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.

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Effects of translational, rotational, and vibrational energy on the dynamics of the D+H2 exchange reaction. A classical trajectory study J. Chem. Phys. 94, 7991 (1991); 10.1063/1.460133Isotope effect in vibrational excitation of H2 by low energy electron impact AIP Conf.The total (elastic plus inelastic) energy unresolved differential cross section for high energy electron scattering, computed previously [J. Chern. Phys. 77, 1323(1982] for H" has now been obtained for D,. The effects of molecular vibrations are discussed and compared with the data derived from experiment. It is shown that the observed difference between H, and D, experimental data was largely an artifact due to an approximate vibrational averaging in the independent atom model employed in the processing of directly measured quantities. However, the effects of vibrational motion of nuclei are large enough to enable an analysis of various approximate molecular potentials.(3)

Generalized oscillator strengths have been computed for high-energy electron scattering by H 2 • The optically allowed transitions from the electronic ground state (X 'I.+) to the B 'I.+ state and the optically forbidden transitions to the E'I/ state have been considered. Accurate wave functions expanded in explicitly correlated Gaussian functions were used to describe the electronic states of H 2 • The dependencies of the form factors on the molecular orientation, the momentum transfer, and the internuclear separation are discussed. For the X -B transitions the form factors have been integrated over the internuclear separation with accurate vibrational wave functions. A comparison of the obtained values with those computed assuming the Franck-Condon approximation has shown that the latter are not always reliable. In the region of a very small momentum transfer, the present results are found to be very close to the accurate values obtained for optical transition probabilities to the B state with wave functions depending explicitly on odd and even powers of the interelectronic distance.