Gunnar Öhrwall

The electronic structure of free water clusters probed by Auger electron spectroscopy

(H2O)(N) clusters generated in a supersonic expansion source with N approximately 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.

The size of neutral free clusters as manifested in the relative bulk-to-surface intensity in core level photoelectron spectroscopy

A new approach for obtaining an estimate of the effective size of the free neutral clusters is proposed. The approach relies on an experimental measure of the surface and interior or bulk cluster atoms provided by the x-ray photoelectron spectroscopy and on a model for the attenuation of photoelectrons ejected from the bulk of the cluster as the result of the ionizing irradiation. The experimental part gives the ratio of the electron signal from the bulk cluster atoms to that from the cluster surface atoms for a wide range of cluster sizes and electron kinetic energies. The attenuated response of the bulk atoms is modeled using an exponential law with the cluster size and kinetic-energy-dependent electron escape depth as parameters. For the experimental size range, model-based calculations for Ar, Kr, and Xe clusters are presented. The cluster size estimates obtained from comparison of the model calculations and experimental results agree well with those determined from the parameters of the cluster creation process. The combination of experiment and modeling also makes it possible to estimate the effective escape depth for electron propagation in free clusters. For Ar, Kr, and Xe clusters of varying mean size, absolute determination of the surface and bulk electron binding energies of the core levels used in the experiments has also been made.

Observation of resonant interatomic coulombic decay in Ne clusters.

We have measured the electron spectra of Ne clusters after excitation with photon energies around the 2s inner valence threshold. At two photon energies below threshold, a resonantly enhanced surplus of low kinetic-energy electrons is observed. The kinetic energy of the peak does not vary with the photon energy and is slightly larger than the transition energy of Interatomic Coulombic Decay (ICD) above threshold. This leads us to assume that an ICD-like process is present. In analogy to the Auger and the resonant Auger decay this new phenomenon is termed resonant ICD.

Observation of elastic scattering effects on photoelectron angular distributions in free Xe clusters

We report an observation of substantial deviations in the photoelectron angular distribution for photoionization of atoms in free Xe clusters compared to the case of photoionization of free atoms. The cross section, however, seems not to vary between the cluster and free atoms. This observation was made in the vicinity of the Xe 4d Cooper minimum, where the atomic angular distribution is known to vary dramatically. The angular distribution of electrons emitted from atoms in the clusters is more isotropic than that of free atoms over the entire kinetic energy range studied. Furthermore, the angular distribution is more isotropic for atoms in the interior of the clusters than for atoms at the surface. We attribute this deviation to elastic scattering of the outgoing photoelectrons. We have investigated two average cluster sizes, (N) approximate to 4000 and 1000 and found no significant differences between these two cases. (Less)

A photoelectron spectroscopic study of aqueous tetrabutylammonium iodide

Photoelectron spectra of tetrabutylammonium iodide (TBAI) dissolved in water have been recorded using a novel experimental set-up, which enables photoelectron spectroscopy of volatile liquids. The set-up is described in detail. Ionization energies are reported for I− 5p, I− 4d, C 1s and N 1s. The C 1s spectrum shows evidence of inelastic scattering of the photoelectrons, that differs from the case of TBAI in formamide.

The local structure of small water clusters: imprints on the core-level photoelectron spectrum

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.

Core excitations of naphthalene: Vibrational structure versus chemical shifts

High-resolution x-ray photoelectron emission (XPS) and near-edge x-ray absorption fine structure (NEXAFS) spectra of naphthalene are analyzed in terms of the initial state chemical shifts and the vibrational fine structure of the excitations. Carbon atoms located at peripheral sites experience only a small chemical shift and exhibit rather similar charge-vibrational coupling, while the atoms in the bridging positions differ substantially. In the XPS spectra, C-H stretching modes provide important contributions to the overall shape of the spectrum. In contrast, the NEXAFS spectrum contains only vibrational progressions from particular C-C stretching modes. The accuracy of ab initio calculations of absolute electronic transition energies is discussed in the context of minute chemical shifts, the vibrational fine structure, and the state multiplicity.

Anion and cation formation following core-level photoexcitation of CO2

Mass-resolved anion and cation partial-yield spectra following photoexcitation of CO2 have been recorded in the vicinity of the C 1s and O 1s thresholds. Anion production was found to be site specific: a weak production of O− was observed on the π* resonance near the C K edge, whereas both C− and O− fragments were observed near the O K edge. Due to suppression of the shape resonances that dominate the cross section above the core thresholds, the anion-partial-yield spectra at the O K edge reveal several previously unobserved electronic states. In addition, several cations such as CO2+ and O2+ were observed for the first time.

Femtosecond pump–probe photoelectron spectroscopy of predissociative Rydberg states in acetylene

We employ a pump–probe approach to molecular photoionization to study fast dissociation of Rydberg states in acetylene. By using time-resolved photoelectron spectroscopy to study the electronic state of the resulting ions we are able to monitor the system continuously during dissociation or rearrangement. We find that the predissociative lifetime for the 3R′′′ (v2′=1) Rydberg state is about 150 fs. We demonstrate a powerful new technique using time-correlated femtosecond harmonic generation and laser light pulses to study the time evolution of ultrafast dynamic processes in molecules.

Very high resolution electron spectroscopy with third generation synchrotron radiation sources: the resonant Auger decay spectrum of the Xe 4d5/2−16p state

Abstract We have recorded an Auger decay spectrum of the resonantly excited Xe 4d 5/2 −1 6p state between 36.4 and 37.3 eV kinetic energy. The spectrum was obtained under resonant Raman conditions, with a bandwidth of ≈6 meV for the exciting radiation, much smaller than the natural lifetime width of 110–120 meV. The full width at half maximum of the peaks in the spectrum was 10 meV, making it possible to completely resolve structure that overlapped in earlier studies.