Spectral and luminescence properties of ethanol solutions of rhodamine 6G under picosecond excitation

Investigations were made of the absorption and luminescence spectra of ethanol solutions of rhodamine 6G as a function of concentration at room and liquid nitrogen temperatures. Light from a steady-state source and picosecond pulses from a neodymium glass laser were used for excitation. Under picosecond excitation, it was found that the luminescence spectrum of solutions with high concentrations ($10^{-2}$ mole or higher) exhibited a long-wavelength band with $\lambda_{\max}\approx670$ nm. At liquid nitrogen temperature, the luminescence spectrum of these solutions had two maxima at 620 and 670 nm. Measurements of the luminescence lifetime of these bands showed that it was of the same order as the fluorescence lifetime of rhodamine 6G molecules in ethanol. The observed long-wavelength luminescence was attributed to associated rhodamine 6G molecules.