Ultrafast change in refractive index after excitation by a femtosecond laser pulse was observed for dilute Auramine O solutions in low-viscosity solvents by detecting the change as an ultrafast lensing effect (ULE). The decay of the ULE signals was found to consist of two components, and both their relaxation times depended on solvents. This is the first reported observation of the faster component by this technique. The slower component had a similar relaxation time to that of the fluorescence lifetime. Molecular orbital calculation results attributed the faster to relaxation in the lowest excited singlet state S 1 by way of internal rotation and the slower to the subsequent relaxation from S 1 to the ground state. Solvent dependence of the experimentally determined relaxation time was strongly correlated with the molecular weight of the solvents rather than their bulk viscosity. A simplified model explaining these results was proposed in which solute and solvent molecules interact only via hydrogen bonding, the strength of which is solvent independent.

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