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  • 1
    Electronic Resource
    Electronic Resource
    High density polarized hydrogen gas target for storage rings (1995)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 66 (1995), S. 28-31 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A target of gaseous polarized hydrogen was formed by injecting polarized hydrogen atoms (produced by Stern–Gerlach spin separation) into a storage cell consisting of a cylindrical tube open at both ends. The target was placed in a storage ring to study the target characteristics (nuclear polarization, target thickness, radiation resistance). A weak transverse guide field (5 G) was applied to define the polarization direction. When atoms in a single hyperfine state were selected, the nuclear polarization of the target was measured to be 0.80±0.02. The areal density of the target under these conditions was (5.5±0.2)×1013 H/cm2, while for two spin states (applicable to experiments in high energy rings where a strong magnetic field can be applied to the target) the target thickness was found to be (8.2±0.3)×1013 H/cm2. The target polarization was unaffected by prolonged exposure of the target to beams up to 1 mA. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1146388
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  • 2
    Electronic Resource
    Electronic Resource
    Optical properties of GaN epitaxial films grown by low-pressure chemical vapor epitaxy using a new nitrogen source: Hydrazoic acid (HN3) (1995)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 66 (1995), S. 2433-2435 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We report results of our growth and characterization of GaN films using low-pressure chemical vapor epitaxy with a new nitrogen source, hydrazoic acid (HN3). This growth technique allows for low-temperature deposition, low III/V ratios, and increased deposition rates (up to ∼2–3 μm/h). The deposited films show Ga:N atomic ratios of 1±0.25 based on our x-ray photoelectron spectroscopy analyses, and the He(II) UPS (ultraviolet photoelectron spectroscopy) spectra compare favorably with the semi-ab initio calculations for the GaN valence bands and with the reported UPS data for single crystal GaN films. X-ray and Raman spectra show deposited films crystallized in the expected wurtzite structure. We find these epitaxial films to be efficient light emitters in the blue or yellow region of the spectrum, depending upon growth conditions. Our photoluminescence time-decay kinetics confirm the excitonic nature of the blue emission. Lastly, far infrared time-domain spectroscopy shows the low carrier concentration of this material. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.113964
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  • 3
    Electronic Resource
    Electronic Resource
    Resonance Raman spectroscopy of the S1 and S2 states of pyrazine: Experiment and first principles calculation of spectra (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 103 (1995), S. 6851-6860 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: New experimental and theoretical data on the resonance Raman (RR) spectroscopy of the S1 and S2 states of pyrazine are presented. Based on recent ab initio CASSCF (complete- active-space-self-consistent-field) and MRCI (multireference configuration interaction) calculations of Woywod et al. [J. Chem. Phys. 100, 1400 (1994)], we construct a vibronic coupling model of the conically intersecting S1 and S2 states of pyrazine, which includes the seven most relevant vibrational degrees of freedom of the molecule. Employing a time-dependent approach that treats the intramolecular couplings in a nonperturbative manner, we calculate RR cross sections for this model, taking explicitly into account the nonseparability of all vibrational modes. The combination of high-level ab initio calculations and multimode propagation techniques makes it possible, for the first time, to make first-principles predictions of RR spectra for vibronically coupled electronic states of an aromatic molecule. The theoretical data are compared to experimental gas-phase RR spectra which have been obtained for five different excitation wavelengths. The comparison reveals that the ab initio predictions match the experimental results in almost every detail. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.470689
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  • 4
    Electronic Resource
    Electronic Resource
    A semiclassical self-consistent-field approach to dissipative dynamics: The spin–boson problem (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 103 (1995), S. 1561-1573 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A semiclassical time-dependent self-consistent-field approach for the description of dissipative quantum phenomena is proposed. The total density operator is approximated by a semiclassical ansatz, which couples the system degrees of freedom to the bath degrees of freedom in a self-consistent manner, and is thus in the spirit of a classical-path description. The capability of the approach is demonstrated by comparing semiclassical calculations for a spin–boson model with an Ohmic bath to exact path-integral calculations. It is shown that the semiclassical model nicely reproduces the complex dissipative behavior of the spin–boson model for a large range of model parameters. The validity and accuracy of the semiclassical approach is discussed in some detail. It is shown that the method is essentially based on the assumption of complete randomization of nuclear phases. In particular, the assumption of phase randomization allows one to perform the trace over the bath variables through quasiclassical sampling of the nuclear initial conditions without invoking any further approximation. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.469778
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  • 5
    Electronic Resource
    Electronic Resource
    Classical description of nonadiabatic photoisomerization processes and their real-time detection via femtosecond spectroscopy (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 103 (1995), S. 10015-10029 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A classical-path approach to the description of photoinduced isomerization dynamics as well as the interrelated electronic and vibrational relaxation processes is outlined. Adopting a three-mode model of photoisomerization that has been recently proposed by Seidner and Domcke (Chem. Phys. 186, 27 (1994)), we perform detailed numerical studies and compare the results of the classical simulations to available exact quantum-mechanical results. It is shown that the classical model reproduces semiquantitatively time-dependent diabatic and adiabatic electronic population probabilities, state-specific torsional wave functions, and energy contents of vibrational degrees of freedom. Furthermore it is demonstrated that the classical approach is able to simulate at least qualitatively time- and frequency-resolved pump-probe spectra of these processes. In accordance with exact quantum calculations, the classical simulations reveal the decay of the stimulated emission of the reactants and the delayed onset of the absorption of the photoproducts. To demonstrate the capability of the classical approach, the three-mode model of Seidner et al. is augmented by a hundred weakly-coupled harmonic modes. This allows to roughly simulate the relaxation dynamics of a chromophore interacting with a solvent. The simulations reveal that the time evolution of the full system within the first few hundred femtoseconds is quite similar to the case of the bare three-mode model. For later times, however, the dynamics of the three-mode model becomes quasistationary, whereas the calculations for the full system reflect the redistribution of the excess energy of the reaction mode into the bath nuclear degrees of freedom. It is found that the quantum yield of the cis-trans photoreaction depends to a large extent on the specific chromophore-solvent coupling employed, as it governs directly the competition of the various relaxation pathways. Simulations of the corresponding time- and frequency-resolved pump-probe spectra reveal that the cooling of the vibrationally hot photoproducts in the solvent is mainly reflected in a blue shift and a narrowing of the width of the absorption spectrum. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.469905
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  • 6
    Electronic Resource
    Electronic Resource
    Nonperturbative approach to femtosecond spectroscopy: General theory and application to multidimensional nonadiabatic photoisomerization processes (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 103 (1995), S. 3998-4011 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A general nonperturbative approach to calculate femtosecond pump-probe (PP) signals is proposed, which treats both the intramolecular couplings and the field-matter interaction (numerically) exactly. Experimentally as well as in a perturbative calculation it is straightforward to distinguish between different spectroscopic processes through the direction of the wave vector of the emitted radiation. A nonperturbative calculation, on the other hand, yields the overall polarization of the system, which is the sum of all these contributions. We present a general and practical method that allows to extract the individual spectroscopic signals, which are resolved in time, frequency, and direction of the emission, from the overall polarization. We briefly derive the basic expressions for the time- and frequency-resolved PP signals under consideration, and discuss in detail the simplifications that arise when the usual assumptions (i.e., weak laser fields, nonoverlapping pulses, slowly-varying envelope assumption and rotating-wave approximation) are invoked. The computational procedure is illustrated by nonperturbative calculations of the polarizations and PP signals for a one-dimensional shifted harmonic oscillator. To demonstrate the capability of the approach we have evaluated the polarization as well as PP signals for a three-dimensional model system with vibronically coupled potential-energy surfaces, which describes ultrafast nonadiabatic isomerization dynamics triggered by the twisting of a double bond. We consider various wavelengths and pulse durations of the laser fields and study integral and dispersed PP spectra as well as coherent photon-echo signals. It is shown that the time- and frequency-resolved PP signals reflect in real time the disappearance of the reactants and the delayed appearance of the products. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.469586
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  • 7
    Electronic Resource
    Electronic Resource
    A semiclassical self-consistent-field approach to dissipative dynamics. II. Internal conversion processes (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 103 (1995), S. 2888-2902 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A semiclassical time-dependent self-consistent-field (TDSCF) formulation is developed for the description of internal conversion (IC) processes in polyatomic molecules. The total density operator is approximated by a semiclassical ansatz, which couples the electronic degrees of freedom to the nuclear degrees of freedom in a self-consistent manner, whereby the vibrational density operator is described in terms of Gaussian wave packets. The resulting TDSCF formulation represents a generalization of familiar classical-path theories, and is particularly useful to make contact to quantum-mechanical formulations. To avoid problems associated with spurious phase factors, we assume rapid randomization of the nuclear phases and a single vibrational density operator for all electronic states. Classically, the latter approximation corresponds to a single trajectory propagating along a "mean path'' instead of several state-specific trajectories, which may become a critical assumption for the description of IC processes. The validity and the limitations of the mean-path approximation are discussed in detail, including both theoretical as well as numerical studies. It is shown that for constant diabatic coupling elements Vkk′ the mean-path approximation should be appropriate in many cases, whereas in the case of coordinate-dependent coupling Vkk′(x) the approximation is found to lead to an underestimation of the overall relaxation rate.As a remedy for this inadequacy of the mean-path approximation, we employ dynamical corrections to the off-diagonal elements of the electronic density operator, as has been suggested by Meyer and Miller [J. Chem. Phys. 70, 3214 (1979)]. We present detailed numerical studies, adopting (i) a two-state three-mode model of the S1−S2 conical intersection in pyrazine, and (ii) a three-state five-mode and a five-state sixteen-mode model of the C˜→B˜→X˜ IC process in the benzene cation. The comparison with exact basis-set calculations for the two smaller model systems and the possible predictions for larger systems demonstrate the capability of the semiclassical model for the description of ultrafast IC processes. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.470502
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