LIGHT QUENCHING AND DEPOLARIZATION OF FLUORESCENCE OBSERVED WITH LASER-PULSES - A NEW EXPERIMENTAL OPPORTUNITY IN TIME-RESOLVED FLUORESCENCE SPECTROSCOPY

We report the first time-resolved studies of quenching of fluorescence by light, i.e., ''light quenching''. The dye yanomethylene)-2-methyl- 6-(p-dimethamino)4H-pyrane (DCM) was excited in the anti-Stokes region from 560-600 nm. At high illumination power the intensities of DCM we re sub-linear with incident power. The extent of light quenching was p roportional to the emission spectrum at the incident wavelength, as ex pected for light-stimulated decay from the excited state. The frequenc y-domain intensity decays indicated the effect was not due to heating or other photochemical effects. Importantly, the decay time was unchan ged, as expected for light quenching with a single pulsed laser beam, while the time-zero anisotropy was decreased due to orientation-depend ent quenching of the excited state population. Light quenching of fluo rescence provides a new method to control the excited state population and orientation of fluorophores, and offers new experimental opportun ities for biophysical applications of time-resolved fluorescence.