constituent molecules of the dimer. In turn, this information can be used to predict structural properties of the dimer. © 1990 American Institute of Physics.