Spectra of the lowest excited singlet states of rhodamine 6G and rhodamine B

Excited states of laser dyes were generated in solution by an intense 6 ns laser pump pulse tuned to the absorption maximum of the longest wavelength, ground state absorption band. They were interrogated by a shorter, simultaneous, monochromatic pulse of smaller diameter, concentric and collinear with the pump, having a crossed polarization. Plots of transmission curves against intensity for the pump beams approached residual absorptions at the highest intensities. Gain or attenuation of the probe was measured throughout the region of the long wavelength absorption bands, at the 578.2 nm copper vapor laser wavelength, and for rhodamine 6G across the lasing band. Gains were determined at different probe intensities, and were extrapolated to small-signal values. The dyes have fluorescences at the ground state absorption maxima, hence pumping at these wavelengths cannot achieve 100 percent S
1
population. Auxiliary saturation transmissions were determined beyond the fluorescence bands and the first experiments were corrected to give a measure of
\sigma_{e} - \sigma^{\ast}
, the difference between the stimulated emission cross section and the excited state absorption cross section. Assumptions in the experiment and improvements in experimental technique are discussed.