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X-ray scattering by molecules, surface-adsorbates and solids with a coupling to the nuclear motion
KTH, Superseded Departments, Chemistry.
2000 (English)Licentiate thesis, comprehensive summary (Other scientific)
Place, publisher, year, edition, pages
Stockholm: KTH , 2000. , vi, 36 p.
National Category
Chemical Sciences
Identifiers
URN: urn:nbn:se:kth:diva-1120ISBN: 91-7170-556-2 (print)OAI: oai:DiVA.org:kth-1120DiVA: diva2:6904
Note
QC 20100622Available from: 2000-09-26 Created: 2000-09-26 Last updated: 2010-06-22Bibliographically approved
List of papers
1. Duration of x-ray Raman scattering
Open this publication in new window or tab >>Duration of x-ray Raman scattering
1999 (English)In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 59, no 1, 380-389 p.Article in journal (Refereed) Published
Abstract [en]

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.

National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-13608 (URN)10.1103/PhysRevA.59.380 (DOI)
Note
QC 20100622Available from: 2010-06-22 Created: 2010-06-22 Last updated: 2017-12-12Bibliographically approved
2. Restoration of selection rules in nonadiabatic resonant inelastic x-ray scattering
Open this publication in new window or tab >>Restoration of selection rules in nonadiabatic resonant inelastic x-ray scattering
1997 (English)In: Journal of Experimental and Theoretical Physics, ISSN 1063-7761, E-ISSN 1090-6509, Vol. 85, no 1, 20-26 p.Article in journal (Refereed) Published
Abstract [en]

Recently a new effect in the Raman scattering of x-ray radiation has been predicted theoretically and discovered in experiments, the effect of restoration of the selection rules for the scattering tensor under strong electron-vibrational interaction. We propose a fairly simple model for describing this effect, a model that allows for an exact solution and takes into account the real vibrational structure of the molecule and electron-vibrational interaction.

Keyword
SYMMETRY-BREAKING, RAMAN-SCATTERING, AUGER-SPECTRA, EXCITATION, FLUORESCENCE, MOLECULES, EMISSION
National Category
Natural Sciences
Identifiers
urn:nbn:se:kth:diva-13634 (URN)A1997XT45000004 ()
Note
QC 20100622Available from: 2010-06-22 Created: 2010-06-22 Last updated: 2017-12-12Bibliographically approved
3. Role of electron-phonon interaction in resonant x-ray Raman scattering by polymers and solids
Open this publication in new window or tab >>Role of electron-phonon interaction in resonant x-ray Raman scattering by polymers and solids
1999 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 59, no 14, 9243-9258 p.Article in journal (Refereed) Published
Abstract [en]

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.

National Category
Natural Sciences
Identifiers
urn:nbn:se:kth:diva-13609 (URN)10.1103/PhysRevB.59.9243 (DOI)
Note
QC 20100622Available from: 2010-06-22 Created: 2010-06-22 Last updated: 2017-12-12Bibliographically approved
4. Continuum modeling of multi-mode vibronic excitations in near-edge x-ray absorption fine structure spectra
Open this publication in new window or tab >>Continuum modeling of multi-mode vibronic excitations in near-edge x-ray absorption fine structure spectra
2000 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 113, no 9, 3734-3740 p.Article in journal (Refereed) Published
Abstract [en]

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.

Keyword
restricted-step, molecules, polymers
National Category
Biological Sciences
Identifiers
urn:nbn:se:kth:diva-13636 (URN)10.1063/1.1287425 (DOI)000088878400030 ()
Note
QC 20100622Available from: 2010-06-22 Created: 2010-06-22 Last updated: 2017-12-12Bibliographically approved
5. Collapse of vibrational structure in spectra of resonant x-ray Raman scattering
Open this publication in new window or tab >>Collapse of vibrational structure in spectra of resonant x-ray Raman scattering
1997 (English)In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 56, no 1, 256-264 p.Article in journal (Refereed) Published
Abstract [en]

Extreme narrowing, or collapse, of electron-vibrational bands is predicted as a new phenomenon in resonant radiative and nonradiative x-ray scattering (RXS) spectra. It is shown that in the inelastic scattering case, that is, when the potential surfaces of ground and final slates are different, a considerable narrowing of the vibronic RXS band results from detuning the excitation Photon frequency omega away from the absorption resonance. By fine tuning the frequency, this may under special circumstances also occur in the region of strong photoabsorption. In the case of elastic Rayleigh scattering, that is, when the potential surfaces of ground and final states are identical, the narrowing results in a total collapse to a single resonance by detuning the frequency. The theory predicts how the differencies in vibrational structure of resonant and nonresonant photoemission spectra depends on the excitation frequency. The effect of frequency detuning on complex spectral multilevel structures due to multiplet: and spin-orbit splittings is discussed qualitatively. It is shown that the notion of: duration time for the x-ray scattering process plays a crucial role in the understanding of RXS spectra.

Keyword
AUGER-SPECTRA, EXCITATION, EMISSION, INTERFERENCE, SYMMETRY, STATES, DECAY
National Category
Natural Sciences
Identifiers
urn:nbn:se:kth:diva-13635 (URN)A1997XL64500030 ()
Note
QC 20100622Available from: 2010-06-22 Created: 2010-06-22 Last updated: 2017-12-12Bibliographically approved
6. Soft and hard x-ray Raman scattering by oriented symmetrical molecules: Selection rules, interference, and dephasing mechanisms
Open this publication in new window or tab >>Soft and hard x-ray Raman scattering by oriented symmetrical molecules: Selection rules, interference, and dephasing mechanisms
1998 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 109, no 12, 5060-5069 p.Article in journal (Refereed) Published
Abstract [en]

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.

National Category
Theoretical Chemistry
Identifiers
urn:nbn:se:kth:diva-13637 (URN)000076054500041 ()
Note
QC 20100622Available from: 2010-06-22 Created: 2010-06-22 Last updated: 2017-12-12Bibliographically approved
7. Temperature dependence and Debye-Waller factors for resonant x-ray Raman scattering in solids
Open this publication in new window or tab >>Temperature dependence and Debye-Waller factors for resonant x-ray Raman scattering in solids
2000 (English)In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 62, no 21, 13996-14005 p.Article in journal (Refereed) Published
Abstract [en]

Resonant x-ray Raman scattering is a strongly coherent process. The well defined phase relations between scattering channels through the core excited states localized at different atoms result in a conservation of the electronic crystal momentum. However, as we show here, the zero-point and thermal vibrations dephase these scattering channels and lead to incoherent contributions with different spectral shapes. The relative strength of the coherent and incoherent contributions are found to strongly depend on temperature via a Debye-Waller factor. This results in a characteristic temperature dependence of the spectral profile of the resonant x-ray Raman scattering.

Keyword
MOLECULES, PHOTOEMISSION, INTERFERENCE, DISPERSION, EMISSION, POLYMERS, SPECTRA, STATES, DECAY
National Category
Theoretical Chemistry
Identifiers
urn:nbn:se:kth:diva-13638 (URN)10.1103/PhysRevB.62.13996 (DOI)000165717200044 ()
Note
QC 20100622Available from: 2010-06-22 Created: 2010-06-22 Last updated: 2017-12-12Bibliographically approved

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