STIMULATED-EMISSION CROSS-SECTIONS IN FLUORESCENT DYE SOLUTIONS - GAIN SPECTRA AND EXCITED-STATE LIFETIMES OF NILE BLUE-A AND OXAZINE-720

Gain spectra of two laser dyes, Nile blue A and oxazine 720, have been recorded with the pump-probe spectroscopic method using a modulated-pump-unmodulated-probe method of high sensitivity. By locating the pump laser within the region of sample absorption with the probe set to shorter wavelengths, where fluorescence is negligible, it is shown how it is possible to determine the absorption cross-section free of the influence of stimulated emission. By contrast, by locating the pump laser away from the region of fluorescence emission but locating the probe laser in those regions where absorption and fluorescence can both occur, it is shown that the gain signal contains contributions from both of these processes. By combining the two sets of measurements, it is shown how both absorption and stimulated emission cross-sections can be separately determined. Measurement of the integrated areas shows that within the experimental error the areas under the spectral profiles of the absorption and stimulated emission cross-sections are identical. Temporal scans of the gain have been performed for both samples in order to determine excited-state lifetimes and orientational relaxation times. These lifetimes have been shown to vary significantly in the different solvents investigated and explanations for this behaviour have been proposed.