Anne B. Myers

A resonance Raman intensity study of electronic spectral broadening mechanisms in CS2/cyclohexane

Complete resonance Raman spectra, including absolute cross sections, have been measured for CS2 in cyclohexane using four excitation wavelengths from 223 to 204 nm, on resonance with the strongly allowed S3←S0 electronic transition. Absolute intensities have also been measured in CS2 vapor using 200 nm excitation. These total Raman cross sections, together with the Rayleigh cross sections calculated from the Kramers–Kronig transform of the absorption spectrum, are used to determine the homogeneous linewidth Γ for the electronic transition under the assumption that the solvent induced broadening may be partitioned into a static, inhomogeneous part and a very rapidly modulated, homogeneous component. Γ is found to be ∼13 cm−1 in the vapor and ∼200 cm−1 in cyclohexane solution, indicating that homogeneous broadening is responsible for much of the increase in spectral breadth between vapor and solution phases. Direct modeling of the solution phase absorption spectrum and the intensities of the lower lying Ram...

Photodissociation of alkyl iodides in solution : substituent effects on the early-time dynamics

Resonance Raman spectra, including absolute scattering cross sections, have been measured for ethyl, isopropyl, and tert‐butyl iodides in cyclohexane solution at seven to ten wavelengths between 303 and 200 nm. Spectra of fully deuterated ethyl iodide have also been obtained at five wavelengths. Spectra excited in the 300–250 nm region, on resonance with the directly dissociative A state, are dominated by long overtone progressions in the nominal C–I stretching mode near 500 cm−1. In all three molecules the fundamental of the C–I stretch is unexpectedly weak relative to the overtones when excited near the peak of the A band. This is shown to arise from interference between the A‐state resonant part of the fundamental Raman amplitude and preresonant contributions from higher electronic states. In addition to the C–I stretching activity, A‐state excitation generates significant intensity in fundamentals, overtones, and combination bands of modes nominally assigned as bending and CC stretching vibrations, su...

Optical studies of single terrylene molecules in polyethylene

Abstract Flourescence excitation techniques have been used to study the optical spectroscopy and dynamics of single impurity molecules of terrylene in a poly(ethylene) matrix at 1.5K. We observe a variety of spectral diffusion effects, including spontaneous resonance frequency changes on the 10–100 MHz scale which lead to apparent fluctuations in the absorption line width and shape, discontinuous jumps in the resonance frequency of 100–2000 MHz on a time scale longer than 2.5 s, and long-lived light-induced changes in resonance frequency of more than a few GHz (single-molecule spectral hole-burning). We also observe for some molecules the unusual effect that the spectral diffusion rate and the frequency range increase for higher probing light intensity. For completeness, we also show the vibrationally resolved dispersed flourescence spectrum in the ensemble-averaged (large N ) limit, since such spectra have not been reported previously to our knowledge.

‘Time-Dependent’ Resonance Raman Theory

This review summarizes theoretical formulations of resonance Raman scattering that express the steady-state spectrum in terms of time-dependent quantities. Time-domain expressions for the Raman or direct one-photon absorption (electronic or vibrational) lineshape are compared and contrasted with time-dependent formulations for the resonance Raman intensity. Time-domain expressions that properly describe the total spontaneous emission of a chromophore interacting with a bath are also discussed. A final section addresses a few aspects of the interpretation and practical application of these theories to real experimental data.

Emission cross sections and line shapes for photodissociating triiodide in ethanol: Experimental and computational studies

Resonance Raman spectra and emission cross sections have been measured for triiodide in ethanol. The spectra show a long progression in the symmetric stretch and a broad background, which we attribute to a combination of solvent‐dephasing‐induced fluorescence and unresolved Raman combination bands of the symmetric and antisymmetric stretches. We also observe the fundamental of the antisymmetric stretch, indicating that the symmetry of the triiodide ion is broken by the environment on the resonance Raman time scale. Using two skewed LEPS potentials to describe the excited states, we are able to calculate emission band shapes and cross sections that agree very well with experiment. However, the time scale for solvent‐induced electronic dephasing is unclear due to the difficulty in partitioning the broad background into separate Raman and fluorescence contributions.

Vibrational analysis of the dispersed fluorescence from single molecules of terrylene in polyethylene

Abstract Vibrationally resolved dispersed fluorescence spectra of single molecules of terrylene in polyethylene at 1.5 K are presented. Most of the ≈ 35 individual molecules examined show spectra very similar to those of the bulk sample, with only small frequency shifts and intensity variations. Several molecules with a different frequency and intensity pattern are also observed. After considering many possibilities, we tentatively conclude that the two types of spectra arise from terrylene molecules located in the amorphous and crystalline regions of the polyethylene matrix.

Resonance Raman quantum yields for CS2 in solution: Dynamics of solvent‐induced spectral broadening

Electronic absorption spectra, resonance Raman spectra and quantum yields, and total emission yields have been measured for the S3←S0 transition of CS2 in cyclohexane, pentane, acetonitrile, hexadecane, and perfluorohexane solvents. The solution‐phase absorption spectra are significantly broadened and redshifted relative to the vapor. The solution‐phase S3 state lifetimes inferred from the total emission yields are 0.6–1.0 ps, close to the vapor‐phase lifetime, while the resonance Raman quantum yields imply electronic dephasing times of 25–50 fs in solution. This rapid dephasing due to intermolecular (solvent–solute) interactions is sufficient to account for almost all of the increased electronic spectral breadth in solution. The data are analyzed quantitatively with the aid of a stochastic theory of line broadening that accounts for solvent memory effects, and evidence is found for non‐Markovian (nonexponential in time) electronic dephasing. Possible physical origins for the rapid electronic dephasing in...

Relating absorption, emission, and resonance Raman spectra with electron transfer rates in photoinduced charge transfer systems: promises and pitfalls

Abstract In molecules or complexes in which a photoinduced electronic transition corresponds to a formal electron transfer process, there exists a close relationship between the theoretical expressions for the absorption, fluorescence, and resonance Raman spectra and the nonphotochemical return electron transfer rate in the coupling limit. In this paper, the theoretical expressions cast in both the sum-over-vibrational-eigenstates picture and the equivalent time-domain picture are reviewed. Several recent efforts by ourselves and other groups to make use of such relationships in the analysis of experimental spectroscopic and electron transfer rate data are then described and critiqued. Finally, prospects for the further development of this approach are discussed, as are some of its limitations and potential pitfalls.

Resonance Raman excitation profiles of methyl iodide in hexane

Resonance Raman spectra, including absolute scattering cross sections, have been measured for methyl iodide in hexane at nine excitation wavelengths from 204.2 to 274.0 nm. Spectra excited in the 246–274 nm region, on resonance with the directly dissociative A state, exhibit overtone progressions in the C–I stretch and its combination bands with the methyl umbrella mode. The fundamental of the C–I stretch is weak relative to the overtones when excited near the peak of the A band, an effect shown to arise from interferences between the resonant amplitude and preresonant contributions from higher electronic states. Both the absorption spectrum and the Raman excitation profiles are quite similar in solution and vapor phases, suggesting that the dissociative A state is not strongly perturbed by solvation. This conclusion is consistent with a simple theoretical estimate of the solvation effect on the A state potential.

Vibrational dephasing and frequency shifts for highly excited vibrations of methyl iodide in solution

Resonance Raman spectra have been obtained for CH3I in hexane, hexadecane, and acetonitrile solvents, excited on resonance with the directly dissociative 3Q0 electronic state near 260 nm. Vibrational bandwidths and vapor to solution frequency shifts have been measured for transitions involving up to fifteen quanta of excitation in the C-I stretching mode. Most of the Raman bands shift to lower frequencies upon solvation in all three solvents, indicating the dominance of attractive solvent-solute interactions. Both the vapor to solution frequency shifts and the solution phase bandwidths scale approximately linearly with vibrational quantum number. The experimental results, particularly regarding the dependence of bandwidth on vibrational quantum number, are compared with predictions of a modified version of the theory of Schweizer and Chandler. Both the vapor to solution frequency shifts and the vibrational bandwidths in hexane are predicted fairly accurately, but only if the correlation time for the forces due to nearest-neighbor atomic repulsions is assumed to be several hundred femtoseconds or longer. The validity of this general approach for describing large amplitude motions in solution is discussed.