Rotational diffusion and triplet state processes in dye laser solutions

Abstract

A cw dye laser with a rotatable intracavity Brewster-angle plate can be forced to oscillate with its signal polarization rotated by an arbitrary angle β about the laser axis from the pumping laser polarization. The threshold pump power R(β) then increases as the signal polarization is rotated from parallel (β = 0°) to perpendicular (β = 90°). By measuring the ratio R(90°)/R(0°) and comparing it with the first excited singlet level fluorescence intensity ratio I(0°)/I(90°) measured in the same laser medium below threshold, it is possible to determine the important dye laser triplet state parameter KT ≡ kSTτTσT/σe. A detailed analysis of this new triplet measurement method is given. Both the orientational distribution of the excited state population and the gain for arbitrary polarizations and pumping geometry are calculated. Triplet state effects have been studied for Rhodamine 6G as a function of dye and soap concentration in H2O. The triplet state parameter KT is found to have the value 0.35 ± 0.02 at 5960 Å for a 5 × 10−4 M concentration of R6G in water with 2% by volume of Triton X-100, and 0.25 ± 0.02 for the same dye concentration with 4% by volume of Ammonyx LO. The very long rotational relaxation times measured (∼ 1–2 nsec) indicate rigid attachment of the R6G molecules to micelle soap complexes. Evidence of intermolecular energy transfer via dipolar coupling at higher dye concentrations is observed.