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X-ray absorption and resonant Auger spectroscopy of O(2) in the vicinity of the O 1s ->sigma* resonance: Experiment and theory
KTH, School of Biotechnology (BIO), Theoretical Chemistry (closed 20110512).
2008 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 128, no 6, 064304Article in journal (Refereed) Published
##### Abstract [en]

We report on an experimental and theoretical investigation of x-ray absorption and resonant Auger electron spectra of gas phase O(2) recorded in the vicinity of the O 1s ->sigma* excitation region. Our investigation shows that core excitation takes place in a region with multiple crossings of potential energy curves of the excited states. We find a complete breakdown of the diabatic picture for this part of the x-ray absorption spectrum, which allows us to assign an hitherto unexplained fine structure in this spectral region. The experimental Auger data reveal an extended vibrational progression, for the outermost singly ionized X (2)Pi(g) final state, which exhibits strong changes in spectral shape within a short range of photon energy detuning (0 eV>Omega>-0.7 eV). To explain the experimental resonant Auger electron spectra, we use a mixed adiabatic/diabatic picture selecting crossing points according to the strength of the electronic coupling. Reasonable agreement is found between experiment and theory even though the nonadiabatic couplings are neglected. The resonant Auger electron scattering, which is essentially due to decay from dissociative core-excited states, is accompanied by strong lifetime-vibrational and intermediate electronic state interferences as well as an interference with the direct photoionization channel. The overall agreement between the experimental Auger spectra and the calculated spectra supports the mixed diabatic/adiabatic picture.

##### Place, publisher, year, edition, pages
American Institute of Physics (AIP), 2008. Vol. 128, no 6, 064304
##### Keyword [en]
Auger electron spectroscopy, Electron scattering, Electronic states, Excited states, Potential energy, Vibrational spectra, X ray absorption, Resonant Auger spectroscopy, Spectral region, Vicinity
##### National Category
Theoretical Chemistry
##### Identifiers
ISI: 000253238200017PubMedID: 18282035ScopusID: 2-s2.0-39349091895OAI: oai:DiVA.org:kth-9734DiVA: diva2:127310
##### Note

QC 20100916

Available from: 2008-12-04 Created: 2008-12-04 Last updated: 2016-07-22Bibliographically approved
##### In thesis
1. Quantum nuclear dynamics in  x-ray scattering and lasing
Open this publication in new window or tab >>Quantum nuclear dynamics in  x-ray scattering and lasing
2008 (English)Doctoral thesis, comprehensive summary (Other scientific)
##### Abstract [en]

This thesis presents a theoretical study of the role of nuclear degrees of freedom in the x-ray absorption, x-ray resonant scattering  and some aspects of the interaction of matter with strong laser fields. Most numerical simulations are performed with a time-dependent wave-packet program that have proved its robustness  in previous investigations. The relevant experimental results are also presented for comparison when available.

The first problem considered in the thesis is the possibility of obtaining x-ray absorption spectra with resolution beyond the natural lifetime broadening of the core-excited electronic states. It is shown that the method of measuring x-ray absorptionin the resonant scattering mode suggested earlier for that purpose exhibits severe limitations originating from the lifetime vibrational interference between the intermediate core-excited vibrational levels. However, a broad class of molecules is found for which spectra with super-high resolution can indeed be obtained. These molecules have parallel potential energy surfaces of the core-excited and final states for the x-ray scattering process.

The interpretations of two interesting cases of x-ray absorption and Auger scattering follow. The first one is related to scattering through a doubly excited Π state in the CO molecule. A Doppler split feature near 299.4 eV and strong scattering anisotropy are  observed. Both features are well explained and reproduced by the theory. Next, theelectron-vibrational fine structure of the $O1s\rightarrow\sigma^{*}$ excitation for O2 is investigated by means of different models. We are able to single out the electronic states and interpotential crossing points responsible for the peculiar absorption profile. Based on these findings we explain and reproduce the x-ray Auger scattering spectra through the same excitation. Here we encounter a rather unusual situation in which the Auger spectra are affected by three types of the interference: Apart from the lifetime vibrational interference, a strong interference between two intermediate electronic states and an interference with the direct-scattering amplitude is also present.

The process of intramolecular vibrational redistribution (IVR) is investigated in the context of formation of amplified spontaneous emissions (ASE) inside laser-pumped gain media. IVR raises to a higher energy region the threshold pump intensity after which blue-shifted ASE is observed.

Finally, we suggest a new scheme of x-ray pump-probe spectroscopy based on the core-hole hopping in N2 induced by an infrared laser field. We investigate the result from the core-hole hoping on the vibrational structure of the x-ray absorption profile. Furthermore, by populating core-excited states with opposite parities, the laser field opens up symmetry forbidden resonant inelastic scattering channels, which can give new insights about the electronic structure of matter.

##### Place, publisher, year, edition, pages
Stockholm: KTH, 2008. x, 63 p.
##### Series
Trita-BIO-Report, ISSN 1654-2312 ; 2008:28
##### National Category
Theoretical Chemistry
##### Identifiers
urn:nbn:se:kth:diva-9728 (URN)978-91-7415-159-6 (ISBN)
##### Public defence
2008-12-17, FB42, AlbaNova, Roslagstullsbacken 21, Stockholm, 10:00 (English)
##### Note
QC 20100917Available from: 2008-12-10 Created: 2008-12-03 Last updated: 2010-09-17Bibliographically approved

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