| 1. |
- Gel'mukhanov, Faris, et al.
(författare)
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Doppler effects in resonant x-ray Raman scattering
- 1998
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Ingår i: Physical Review A. Atomic, Molecular, and Optical Physics. - 1050-2947 .- 1094-1622. ; 57:4, s. 2511-2526
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Tidskriftsartikel (refereegranskat)abstract
- Theory for Doppler effects in resonant x-ray Raman scattering (RXS) is presented. It is shown that the "electron" Doppler effect is important in nonradiative RXS for decay transitions between continuum nuclear states lying above the dissociation threshold, and that the averaging of the RXS cross section over molecular orientations can lead to strong non-lorentzian broadenings of the atomiclike resonances. The Doppler effect is found to give a unique possibility to distinguish dissociating identical atoms, because different peaks correspond to atoms with opposite Doppler shifts. Spectral features of the atomiclike profile are predicted and analyzed. Strong oscillations of the RXS cross section will occur as a consequence of the interference of the Auger electrons. Due to the Doppler effect and the interference, the atomiclike profile can be associated with supernarrow spectral features, the width of which goes below the lifetime broadening and is practically independent of the spectral distribution of the incident radiation. As another consequence of the oscillations and strong anisotropy caused by the interference, we predict parity selection rules for Auger decay transitions in both bound and dissociative systems. The corresponding experiments can be realized by measurements of resonant Auger of surface adsorbed molecules and for molecules by the electron-ion coincidence technique.
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| 2. |
- Gel’mukhanov, Faris, et al.
(författare)
-
Duration of x-ray Raman scattering
- 1999
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Ingår i: Physical Review A. Atomic, Molecular, and Optical Physics. - 1050-2947 .- 1094-1622. ; 59:1, s. 380-389
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Tidskriftsartikel (refereegranskat)abstract
- There has recently been much interest in using the notion of a duration time to analyze resonant x-ray Raman scattering (RXS) of atoms, molecules, and solids. This notion implies a selection of processes with different time scales responsible for the formation x-ray Raman spectra, and has been useful for actual predictions of various phenomena associated with RXS and that subsequently have been experimentally verified. However, the notion of a duration time for the x-ray scattering event can also have some paradoxical consequences, as when comparing the RXS duration with the relaxation time of the wave packet evolution in the case when the inverse detuning of the excitation energy is shorter than the time of flight or the lifetime of the core excited state. We present here a solution of this contradiction and give a detailed analysis of the notion of the duration time for RXS. It is shown that this time is complex and consists of two qualitatively different contributions. The first originates in the irreversible decay of the core excited state, while the imaginary part is caused by a reversible dephasing in the time domain. We investigate also the evolution of the wave packets of bound and dissociative states to stationary distributions. The theoretical analysis is accompanied by numerical examples of the time evolution of the wave packet in bound and dissociative core excited states of the N2 and HCl molecules.
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| 3. |
- Privalov, Timofei, et al.
(författare)
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Role of electron-phonon interaction in resonant x-ray Raman scattering by polymers and solids
- 1999
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 59:14, s. 9243-9258
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Tidskriftsartikel (refereegranskat)abstract
- We present a theory of radiative resonant x-ray Raman scattering by solids and polymers beyond the Born-Oppenheimer approximation. The role of electron-phonon coupling of the intermediate core excited and final electronic states is analyzed in detail for π-electron systems and solids assuming the frozen-orbital approximation. The momentum exchange processes between valence electrons and phonons and with x-ray photons are analyzed, showing the similarities and differences between the two processes. The resonant inelastic x-ray scattering by a π-electron system is quenched up to zero in the Born-Oppenheimer approximation with frozen orbitals if the momentum exchange between valence electrons and x-ray photons is neglected. The electron-phonon and electron-photon interactions open the scattering channels for all occupied states in π systems. The frequency dependence of these effects is analyzed, showing that when the duration of the scattering is shortened by a large detuning of the excitation frequency, the role of electron-phonon coupling of both core-excited and final states is suppressed, depleting the cross section for π systems up to zero. The detuning quenches the symmetry breaking of the core-excited electronic states and results in a restoration of the selection rules and a conservation of electron momentum. Specific selection rules for the zero-phonon line in x-ray Raman spectra of linear polyenes are found. A detailed investigation of the narrowing, or collapse, of the electron-vibrational bands is given. When the detuning is large, the spectral profile is described by a joint density of states. It is predicted that the singularities of this joint density of states follows the Raman-Stokes dispersion law, something that allows a mapping of the band structure. We found that the phonon broadening of these singularities is completely quenched by detuning. A detailed investigation of the spectral shape versus detuning of the so-called excitonic band is given.
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| 4. |
- Salek, Pawel, et al.
(författare)
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Generalized Franck-Condon principle for resonant photoemission
- 1999
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Ingår i: Physical Review A. Atomic, Molecular, and Optical Physics. - 1050-2947 .- 1094-1622. ; 60:4, s. 2786-2791
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Tidskriftsartikel (refereegranskat)abstract
- A generalized Franck-Condon (GFC) principle for resonant x-ray Raman scattering and for resonant photoemission in particular is derived and numerically investigated. The GFC amplitudes differ from ordinary FC amplitudes by the presence of photon and photoelectron phase factors which describe the coupling-or interference-of the x-ray photons or Anger electrons with the nuclear motion. With the GFC amplitudes, a Kramers-Heisenberg relation is obtained for vibronic transitions that corrects the so-called lifetime-vibrational interference formula. For resonant photoemission in the soft-x-ray region involving typical bound potential surfaces, the generalization gives a contribution to the FC factors that can amount to 20%. For core excitation above the dissociation threshold, the GFC principle relates to Doppler effects on the ejected photoelectron both for the so-called "molecular'' and "atomic" bands. The role of the GFC principle in direct photoionization is briefly discussed.
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| 5. |
- Salek, Pawel, et al.
(författare)
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Wave-packet dynamics of resonant x-ray Raman scattering : Excitation near the Cl L-II,L-III edge of HCl
- 1999
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Ingår i: Physical Review A. Atomic, Molecular, and Optical Physics. - 1050-2947 .- 1094-1622. ; 59:2, s. 1147-1159
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Tidskriftsartikel (refereegranskat)abstract
- A theory of radiative and nonradiative x-ray Raman scattering based on nuclear wave-packet dynamics is presented. The theory is evaluated with special emphasis on the cases when the intermediate and/or final state potentials are dissociative. Different "one-step" and "two-step" time-dependent wave-packet formalisms are proposed and evaluated, giving different interpretational content and computational efficiency. An interference between the molecular background and the narrow atomiclike contributions is predicted and evaluated. Due to this interference the atomiclike spectral feature manifests itself as a peak or as a spectral hole depending on the circumstances in terms of excitation energy, spectral width of incident radiation, and the form of the interatomic potentials. The counterintuitive situation may even arise that a narrow peak is formed by increasing the spectral photon width. The duration of the resonant x-ray Raman scattering influences in a qualitatively different manner the space distributions of the wave packets in the molecular and the dissociative domains, something that is crucial for the formation of the cross section profile. It is shown that the scattering cross section is proportional to the square of the core excited wave packet and inversely proportional to the derivative of the difference between core excited and final state potentials. The atomiclike profile is shown to consist of a Lorentzian inner part and red or blue wings which give direct information about the long-range regions of the potentials; red wings for diverging potentials, and blue for converging. A technique of mapping of the space distribution of the squared core excited wave packet and the interatomic potentials is suggested. The various features are demonstrated by an ab initio computational study of the resonant Auger spectra of the HCl molecule close to the Cl L-II,L-III edge.
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| 6. |
- Ågren, Hans, et al.
(författare)
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Dynamics of Inner Shell Resonant Raman Scattering.
- 1999
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Ingår i: Journal of the Japanese Society for Synchrotron Radiation Research. - 0914-9287. ; 12:4, s. 257-267
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Tidskriftsartikel (refereegranskat)abstract
- Some recent advances in the theory of the resonant Raman process for atoms, molecules and solids involving inner shell electrons are reviewed. Special emphasis is put on the dynamical aspects and on the notion of a duration time for the x-ray Raman process (RXS), which brings about a distinetion of processes with different time scales responsible for the formation of the spectral profile. This notion has been useful for actual predietions of various phenomena associated with RXS such as "symmetry restoration" "vibrational collapse", and "control of dissociation". The temporal theory of RXS is based on the wave packet formalism. The increase of the broad "molecular" parts relative to the atomic, or fragment, decay upon frequency detuning is demonstrated. The atomic-like resonance and molecular parts are proved to show different dispersion relations. Under certain conditions the interference between the molecular and atomic parts produces conspicuous "spectral holes". These conceptual tools are applicable also in the case of solids; there is an analogous restoration of momentum selection rules and a collapse effect upon detuning the frequency. Thus when the duration of the scattering is shortened by a large detuning, the role of electron-phonon coupling of the core excited states is suppresssed. The shortening of the RXS duration in a certain sense leads to a deloealization of the core hole in a solid. We describe also some new features that derive from the Doppler effect on ejected Auger electrons.
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