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  • American Institute of Physics (AIP)  (66)
  • 1
    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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  • 2
    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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  • 3
    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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  • 4
    Electronic Resource
    Electronic Resource
    Time dependence of donor–acceptor electron transfer and back transfer in solid solution (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 90 (1989), S. 159-170 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Electron transfer from an optically excited donor to randomly distributed acceptors followed by electron back transfer is treated theoretically for donors and acceptors in a rigid solution. The forward electron transfer process is described in terms of the excited state population probabilityPex(t) of the donor molecules, while the electron back transfer from the radical anion to the radical cation is characterized by Pct(t), the donor cation state population probability. Exact expressions for the ensemble averages 〈Pex(t)〉 and 〈Pct(t)〉 are derived. Numerical calulations are presented for the cation probabilities, the average cation–anion separation distance 〈R(t)〉, and the average cation existence time 〈τ(R)〉, using parameters which characterize the forward and back transfer distance dependent rates. Relationships among 〈Pex(t)〉, 〈Pct(t)〉 and the intermolecular interaction 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. In particular, picosecond transient grating experiments are analyzed, and it is shown that by combining grating experiments (or other ground state recovery experiments) with fluorescence experiments it is possible to obtain the intermolecular interaction parameters for both forward and back transfer and a detailed description of the dynamics. The calculations presented here for rigid solutions are the precursor to the inclusion of diffusive motion of donors and acceptors to describe the dynamics of coupled electron transfer and back transfer in liquid solutions.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.456509
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  • 5
    Electronic Resource
    Electronic Resource
    Gas phase dynamics and spectroscopy probed with picosecond transient grating experiments (1987)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 86 (1987), S. 5370-5391 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Picosecond transient grating experiments in sodium and iodine vapors, involving the 3S→3P and X→B transitions, respectively, are discussed in detail. Population gratings in sodium demonstrate that the technique can be used to measure velocity distributions in the gas phase. It is shown that the time dependent transient grating signal is related to the Fourier transform of the velocity distribution. Similar experiments on iodine illustrate the effect of state changing collisions on the grating signal. Theoretical calculations for a model in which the change of state is caused by a single collision are given. Close agreement with the data is observed for the situation in which the collision takes the initial velocity into a random velocity distribution. From this model a collision cross section is determined. The results demonstrate that information on collision dynamics can be obtained from grating experiments. In addition, the sodium experiments are used to illustrate a new type of time domain high resolution spectroscopy. When the grating excitation pulses have perpendicular polarizations, a polarization grating, rather than the usual population grating, is formed. Diffraction from the sodium polarization grating shows larger time dependent oscillations in the diffraction efficiency. These oscillations yield the ground state and excited state hyperfine frequencies (1.77 GHz and 189 MHz, respectively). A detailed theoretical description of the origin of the oscillations is presented. The results suggest that polarization grating spectroscopy can have applications in other areas, such as molecular rotational dynamics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.452773
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  • 6
    Electronic Resource
    Electronic Resource
    A picosecond photon echo study of a chromophore in an organic glass: Temperature dependence and comparison to nonphotochemical hole burning (1987)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 86 (1987), S. 77-87 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The first two-pulse photon echo experiments on a chromophore in an organic glass are reported. The homogeneous electronic dephasing of resorufin in ethanol glass is measured from 1.5–11.4 K. The temperature dependence of the dephasing time does not fit the power law frequently predicted by theory for the dephasing characteristic of glasses. However, the temperature dependence can be accounted for by including dephasing from librations or acoustic phonons, mechanisms known to be important in crystals. The dephasing decay is found to be a single exponential for over six factors of e. The dephasing is also shown to be uncorrelated with the extent of nonphotochemical hole burning (NPHB). However, the homogeneous linewidth deduced from the photon echo is four times narrower than the linewidth obtained from NPHB, demonstrating that the hole is broadened by additional processes.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.452595
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  • 7
    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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  • 8
    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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  • 9
    Electronic Resource
    Electronic Resource
    Solvation shell effects and spectral diffusion: Photon echo and optical hole burning experiments on ionic dyes in ethanol glass (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 92 (1990), S. 4125-4138 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Results of picosecond photon echo and optical hole burning experiments are reported for four ionic dyes in ethanol glass. At low temperatures, the dephasing times deduced from the hole widths are as much as nine times shorter than those measured by the two-pulse echo because of the effect of spectral diffusion. The temperature dependences found are of the form aTα+b exp (−ΔE/kT) due to glass two level system dynamics (T〈4 K) and a process that activates exponentially at higher temperatures, possibly from a pseudolocal mode or glass optical phonon. Comparing the ratios of echo to hole burning measured dephasing times for the four dyes suggests that the dephasing is influenced by the existence of distinct local ethanol solvation shells in addition to the dynamics of the bulk solvent. A theoretical description of solvent shell effects is achieved through the use of a two spatial domain model of the glass dynamics. Calculations of dynamic perturbations from distinct solvation shell and bulk solvent regions show that the observed differences between the dyes' dephasing ratios can be explained if the ionic chromophores alter glass dynamics locally.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.457772
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  • 10
    Electronic Resource
    Electronic Resource
    Dispersive excitation transport at elevated temperatures (50–298 K): Experiments and theory (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 92 (1990), S. 5622-5635 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Time-resolved fluorescence depolarization has been used to measure electronic excitation transport among naphthyl chromophores in polymeric glasses. 2-ethylnaphthalene randomly distributed in PMMA and 2-vinylnaphthalene/methyl methacrylate copolymer in PMMA were studied. It was found that excitation transport is dispersive at all temperatures studied, from 50 K to room temperature, i.e., the extent of transfer depends on the excitation wavelength within the S0–S1 absorption band. A theory based on the nondispersive, Förster mechanism for excitation transfer has been developed to describe dispersive transport. Good agreement between the theoretical and experimental results are achieved without resorting to adjustable parameters. Both the theory and experiment show that, for the observable used here, excitation at a certain wavelength, called the "magic wavelength,'' results in a time dependence that is identical to the Förster nondispersive result, i.e., dispersive transport appears to vanish.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.458494
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