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Preferential site occupancy of krypton atoms on free argon-cluster surfaces
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics.
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Materials Science.
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Materials Science.
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics.
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2006 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 125, no 1, 014305Article in journal (Refereed) Published
Abstract [en]

Argon clusters have been doped with krypton atoms in a pick-up setup and investigated by means of ultraviolet and x-ray photoelectron spectroscopy (UPS and XPS). The width of the krypton surface feature in the XPS spectra from mixed krypton/argon clusters has been studied and found to be narrower than in the case of homogeneous krypton clusters. By considering known spectral broadening mechanisms of the cluster features and the electron binding energy shift of the cluster surface feature relative to the atomic signal, we conclude that krypton ad-atoms preferentially occupy high-coordination surface sites on the argon host-cluster.

Place, publisher, year, edition, pages
2006. Vol. 125, no 1, 014305
National Category
Physical Chemistry
Identifiers
URN: urn:nbn:se:uu:diva-96753DOI: 10.1063/1.2209677ISI: 000238849500017PubMedID: 16863296OAI: oai:DiVA.org:uu-96753DiVA: diva2:171432
Available from: 2008-02-28 Created: 2008-02-28 Last updated: 2017-12-14Bibliographically approved
In thesis
1. A Treatise on the Geometric and Electronic Structure of Clusters: Investigated by Synchrotron Radiation Based Electron Spectroscopies
Open this publication in new window or tab >>A Treatise on the Geometric and Electronic Structure of Clusters: Investigated by Synchrotron Radiation Based Electron Spectroscopies
2008 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Clusters are finite ensembles of atoms or molecules with sizes in the nanometer regime (i.e. nanoparticles). This thesis present results on the geometric and electronic structure of homogeneous and heterogeneous combinations of atoms and molecules. The systems have been studied with synchrotron radiation and valence, core and Auger electron spectroscopic techniques.

The first theme of the thesis is that of mixed clusters. It is shown that by varying the cluster production technique both structures that are close to that predicted by equilibrium considerations can be attained as well as far from equilibrium structures.

Electronic processes following ionization constitute the second theme. The post-collision interaction phenomenon, energy exchange between the photo- and the Auger electrons, is shown to be different in clusters of argon, krypton and xenon. A model is proposed that takes polarization screening in the final state into account. This result is of general character and should be applicable to the analysis of core level photoelectron and Auger electron spectra of insulating and semi-conducting bulk materials as well.

Interatomic Coloumbic Decay is a process that can occur in the condensed phases of weakly bonded systems. Results on the time-scale of the process in Ne clusters and mixed Ar/Ne clusters are herein discussed, as well observations of resonant contributions to the process. In analogy to Auger vis-à-vis Resonant Auger it is found that to the ICD process there is a corresponding Resonant ICD process possible. This has later been observed in other systems and by theoretical calculations as well in subsequent works by other groups.

Delocalization of dicationic valence final states in the hydrogen bonded ammonia clusters and aqueous ammonia has also been investigated by Auger electron spectroscopy. With those results it was possible to assign a previously observed feature in the Auger electron spectrum of solid ammonia.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2008. 103 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 393
Keyword
Cluster, nanoparticle, Heterogeneous, Homogeneous, Mixed, Cluster production methods, co-expansion, pick-up, doping, Cluster geometry, radial segregation, Interatomic Coulombic Decay, ICD, Auger electron spectroscopy, AES, X-ray photoelectron spectroscopy, XPS, Ultra-violet spectroscopy, UPS, Noble gas, Rare gas, Ne, Ar, Kr, Xe, Ammonia, NH3, Sulphur hexafluoride, SF6, MAX-lab, BESSY
National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-8463 (URN)978-91-554-7095-1 (ISBN)
Public defence
2008-03-14, Häggsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 10:15
Opponent
Supervisors
Available from: 2008-02-28 Created: 2008-02-28Bibliographically approved
2. Rare-gas Clusters Studied by Electron Spectroscopy: Structure of Heterogeneous Clusters and Effects of Electron Scattering on Auger Decay
Open this publication in new window or tab >>Rare-gas Clusters Studied by Electron Spectroscopy: Structure of Heterogeneous Clusters and Effects of Electron Scattering on Auger Decay
2007 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In this Thesis experimental studies of nano-clusters using synchrotron radiation based photoelectron (UPS and XPS) and Auger Electron Spectroscopy (AES) are presented. The investigations may be divided into two parts where the first reports on the structure of heterogeneous two component clusters, and the second concerns electronic decay processes.

Using photoelectron spectroscopies as investigative tools the radial composition of heteroclusters of argon mixed with xenon, krypton or neon has been determined. Two methods of heterogeneous cluster production were employed: co-expansion and doping/pick-up. By analyzing the line shapes, energy positions, and widths of the spectral cluster features the radial composition of the clusters produced by co-expansion were found to form close-to-equilibrium structures, placing the component with larger cohesive energy in the cluster core while the second component was to varying degree segregated toward the surface. By instead using the doping/pick-up technique the opposite radial structures, i.e. far-from-equilibrium structures, may be formed. In the case of argon/krypton clusters a similar surface structure is formed regardless of production technique.

The second part of the Thesis concerns post-ionization decay processes. Experimental evidence for the Interatomic Coulombic Decay process, a theoretically predicted decay channel, is presented in a study of homogeneous neon clusters. The time scale of the decay was determined to 6±1 fs for bulk atoms and >30 fs for surface atoms in the neon cluster, showing the connection between local geometry and dynamics of the decay.

Another channel for electronic relaxation is Auger decay. This Thesis presents a method of disentangling the spectral surface and bulk responses from clusters in Auger spectra. Studies of argon clusters show that the AES technique is more surface sensitive than XPS, even at the same electron kinetic energy. Furthermore, the effect scattering of the photoelectron has on the Auger spectra was investigated. Special effort was put into explaining an experimentally observed photon energy dependent intensity appearing on the high-kinetic energy side on the Auger signal. We propose that this intensity is due to a solid state-specific photoelectron recapture process we name Pre-Auger Recapture (PAR), which affects the kinetic energy of the Auger electrons.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2007. 85 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 260
Keyword
Physics, Nano-particles, Clusters, Heterogeneous clusters, Core-shell clusters, XPS, UPS, AES, Electron scattering, Radial structure, Surface structure, Surface sensitivity, Synchrotron radiation, Ne, Ar, Kr, Xe, Fysik
National Category
Natural Sciences
Identifiers
urn:nbn:se:uu:diva-7431 (URN)978-91-554-6769-2 (ISBN)
Public defence
2007-02-09, Polhemsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 10:15 (English)
Opponent
Supervisors
Available from: 2007-01-16 Created: 2007-01-16 Last updated: 2016-08-24Bibliographically approved

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