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  • 1
    Electronic Resource
    Electronic Resource
    X-ray Raman scattering under pulsed excitation (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 5593-5603 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Accounting for actual and anticipated developments in synchrotron radiation- and in x-ray and free electron laser techniques, we present a theory of resonant scattering of short-pulsed x rays. Both the instantaneous and integrated-over-observation-time x-ray scattering spectra are studied. Contrary to the commonly accepted notion of an instantaneous probability, we find that the correct quantity describing the resonant x-ray scattering (RXS) experiment is the probability integrated over the time of measurement. It is shown that the integral RXS probability is given by the convolution of the RXS probability for monochromatic excitation with the Wiener–Khintchine spectrum of the nonstationary incident radiation. We have found that short pulses yield a new spectral band following a non-Raman dispersion law. The finite duration of pulses results in a broadening of the Raman resonance. Some aspects of the theoretical analysis of the RXS spectra as functions of the pulse duration time are analyzed by numerical calculations. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.481134
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  • 2
    Electronic Resource
    Electronic Resource
    Soft and hard x-ray Raman scattering by oriented symmetrical molecules: Selection rules, interference, and dephasing mechanisms (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 109 (1998), S. 5060-5069 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Resonant x-ray Raman scattering in chaotical, partial, and fixed oriented symmetric molecules is analyzed for x-ray photon excitation frequencies in both the soft and the hard x-ray regions. Different dephasing mechanisms and their connection with channel interference and the observation of selection rules are investigated. It is predicted that for harder x-ray energies the scattering cross sections become strongly anisotropic and oscillatory due to channel interference. The orientational dephasing is predicted to be an important coherence-blocking mechanism and can be introduced even by zero-point vibrational or librational motions. The connections between selection rules, symmetry and phase factors of the photon wave function, Bragg conditions and the channel interference show that the selection rules may operate for oriented, surface adsorbed, molecules even in the hard x-ray region. The possibility of using the interference effect for structure determination of adsorbates is discussed. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.477120
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  • 3
    Electronic Resource
    Electronic Resource
    Erratum: "Resonant x-ray scattering beyond the Born–Oppenheimer approximation: Symmetry breaking in the oxygen resonant x-ray emission spectrum of carbon dioxide" [J. Chem. Phys. 106, 3439 (1997)] (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 107 (1997), S. 2699-2699 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.475329
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  • 4
    Electronic Resource
    Electronic Resource
    Resonant x-ray scattering beyond the Born–Oppenheimer approximation: Symmetry breaking in the oxygen resonant x-ray emission spectrum of carbon dioxide (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 106 (1997), S. 3439-3456 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Although resonant x-ray scattering of molecules fulfills strict electronic symmetry selection rules, as now firmly proven by spectra of diatomic molecules, the accumulated body of data for polyatomic molecules indicates that an apparent breaking of these rules represents the common situation rather than the exception. The CO2 molecule provides a good example of symmetry breaking, with the oxygen x-ray emission spectra showing strong intensity for transitions that are forbidden by the parity selection rule. We present time-independent and time-dependent theories for frequency-dependent resonant x-ray scattering beyond the Born–Oppenheimer approximation in order to explore under what circumstances one can anticipate symmetry breaking in the spectra of polyatomic molecules. The theory starts out from the Kramers–Heisenberg dispersion relation and is generalized for vibrational degrees of freedom and for nonadiabatic coupling of the electronic (vibronic) states, including the frequency dependency of the scattering cross section. Different limiting cases and few-level models are considered. The symmetry breaking is proven to be the result of pseudo-Jahn–Teller-like vibronic coupling between near-degenerate core-excited states. Thus vibronic interaction over the antisymmetric vibrational mode between the "bright" 1σg−12πu1 and "dark" 1σu−12πu1 intermediate states of CO2 allows transitions otherwise forbidden. The measurements and theory demonstrate that the symmetry-selective character of the resonant x-ray emission is strongly frequency dependent. The strong intensity of "dipole-forbidden" transitions in the π* oxygen K spectrum at resonance is reduced monotonically with the detuning of the excitation energy from resonance, and the spectra become "symmetry purified." Simulations with full vibronic coupling predict this feature of the x-ray scattering experiment and a few-level model explains the energy dependence of the symmetry selection and the symmetry purification at large detuning energies in the limit of narrowband photon excitation. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.474111
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  • 5
    Electronic Resource
    Electronic Resource
    Core electron chemical shifts in conjugated molecules and polymers (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 103 (1995), S. 5848-5859 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Modern experiments and quantum calculations have revealed interesting trends in core electron spectra of oligomers and polymers. In particular, the core electron chemical shifts show salient features that are structurally dependent, but which have not been completely understood. From π-electron theory we derive a model for core electron binding energy shifts in conjugated molecules and polymers. The alternant behavior of the site dependent shifts in the linear and polycyclic hydrocarbons are predicted by the model. The alternations show end-to-bulk amplification, respectively, damping for the cyclic and the linear polyenes. The polycyclic hydrocarbons show the strongest alternation effects for the bulk atoms, while for the polyenes the alternation is strongest for the terminal atoms. The analysis is based on renormalized perturbation theory and on the analytic solutions of the Hückel equations. The theory and the proposed model can be applied also for investigations of the influence of impurities on physical properties of polymers. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.470464
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  • 6
    Electronic Resource
    Electronic Resource
    Theory of x-ray emission of conjugated molecules (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 105 (1996), S. 5224-5232 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A theory of nonresonant x-ray emission spectra of conjugated molecules is presented. The role of relaxation effects on the spectral shape of x-ray emission is advanced and examined in detail for the transpolyenes. In some contrast to unconjugated systems, the relaxation of valence charge around the core hole is shown to influence the distribution of x-ray intensities significantly. Site and size dependences of the spectral shape of x-ray emission are predicted; the site dependence as being alternant among the π levels. Comparative calculations of x-ray emission spectra at the ab initio Hartree–Fock and the semiempirical levels show that the relaxation qualitatively changes the spectral distribution and shifts intensity from the σ to the π parts of the spectra, creating excitonlike features in the latter. The exciton character is site and size dependent; for longer polyenes it is stronger for mid-atoms than for end-atoms, and is found stronger at the top than at the bottom of the π band. Complication/simplification trends of the spectra along the oligomer sequence are predicted for both the relaxed and frozen orbital models. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.472365
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  • 7
    Electronic Resource
    Electronic Resource
    Continuum modeling of multi-mode vibronic excitations in near-edge x-ray absorption fine structure spectra (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 3734-3740 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The shape of electro-vibrational bands in near-edge x-ray absorption fine structure (NEXAFS) spectra is often made up by a few strong lines near a phononless resonance. In addition—as predicted here—there can be extended tails due to the large density of vibrational states in the short-wavelength region of the x-ray spectrum where the spacings between vibrational levels are smaller than the lifetime broadening. We propose a continuum approach for simulating vibronic bands in x-ray absorption spectra for systems with a high density of states. This approach is found to provide an accurate description of the spectral shape of the multi-mode vibrational bands in the NEXAFS spectra of polyenes having even a relatively small number of vibrational modes. Ab initio simulations using the discrete linear coupling model confirm the common trends obtained by the continuum approach. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1287425
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  • 8
    Electronic Resource
    Electronic Resource
    An ab initio calculation of K-spectra in molecules HCl and HF (1977)
    Springer
    Theoretical chemistry accounts 44 (1977), S. 257-263 
    Details
    ISSN: 1432-2234
    Keywords: X-ray spectrum of HCl and HF ; Vibrational structure ; HCl ; HF
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract An ab initio calculation of energies and intensities of K-emission and K-absorption spectra in molecules HCl and HF are carried out. An electronic readjustment due to a hole in a molecular core is taken into account. A vibrational structure of K-emission and K-absorption spectra is also calculated. The calculation shows that it is possible to detect a vibrational structure of valence bonding levels in X-ray emission spectra. A good agreement with experiment is obtained.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00551168
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  • 9
    Electronic Resource
    Electronic Resource
    Band theory vs. Exciton theory interpretations of x-ray spectra of oligomers and polymers (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 63 (1997), S. 313-332 
    Details
    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We interpret x-ray absorption and emission spectra of conjugated molecules from two conceptually different approaches, band theory and exciton theory. We use a few polymers and their associate oligomer sequences as test grounds to characterize and compare x-ray spectra simulated by these two approaches. © 1997 John Wiley & Sons, Inc.
    Additional Material: 14 Ill.
    Type of Medium: Electronic Resource
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  • 10
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    Compatibility of quantitative X-ray spectroscopy with continuous distribution models of water at ambient conditions (2019)
    National Academy of Sciences
    Details
    Publication Date: 2019-02-19
    Description: The phase diagram of water harbors controversial views on underlying structural properties of its constituting molecular moieties, its fluctuating hydrogen-bonding network, as well as pair-correlation functions. In this work, long energy-range detection of the X-ray absorption allows us to unambiguously calibrate the spectra for water gas, liquid, and ice by the experimental atomic ionization cross-section. In liquid water, we extract the mean value of 1.74 ± 2.1% donated and accepted hydrogen bonds per molecule, pointing to a continuous-distribution model. In addition, resonant inelastic X-ray scattering with unprecedented energy resolution also supports continuous distribution of molecular neighborhoods within liquid water, as do X-ray emission spectra once the femtosecond scattering duration and proton dynamics in resonant X-ray–matter interaction are taken into account. Thus, X-ray spectra of liquid water in ambient conditions can be understood without a two-structure model, whereas the occurrence of nanoscale-length correlations within the continuous distribution remains open.
    Print ISSN: 0027-8424
    Electronic ISSN: 1091-6490
    Topics: Biology , Medicine , Natural Sciences in General
    Published by National Academy of Sciences
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