Experimental determination of the frequency spectrum of the rose Bengal picosecond fluorescence using two beating ring dye lasers

Abstract

In this thesis two ring dye lasers have been used to achieve amplitude modulated light of variable frequency. The amplitude modulated light was used to study the frequency response of an 800 ps risetime photomultiplier against different parameters. The photomultiplier was found not to have a frequency response dependence on intensity of incident light, photocathode illumination shape, laser wavelength (in the range 570-593 nm), and polarization of incident laser beam. The higher frequency components of the response were enhanced with increasing photomultiplier operating voltage. The amplitude modulated light was also used to perform demodulation spectroscopy on Rose Bengal. By measuring the demodulation in the fluorescence signal, the fluorescence lifetime of Rose Bengal in pure ethanol was found to be 740 … 4 ps. The lifetime was then determined in mixtures of water and ethanol. The fluorescence lifetime of Rose Bengal was found to monotonically decrease with increasing water concentration, and was found to be 80 … 22 ps in pure water. The decay rates of Rose Bengal fluorescence showed a nonlinear dependence on water concentration. This could be explained by a water cluster model.