Vibrational relaxation of dye molecules investigated by ultrafast induced dichroism

Theoretical and experimental data are presented on the propagation of picosecond pulses in weakly excited dye solutions. It is shown that the so called coherence peak of the induced dichroism observed at short delay times depends on the vibronic relaxation and the excited state absorption of the dye molecules. Interpretation of the experimental results is considerably facilitated at low excitation level. The vibrational relaxation time in theS ₁ state of phenoxazone 9 was measured to be _v =0.8±0.3 ps, 0.7±0.2 ps and 0.7±0.2 ps, respectively, for the solvents dioxane, solid polystyrene and CCl₄. For rhodamine 6 G in ethanol we find _v =0.5±0.2 ps.