LIGHT QUENCHING AND FLUORESCENCE DEPOLARIZATION OF RHODAMINE-B AND APPLICATIONS OF THIS PHENOMENON TO BIOPHYSICS

The fluorescence intensity of rhodamine B (RhB) was found to display a sublinear dependence on incident power when excited with the focused output of a cavity-dumped dye laser. This effect was found to be propo rtional to the amplitude of the emission spectrum at the incident wave length and to be associated with a decrease in the time-zero anisotrop y of RhB. The absence of changes in the intensity decay law or rotatio nal correlation time indicates the absence of photochemical processes. These results are consistent with ''light quenching'' of RhB due to s timulated emission. In viscous solution the extent of depolarization o f the emission was found to be in agreement with theoretical expressio ns which account for photoselective light quenching and for spatial in homogeneities in the incident laser beam. The phenomenon of light quen ching has numerous potential applications in biophysics, such as studi es of the orientation and dynamics of fluorescent macromolecules.