Philip J. Reid

Direct pump/probe spectroscopy of the near-IR band of the solvated electron in alcohols

Abstract We report femtosecond direct pump/probe data on the s-state to p-state transition of the solvated electron in alcohols. The data reveal a p-state lifetime of ≈ 0.5 ps and a large spectral component due to displacement along the ground-state solvent coordinate involving slow diffusive solvent motions and subsequent relaxation. The large displacement is surprising considering the short excited-state lifetime and the long relaxation times of these modes. This effect is apparently the result of a previously unknown p-state to s-state nonadiabatic rearrangement of the solvent coordinate of solvated electrons in alcohols.

Direct measurement of the photochemical ring opening in 1,3-cyclohexadiene by picosecond time-resolved UV resonance Raman

The kinetics of the photochemical ring opening of 1,3-cyclohexadiene to form cis-hexatriene was studied by time-resolved, picosecond UV resonance Raman spectroscopy. The frequency-doubled 283.5 nm output of an amplified, synchronously pumped dye laser was used to perform pump-probe Raman experiments with 2 ps time resolution. Transient Raman spectra were obtained from 0 ps to 3.1 ns after the excitation pulse. The intensity of the 1624 cm−1 ethylenic line of the cis-hexatriene product was monitored versus time. The ring-opened photoproduct appeared with a time constant of 8±1 ps. Analysis of the vibrational spectra suggests that after crossing to the ground state surface, a thermal single-bond isomerization occurs in < 20 ps to produce, e.g. s-cis, cis, s-trans-hexatriene.

Femtosecond absorption anisotropy of the aqueous solvated electron

Abstract Femtosecond near-infrared pump/visible probe absorption dichroism studies on the aqueous solvated electron are reported. Experiments performed with 300 fs time resolution demonstrate that the anisotropy of the transient absorption observed at 740, 800 and 860 nm is established concomitant with excitation and decays in ≈ 3 ps. This observation is consistent with recent theoretical predictions (B.J. Schwartz and P.J. Rossky, Phys. Rev. Letters (1994), submitted) and demonstrates that anisotropic solvent fluctuations relax on the picosecond timescale. However, the magnitude and spectral manifestations of the absorption anisotropy are not consistent with theory.

Evidence for intermolecular hydrogen-bond rearrangement in the electron transfer dynamics of betaine-30 in n-butanol

The electron transfer and ground-state solvation dynamics of betaine-30 (B-30) in n-butanol at low temperatures are investigated by femtosecond pump-probe spectroscopy. Two separate components of the solvent response to solute excitation are observed. At higher temperatures (292-273 K), spectral evolution corresponding to diffusional solvation is evident. However, low-temperature studies (273-193 K) show the presence of a second process characterized by a ≈ 100 ps decay time and weak temperature dependence demonstrating that specific intermolecular hydrogen-bond dynamics are involved in the solvation of B-30 in protic solvents.

Picosecond Time-Resolved Resonance Raman Spectroscopy of Bacteriorhodopsin: Structure and Kinetics of the J, K, and KL Intermediates

Picosecond resonance Raman spectroscopy has been used to obtain structural information on the primary photointermediates of bacteriorhodopsin. A synchronously pumped dye laser was amplified at 50 Hz to produce a probe pulse at 589 nm. A second, spectrally distinct, pump pulse at 550 nm was generated by amplification of a 10 nm portion of a continuum produced from the probe pulse. This apparatus was used to record spectra of the J, K, and KL intermediates. The J spectrum exhibits strong hydrogen out-of-plane (HOOP) intensity and the fingerprint region consists of a broad series of lines centered at 1180 cm-1. By 3 ps, K has formed and the relative HOOP intensity decreases while the fingerprint collapses to a single mode at 1190 cm-1, characteristic of a 13-cis chromophore. These results argue that J contains a highly twisted chromophore which relaxes upon conversion to K and that isomerization is complete within 3 ps. Between 3 ps and 3.7 ns there is a resurgence in HOOP intensity and the ethylenic frequency rises from 1518 to 1521 cm-1 indicating the conversion of K to KL.

Photochemical Ring Openings are Complete in Picoseconds: A Time-Resolved UV Resonance Raman Study

A classic electrocyclic ring-opening reaction is the photoconversion of 1,3-cyclohexadiene (CHD) to cis-hexatriene (cis-HT)[1]. Our time-resolved resonance Raman studies have shown that the timescale for cis-HT formation is 8 ± 1 ps[2]. To determine if the kinetics of CHD are representative of other photochemical ring-opening reactions, we have studied the photoconversions of 1,3,5-cyclooctatriene (COT) to octatetraene and of α-phellandrene (α-PHE) to 3,7-dimethyl-1,3,5-octatriene.[3] COT is known to photochemically ring open with a disrotatory motion of the methylene portion of the ring as opposed to the conrotatory ring opening of CHD[4]. α-PHE is a substituted 1,3-cyclohexadiene in which one of the carbons undergoing conrotatory rotation has a hydrogen substituted for an isopropyl group.[5]

Pericyclic photochemical ring-opening reactions are complete in picoseconds: a time-resolved UV resonance Raman study

The kinetics of the photochemical ring openings of 1,3-cyclohexadiene (CHD), 1,3,5- cyclooctatriene (COT) and ((alpha) -PHE) were determined by picosecond, time-resolved UV resonance Raman spectroscopy. The time evolution of the photoproduct ethylenic intensity demonstrates that the photolysis of CHD produces ground state cis-hexatriene in 8 +/- 1 ps. Similarly, the photoproducts of COT and (alpha) -PHE appear in 12 +/- 2 ps and 11 +/- 2 ps, respectively. The similar ground state photoproduct formation times of these reactions indicates that the

Picosecond time-resolved resonance Raman spectroscopy of bacteriorhodopsin's J, K, and KL intermediates

OM10 ps timescale is a general feature of photochemical electrocyclic ring-opening reactions.