Absorption processes associated with anti-Stokes fluorescence in rhodamine B solutions

Abstract

The absorption coefficient of several rhodamine B solutions excited at 632.8 nm has been measured for temperatures ranging from 200 to 385 °K. The weak monomer absorption coefficient (∼ 10⁻⁵ times the peak coefficient) at this anti‐Stokes excitation wavelength facilitates a study of the other components of absorption in the dye solution, yielding valuable information on solvent effects and factors affecting the quantum efficiency of fluorescence, such as dimer absorption, impurity content, etc. Whereas at high temperatures (above 30 °C), linear singlet‐singlet monomer absorption is observed to dominate, dimer formation is evident at temperatures below 30 °C, and the impurity absorption appears negligible in either case. From a least‐squares fit of the experimental data with the theoretical dependence of the total (monomer and dimer) absorption upon temperature, the association energy for the dimer is found to be 5.0 kcal/mole. It also seems possible that at temperatures above 50 °C, for solutions with low impurity absorption, quantum efficiencies high enough for radiative cooling of the solution may be attained.