Michael A. Kahlow

Femtosecond resolved solvation dynamics in polar solvents

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 f...

Transient solvation of polar dye molecules in polar aprotic solvents

Subpicosecond fluorescence spectroscopy of the polar dye molecules coumarin 311 and coumarin 102 has been used to measure the microscopic solvation dynamics of several polar aprotic solvents. The measured solvation times are significantly longer than the longitudinal relaxation times τ1 of the solvents. τ1 is the predicted time for solvation according to dielectric continuum theory. The experiments were made with a newly constructed subpicosecond ultraviolet emission apparatus that takes advantage of recent advances in ultrafast laser technology. The newly developed, time saving procedure [Nagarajan et al., J. Chem. Phys. 86, 3183 (1987)] for measuring the microscopic solvation response functions was used in this research.

Ultrafast emission spectroscopy in the ultraviolet by time‐gated upconversion

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 stud...

Double well isomerization in solution: A new comparison of theory and experiment

The first‐excited‐singlet state of each of the compounds 2‐vinylanthracene (2VA) and 2‐(2’‐propenyl) anthracene (22PA) undergoes an electronically adiabatic torsional double‐well isomerization which interconverts a s‐cis conformation and a s‐trans conformation. The forward rate constant kf for the isomerization was measured by picosecond fluorescence spectroscopy as a function of temperature and solvent viscosity η in the range 0.23–3.25 cP, i.e., in the C5–C16 normal alkanes. For 2VA, kf is independent of η within experimental uncertainty (∼5%), while for 22PA kf≂cη−0.23. An effective torsional isomerization potential of the form V(φ)=(V1/2)(1−cos φ)+(V2/2)(1−cos 2φ) was determined from thermochemical data on the isomerization. Torsional well (ω0) and barrier (ωb) frequencies were calculated from the effective potential and the appropriate moments of inertia. With an estimate in hand for ωb, it was possible to directly evaluate theoretical predications from the (i) theory of Kramers and alternatively fro...

Subpicosecond emission spectroscopy in the ultraviolet by time gated upconversion

We have built an apparatus to measure emission transients with ultrafast time resolution, following ultraviolet excitation. This system uses fluorescence upconversion to attain nearly pulsewidth limited time resolution. Laser pulses from a synchronously pumped dye laser are amplified at 8.2 kHz to ≥1 μJ/pulse to increase the efficiency of the frequency doubling and sum frequency generation processes. Results are shown for the same system with two dye lasers. The first is a commercially available laser with a pulsewidth of 1 picosecond; the other is a home built laser with a pulsewidth of 70 femtoseconds.

Ultrafast electron transfer: The role of solvent motion

This paper shows that the electron transfer (ET) time τET of the intramolecular ET reaction of electronically excited bianthryl (BA) is not equal to the longitudinal relaxation time τ1 of the solvent in various polar aprotic solvents. It has been observed that microscopic solvation time τs is very similar to τET for a broad range of polar aprotics.