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  • 11
    Electronic Resource
    Electronic Resource
    Solvent relaxation and electron back transfer following photoinduced electron transfer in an ensemble of randomly distributed donors and acceptors (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 3550-3561 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The role of solvent relaxation in electron back transfer following electron transfer from an optically excited donor to randomly distributed acceptors is treated theoretically. The solvent dynamics are included by using a time dependent electron back transfer rate function, Keff(R,t). The solvent relaxation is parameterized by τr, the relaxation time, D, the solvent energy diffusion constant, and Δq, the potential barrier height difference between the nonequilibrium solvent state formed upon ion creation and the relaxed solvent state. The expression for the ensemble averaged donor cation state population probability, 〈Pct(t)〉, as a function of these solvent relaxation parameters is derived. Numerical calculations are presented. Relationships among 〈Pct(t)〉, the intermolecular interaction parameters, and solvent relaxation parameters provide detailed insights into the distance and time dependence of the flow of electron probability in an ensemble of donors and acceptors. The theoretical expressions can be used to calculate experimental observables such as the transient grating signal.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.458787
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  • 12
    Electronic Resource
    Electronic Resource
    Reply to the Comment on: "Anisotropic reorientational relaxation in biphenyl: Transient grating optical Kerr effect measurements'' (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 2162-2163 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.459047
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  • 13
    Electronic Resource
    Electronic Resource
    Anisotropic reorientational relaxation of biphenyl: Transient grating optical Kerr effect measurements (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 90 (1989), S. 6893-6902 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Subpicosecond transient grating optical Kerr effect measurements have been used to evaluate the reorientation of biphenyl molecules in neat biphenyl and n-heptane solutions. Besides an ultrafast (100 fs time scale) component associated with librational damping/dephasing, two reorientational relaxation components are observed. The slow reorientation is due to rotation around the short axes of the molecule (tumbling motion), the fast reorientation is associated with internal rotation around the central C–C bond and/or rotation of the whole molecule around its long axis (spinning motion). Whereas the tumbling motion has been observed in earlier depolarized light scattering data, the time resolved Kerr data presented here are the first ones to reveal the dynamics of the fast reorientation component and the ultrafast librational dynamics. It is shown that the diffusive reorientational relaxation must be coupled to the ultrafast librational dynamics, and implications of this coupling are pointed out.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.456264
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  • 14
    Electronic Resource
    Electronic Resource
    Theory of the phonon perturbed photon echo experiment: Direct determination of electronic excitation–phonon coupling (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 88 (1988), S. 3407-3416 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A theory is presented describing the proposed phonon perturbed photon echo experiment. This experiment is a combination of picosecond time scale stimulated Raman pumping and photon echo experiments. The theory demonstrates that the phonon perturbed photon echo can directly measure electronic excitation–phonon coupling matrix elements by observing the influence of a well defined coherent phonon wave on the photon echo signal. The theory predicts that the echo pulse area (integrated intensity) is reduced. The size of this change is related to the strength of the excitation phonon coupling. In addition, a realistic estimate is made of the size of the effect, and it is shown that the experiment is feasible with available laser equipment.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.453889
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  • 15
    Electronic Resource
    Electronic Resource
    Reorientational motion of a cross-link junction in a poly(dimethylsiloxane) network measured by time-resolved fluorescence depolarization (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 96 (1992), S. 3269-3278 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The reorientational dynamics of a cross-link junction in poly(dimethylsiloxane) networks, measured by the fluorescence anisotropy decay of a chromophore tagged to the cross-link, have been investigated over a range of temperatures from Tg+75 to Tg+150. The probe chromophore, 1-dimethylamino-5-sulfonylnaphthalene amide (dansyl amide), is pendant to a trifunctional silane that acts as a cross-linking molecule. In cyclohexanol, the fluorescence anisotropy decay is in agreement with Debye–Stokes–Einstein hydrodynamic theory (rotational diffusion) demonstrating that the cross-linker can be used as a probe of orientational relaxation. The fluorescence anisotropy decays at a rapid rate in an end-linked poly(dimethyl siloxane) network reflecting fast reorientational motion of the cross-link junction. This reorientation appears diffusive and has a temperature dependence in accord with the Williams–Landel–Ferry equation. A model is proposed that suggests that reorientation and translational motion of the cross-link occur simultaneously and are both coupled to fluctuations of the polymer chain ends.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.461972
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  • 16
    Electronic Resource
    Electronic Resource
    The influence of diffusion on photoinduced electron transfer and geminate recombination (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 96 (1992), S. 7410-7422 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The influence of diffusion on photoinduced electron transfer and geminate recombination in solutions of randomly distributed donors and acceptors is explored. The focus is on the effect diffusional motion has on geminate recombination. The reactive state (state following photoinduced electron transfer) probability is calculated as a function of diffusion constant and relative permittivity for three intermolecular potential cases: attractive, repulsive, and no Coulomb potentials. Also calculated are the reactive state yield and reactive state survival fraction. Both forward and back electron-transfer rates are distance dependent (not contact transfer). Any diffusion constant can be investigated, and donor–acceptor and acceptor–acceptor excluded volumes are taken into account. The model developed here is compared with slow and fast diffusion limits as well as with the theories of Smoluchowski, and Collins and Kimball.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.462391
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  • 17
    Electronic Resource
    Electronic Resource
    Solvent relaxation effects on the kinetics of photoinduced electron transfer reactions (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 94 (1991), S. 1081-1092 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The three-potential surface problem of electron transfer in solution is analyzed using Zusman-type kinetic equations. The model describes ultrafast formation and recombination of radical–ion pairs limited by solvent dielectric relaxation. The problem begins with a donor on an electronic excited state surface. The system evolves with crossing to the radical–ion pair surface (with the possibility of recrossing to the excited donor surface included). Solvent relaxation moves the system to lower energy on the radical–ion pair surface where crossing to the ground state neutral surface occurs (with the possibility of recrossing to the radical–ion surface included). Model calculations of the transient radical–ion pair populations are presented. The time dependent results that are presented show a dramatic dependence on the relative free energy differences (ΔG's) among the three potential surfaces. Comparisons to other formalisms and to less detailed approximations are made. The mean populations of the transient species for a system of a donor and many acceptors in the absence of spatial diffusion are also derived.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.460064
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  • 18
    Electronic Resource
    Electronic Resource
    Nonhydrodynamic molecular motions in a complex liquid: Temperature dependent dynamics in pentylcyanobiphenyl (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 3503-3514 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The isotropic phase of nematogenic substances above the nematic–isotropic phase transition temperature Tni is characterized by short range orientational correlation of the molecules and the existence of pseudonematic domains. This microscopic structure has a strong influence on reorientational dynamics in the liquid. We present time resolved transient grating optical Kerr effect measurements of pentylcyanobiphenyl between 35 and 120 °C, which allow us to monitor dynamics on timescales from subpicoseconds to tens of nanoseconds. The well-known collective component characterizing the reorientational dynamics of the pseudonematic domains follows Landau–deGennes behavior up to 30–35 degrees above Tni where the theoretical correlation length ξ≈three molecular lengths. Additionally fast relaxation channels (with time constants between 1 and 300 ps) are observed which are associated with local reorientational and intermolecular dynamics within the pseudonematic domains. The local dynamics are temperature independent in the range where the pseudonematic domains dominate the liquid structure, demonstrating a decoupling of the molecular motions from the bulk viscosity. Only at high temperatures, T(approximately-greater-than)70 °C, where the pseudonematic domains cease to exist, is a viscosity dependence observed. At these elevated temperatures, the slowest contribution to the fast molecular dynamics approximately follows hydrodynamic theory.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.458832
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  • 19
    Electronic Resource
    Electronic Resource
    Two-level systems and low-temperature glass dynamics: Spectral diffusion and thermal reversibility of hole-burning linewidths (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 92 (1990), S. 4145-4158 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Intermediate time-scale time-dependent hole-burning measurements are reported on three glassy organic systems which undergo spectral diffusion: cresyl violet in ethanol at 1.30 and 2.13 K, resorufin in ethanol at 2.13 K, and resorufin in glycerol at 2.13 K. The hole width is observed to broaden on a log time scale from 0.1 to 5000 s for each ethanol system while no broadening is observed in the system of resorufin in glycerol. A detailed theoretical treatment is introduced which allows the raw data to be converted to the fluctuation rate distribution of the underlying modes responsible for dephasing. Using this theory, the broadening in ethanol is found to be the result of a distribution of glassy modes which is Gaussian on a log R scale with a center rate at ∼0.02 s−1. In addition, temperature cycling hole-burning results are reported on the system cresyl violet in ethanol. A hole is burned at 1.30 K and detected before, during, and after a temperature cycle to 2.13 K and back. The hole width is observed to broaden at the high temperature and then narrow again in a completely reversible manner when the temperature is again lowered. Theoretical calculations show this behavior to be entirely consistent with the tunneling two-level-system (TLS) model of glass dynamics but incompatible with other models such as particle or defect diffusion. The cycling data is shown to fall exactly on the theoretical curve calculated from the TLS model using no adjustable parameters.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.457773
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  • 20
    Electronic Resource
    Electronic Resource
    Pseudolocal modes of guest molecules in mixed molecular crystals: Photon echo experiments and computer simulations (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 92 (1990), S. 3323-3334 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The temperature-dependent optical dephasing of anthracene, 9-methylanthracene, and 2-methylanthracene monomers in phenanthrene host crystals has been measured using photon echo experiments. Despite large linear electron–acoustic phonon coupling, all three systems dephase because of coupling to pseudolocal modes (local motions of the guest molecule). Computer simulations of the three systems calculate the pseudolocal mode eigenvalues and eigenvectors. In contrast to previous discussions in the literature which describe pseudolocal modes as librations, the predicted eigenvalues are in reasonable agreement with the measured pseudolocal mode energies. The predicted eigenvectors are combinations of translational motion along the long molecular axis and rotational motion about the out-of-plane axis of the guest. Differences in site energies for various locations and orientations of the methyl group are calculated.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.457844
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