The transient solvation of a polar fluorescent probe has been studied by the time resolved Stokes shift technique with roughly five times shorter time resolution than previously reported. New shorter time components in the solvation relaxation function C(t) have been discovered for methanol, propionitrile, and propylene carbonate; the C(t) function for acetonitrile is singly exponential within the limitations of the instrument. The observed C(t) has been compared to theoretical calculations using the dielectric continuum (DC) model for each solvent, with non-Debye expressions for the solvent dielectric response. For methanol the DC model predictions agree closely with experiment. For the polar aprotic solvents propylene carbonate and propionitrile, the shape of the experimental decay is different from the DC predictions, but the average decay times 〈τs〉 are closer to the DC predictions than previously reported. The comparison of theory and experiment is clearly limited by the inconsistencies and limited frequency range of the dielectric relaxation data found in the literature. The dynamic solvation measurements have also been compared to predictions of the mean spherical approximation as applied to solvation dynamics, which appear to give slower solvation rates than are observed experimentally.

We have built a new apparatus to time resolve ultrafast fluorescence following ultraviolet excitation. A synchronously pumped dye laser produces optical pulses of 1-ps or 70-fs full-width half-maximum, depending upon dyes and optical configuration. These pulses are amplified at a 8.2-kHz repetition rate using a copper vapor laser-pumped multipass amplifier. The resulting amplified laser pulses are frequency doubled to obtain ultrafast pulses in the ultraviolet. This ultraviolet light is used to electronically excite a sample; the resulting fluorescence is time resolved using fluorescence upconversion as the optical gating technique. A minimum 300-fs full-width half-maximum instrument response function is obtained. After a brief introduction, we discuss the principles involved in this method of time resolving fluorescence. We review special considerations for femtosecond laser experimentation, and discuss the construction of our apparatus. Finally, as an example, we show how this system can be used to study fundamental physical processes in solution.