| 1. |
- Martensson, Nils, et al.
(författare)
-
On the relation between X-ray Photoelectron Spectroscopy and XAFS
- 2013
-
Ingår i: 15th International Conference On X-Ray Absorption Fine Structure (XAFS15). - : IOP Publishing.
-
Konferensbidrag (refereegranskat)abstract
- XAFS and X-ray Photoelectron Spectroscopy (XPS) are element specific techniques used in a great variety of research fields. The near edge regime of XAFS provides information on the unoccupied electronic states of a system. For the detailed interpretation of the XAFS results, input from XPS is crucial. The combination of the two techniques is also the basis for the so called core-hole clock technique. One of the important aspects of photoelectron spectroscopy is its chemical sensitivity and that one can obtain detailed information about the composition of a sample. We have for a series of carbon based model molecules carefully investigated the relationship between core level photoelectron intensities and stoichiometry. We find strong EXAFS-like modulations of the core ionization cross sections as function of photon energy and that the intensities at high photon energies converge towards values that do not correspond to the stoichiometric ratios. The photoelectron intensities are dependent on the local molecular structure around the ionized atoms. These effects are well described by molecular calculations using multiple scattering theory and by considering the effects due to monopole shake-up and shake-off as well as to intramolecular inelastic scattering processes.
|
|
| 2. |
- Söderström, Johan, et al.
(författare)
-
Nonstoichiometric Intensities in Core Photoelectron Spectroscopy
- 2012
-
Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 108:19, s. 193005-
-
Tidskriftsartikel (refereegranskat)abstract
- X-ray photoemission spectroscopy is used in a great variety of research fields; one observable is the sample's stoichiometry. The stoichiometry can be deduced based on the expectation that the ionization cross sections for innershell orbitals are independent of the molecular composition. Here we used chlorine-substituted ethanes in the gas phase to investigate the apparent carbon stoichiometry. We observe a nonstoichiometric ratio for a wide range of photon energies, the ratio exhibits x-ray-absorption fine structure spectroscopy (EXAFS)-like oscillations and hundreds of eV above the C1s ionization approaches a value far from 1. These effects can be accounted for by considering the scattering of the outgoing photoelectron, which we model by multiple-scattering EXAFS calculations, and by considering the effects of losses due to monopole shakeup and shakeoff and to intramolecular inelastic scattering processes.
|
|
| 3. |
- Travnikova, O., et al.
(författare)
-
Energy dependent relative cross sections in carbon 1s photoionization
- 2020
-
Ingår i: 31ST INTERNATIONAL CONFERENCE ON PHOTONIC, ELECTRONIC AND ATOMIC COLLISIONS (ICPEAC XXXI). - : IOP PUBLISHING LTD.
-
Konferensbidrag (refereegranskat)abstract
- We demonstrate that the possibility of monitoring relative photoionization cross sections over a large photon energy range extending to hard x-rays allows studying and disentangling shake processes and intramolecular inelastic scattering effects. An "EXAFS"-type of oscillatory behaviour of the relative C is cross sections is observed in a wide photon energy range up to 1 keV due to multiple intramolecular elastic scattering of the photoelectron. These effects are also present in solids and constitute a significant part of all elastic and inelastic scattering effects occurring in solids.
|
|
| 4. |
- Travnikova, Oksana, et al.
(författare)
-
Selective vibrational excitation in the resonant Auger decay following core-to-pi transitions in N2O
- 2010
-
Ingår i: Journal of Electron Spectroscopy and Related Phenomena. - : Elsevier BV. - 0368-2048 .- 1873-2526. ; 181:2-3, s. 129-134
-
Tidskriftsartikel (refereegranskat)abstract
- In N2O a detailed study of the vibrational distribution of the (X) over tilde state reached after decay of core-to-pi* excitation of N terminal, N central and 0 1s core levels is reported. We observe a change in the relative intensity of bending versus stretching modes while scanning the photon energy across all three resonances. While this effect is known to be due to the Renner-Teller splitting in the core-excited states, we could derive that the antisymmetric stretching is excited mainly in the decay of the N terminal 1s-to-pi* excitation. An explanation for such selectivity is provided in terms of interplay of vibrational structure on potential energy surfaces of different electronic states involved in the process.
|
|
| 5. |
- Abu-samha, M, et al.
(författare)
-
The local structure of small water clusters : imprints on the core-level photoelectron spectrum
- 2009
-
Ingår i: Journal of Physics B. - : IOP Publishing. - 0953-4075 .- 1361-6455. ; 42:5, s. 055201-
-
Tidskriftsartikel (refereegranskat)abstract
- We report on an O 1s photoelectron-spectroscopy study of small neutral water clusters produced by adiabatic expansion. The photoelectron spectra were acquired under two different experimental conditions. At intermediate resolution, the cluster signal was characterized by a very broad O 1s peak with a flat top. In the second set of measurements, resolution was significantly increased at the cost of lower count rates. The cluster signal was now partly resolved into a bimodal structure. Extensive theoretical calculations were undertaken to facilitate an interpretation of the spectrum. These results suggest that the bimodal feature may be ascribed to ionization of water molecules in different hydrogen-bonding configurations, more specifically, molecules characterized by donation of either one or both hydrogen atoms in H-bonding.
|
|
| 6. |
- Harnes, J., et al.
(författare)
-
Neutral CH3Cl and CH3Br clusters studied by X-ray photoelectron spectroscopy and modeling : Insight to intermolecular interactions and structure
- 2008
-
Ingår i: Journal of Electron Spectroscopy and Related Phenomena. - : Elsevier BV. - 0368-2048 .- 1873-2526. ; 166, s. 53-64
-
Tidskriftsartikel (refereegranskat)abstract
- Single-component clusters of methyl chloride and methyl bromide have been produced by adiabatic expansion and their C 1s, Cl 2p and Br 3d photoelectron spectra recorded using synchrotron radiation and a high-resolution electron analyzer. The experimentally observed cluster-to-monomer shifts in core-level ionization energies are interpreted in terms of theoretical models based on molecular dynamics (MD) in conjunction with polarizable force fields developed and validated in the course of this work. MD simulations have also been used to explore the global and local structure of the clusters, providing evidence for a predominance of anti-parallel, head-to-tail arrangement of neighboring molecules. Whereas the cluster-to-monomer shifts are strongly dominated by polarization effects, the polarization contribution is very similar for ionization of carbon and the halogen, respectively. The difference in cluster shifts between the two ionization sites within the same molecule, C vs. Cl or C vs. Br, is thus determined by permanent electrostatic moments, i.e. the magnitude and direction of the permanent dipole moment and the local alignment of dipoles.
|
|
| 7. |
- Harnes, J., et al.
(författare)
-
Size of Free Neutral CO(2) Clusters from Carbon 1s Ionization Energies
- 2011
-
Ingår i: Journal of Physical Chemistry A. - : American Chemical Society (ACS). - 1089-5639 .- 1520-5215. ; 115:38, s. 10408-10415
-
Tidskriftsartikel (refereegranskat)abstract
- Free neutral CO(2) clusters were produced by adiabatic expansion and characterized by carbon 1s (C1s) photoelectron spectroscopy using synchrotron radiation. The shift in C1s ionization energy (IE) between the cluster and the monomer, i.e., Delta IE = IE(cluster) - IE(monomer), was found to vary systematically with the experimental conditions. A functional relationship is established between the mean cluster size in the beam, < N >, and Delta IE, in good agreement with theoretical calculations of shifts in ionization energy for model clusters. This makes it possible to use core-level photoelectron spectroscopy to monitor the mean cluster size and also to estimate < N > from expansion conditions.
|
|
| 8. |
- Harnes, J., et al.
(författare)
-
The structure of mixed methanol/chloroform clusters from core-level photoelectron spectroscopy and modeling
- 2011
-
Ingår i: New Journal of Chemistry. - : Royal Society of Chemistry (RSC). - 1144-0546 .- 1369-9261. ; 35:11, s. 2564-2572
-
Tidskriftsartikel (refereegranskat)abstract
- Mixed neutral molecular clusters have been produced by co-expansion of chloroform and methanol and characterized by carbon 1s photoelectron spectroscopy and theoretical modeling. The produced clusters are in the range 50-150 molecules and the clusters are not homogeneously mixed: chloroform is enriched in the bulk and methanol is found on or close to the surface. This is based on evidence from photoelectron depth profiling and molecular dynamics simulations of mixed clusters. The simulations suggest that methanol forms cyclic or linear oligomers in the surface layers of the mixed clusters. From thermodynamic models, the enrichment of methanol on the cluster surface can be rationalized from the difference in surface tension between the two pure components, which is connected to the qualitative differences in the respective bonding patterns.
|
|
| 9. |
|
|
| 10. |
- Patanen, M., et al.
(författare)
-
Valence photoionization and photoelectron-photoion coincidence (PEPICO) study of molecular LiCl and Li2Cl2
- 2012
-
Ingår i: Journal of Electron Spectroscopy and Related Phenomena. - : Elsevier BV. - 0368-2048. ; 185:8-9, s. 285-293
-
Tidskriftsartikel (refereegranskat)abstract
- Molecular LiCl and Li2Cl2 have been studied in the vapor phase with valence photoelectron and photoelectron-photoion coincidence spectroscopies. These two techniques determine the binding energies in fundamentally different ways. Binding energies obtained from photoelectron spectra are usually taken as the vertical ionization energies of the corresponding electronic states. In cases with several overlapping bands, corresponding to different electronic states, the coincidence measurement can separate the bands if the respective final states fragment differently. This applies well to the monomer case. To facilitate the determination of state-specific ionization energies in the dimeric molecule, a theoretical Franck-Condon analysis has been carried out. Moreover, ab initio coupled-cluster and density-functional-theory calculations have been used to analyze the fragmentation pattern based on asymptotic dissociation energies. The fragmentation pattern is largely common to all the accessible valence-ionized states of the chiller, consistent with rapid conversion to the ionic ground state before fragmentation. However, the highest-lying state of Li2Cl2+, (2)A(g). shows enhanced propensity for Li+ as dissociation product. (C) 2012 Elsevier B.V. All rights reserved.
|
|
| 11. |
- Sorensen, S. L., et al.
(författare)
-
The O 1s photoelectron spectrum of molecular oxygen revisited
- 2008
-
Ingår i: Journal of Physics B. - : IOP Publishing. - 0953-4075 .- 1361-6455. ; 41:9, s. 095101-
-
Tidskriftsartikel (refereegranskat)abstract
- High-resolution photoelectron spectra of the inner-shell levels of molecular oxygen have been measured using synchrotron radiation. The vibrational structure of the two magnetically-split core-shell components is analyzed based upon ab initio calculations. The ratio between the intensities of the two components was analyzed at several different ionization energies up to about 600 eV, and the same is discussed and compared to high-energy ionization intensities. A theoretical calculation shows very good agreement with the measured spectra. The calculation implements a model where two parity components make up the 4Σ peak profile. The gerade-ungerade energy split for this state is found to be 50 meV.
|
|
| 12. |
- Öhrwall, Gunnar, et al.
(författare)
-
The electronic structure of free water clusters probed by Auger electron spectroscopy
- 2005
-
Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 123, s. 054310-
-
Tidskriftsartikel (refereegranskat)abstract
- (H2O)(N) clusters generated in a supersonic expansion source with N similar to 1000 were core ionized by synchrotron radiation, giving rise to core-level photoelectron and Auger electron spectra (AES), free from charging effects. The AES is interpreted as being intermediate between the molecular and solid water spectra showing broadened bands as well as a significant shoulder at high kinetic energy. Qualitative considerations as well as ab initio calculations explain this shoulder to be due to delocalized final states in which the two valence holes are mostly located at different water molecules. The ab initio calculations show that valence hole configurations with both valence holes at the core-ionized water molecule are admixed to these final states and give rise to their intensity in the AES. Density-functional investigations of model systems for the doubly ionized final states-the water dimer and a 20-molecule water cluster-were performed to analyze the localization of the two valence holes in the electronic ground states. Whereas these holes are preferentially located at the same water molecule in the dimer, they are delocalized in the cluster showing a preference of the holes for surface molecules. The calculated double-ionization potential of the cluster (22.1 eV) is in reasonable agreement with the low-energy limit of the delocalized hole shoulder in the AES.
|
|
