R. Püttner

Detailed study of pyridine at the C 1s and N 1s ionization thresholds: The influence of the vibrational fine structure

High resolution, vibrationally resolved, near-edge x-ray absorption fine structure (NEXAFS) spectra at the C 1s and N 1s ionization thresholds of pyridine and deuterated d5-pyridine in the gas phase have been recorded. The high resolution of 65 meV (150 meV) at the C s (N 1s) ionization thresholds reveals vibrational structures in the spectra. Detailed ab initio and density functional theory (DFT) calculations were performed to interpret the experimental spectra and to assign the observed peaks. In particular we focused on the previously unexplained intensity ratio for the two components of the C 1s→1π* transition. For this transition the vibrational structure is included through a linear coupling model in the DFT calculations and leads to the experimentally observed ∼2:3 intensity ratio between the two π* components in the C 1s spectrum rather than the ∼3:2 ratio obtained without vibrational effects. After inclusion of relaxation effects in the excited states, in addition to the vibrational effects, both...

Commissioning results and performance of the high-resolution Russian-German Beamline at BESSY II

Excellent results were obtained during commissioning of the Russian–German Beamline at bending-magnet D16-1A of the electron-storage ring BESSY II. The beamline is based on a Petersen-type plane-grating monochromator covering the XUV and soft X-ray photon-energy range. Photoionization spectra taken from the 2,–13 resonance of doubly excited He reveal a resolving power of E=DED100,000 at hn=64.12 eV. To our knowledge, this represents the best energy resolution achieved up to now on a bending-magnet beamline in the grazing-incidence photon-energy range. We present results on photon flux and dimensions of the light spot at the sample position, and show representative photoionization spectra of several gases studied in the course of the characterization of the beamline at various photon energies. The high stability of the beamline is demonstrated by the first observation of the weak 4,� 24 resonance of doubly excited He, which had not been observed before. r 2003 Elsevier Science B.V. All rights reserved.

Spectroscopic analysis of small organic molecules: A comprehensive near-edge x-ray-absorption fine-structure study of C6-ring-containing molecules

We report high-resolution C 1s near-edge x-ray-absorption fine-structure (NEXAFS) spectra of the C6-ring-containing molecules benzene (C6H6), 1,3- and 1,4-cyclohexadiene (C6H8), cyclohexene (C6H10), cyclohexane (C6H12), styrene (C8H8), and ethylbenzene (C8H10) which allow us to examine the gradual development of delocalization of the corresponding pi electron systems. Due to the high experimental resolution, vibrational progressions can be partly resolved in the spectra. The experimental spectra are compared with theoretical NEXAFS spectra obtained from density-functional theory calculations where electronic final-state relaxation is accounted for. The comparison yields very good agreement between theoretical spectra and experimental results. In all cases, the spectra can be described by excitations to pi*- and sigma*-type final-state orbitals with valence character, while final-state orbitals of Rydberg character make only minor contributions. The lowest C 1s-->1pi* excitation energy is found to agree in the (experimental and theoretical) spectra of all molecules except for 1,3-cyclohexadiene (C6H8) where an energy smaller by about 0.6 eV is obtained. The theoretical analysis can explain this result by different binding properties of this molecule compared to the others.

X-ray photoelectron studies of clean and oxidized α-GeTe(111) surfaces

Clean and oxidized (104–1015 L of O2) surfaces of α-GeTe have been investigated with x-ray photoelectron spectroscopy by using the synchrotron radiation facility BESSY II as well as an Al Kα source. To understand the first steps of oxidation, complementary quantum chemical calculations were performed. The cleaved surfaces of α-GeTe were found to be rumpled with (111) domains that can be related to the domain (twin) structure of the bulk. Both the Ge 3d and the Te 4d spectra of freshly cleaved surfaces exhibit at least three components, which are explained by a Ge or Te termination of the surface domains with possible contributions of a surface reconstruction. The surface oxidation starts at exposures of 104 L and proceeds via several steps. At low exposures, only changes in the Ge spectra are observed. Consequently, the first step of the reaction is associated with the formation of intermediate peroxidelike structures, wherein both oxygen atoms are bonded to germanium atoms. In the range of exposures between 1010 and 1015 L, a layer of a relatively stable oxidation product with the approximate stoichiometry Ge1+δ+4Te1−δ0O2(1+δ)2− is formed, which shows growth kinetics that obey a time-logarithmic law. At this stage, the peroxidelike structures are still present at the oxide/crystal interface. Once the oxidized layer exceeds a thickness of ≈2.5 nm at ∼1013 L, a transformation of the Te0 state into the Te+4 state is observed at the surface of the oxide layer. The final oxidation product can be described as mGeO2×nTeO2.Clean and oxidized (104–1015 L of O2) surfaces of α-GeTe have been investigated with x-ray photoelectron spectroscopy by using the synchrotron radiation facility BESSY II as well as an Al Kα source. To understand the first steps of oxidation, complementary quantum chemical calculations were performed. The cleaved surfaces of α-GeTe were found to be rumpled with (111) domains that can be related to the domain (twin) structure of the bulk. Both the Ge 3d and the Te 4d spectra of freshly cleaved surfaces exhibit at least three components, which are explained by a Ge or Te termination of the surface domains with possible contributions of a surface reconstruction. The surface oxidation starts at exposures of 104 L and proceeds via several steps. At low exposures, only changes in the Ge spectra are observed. Consequently, the first step of the reaction is associated with the formation of intermediate peroxidelike structures, wherein both oxygen atoms are bonded to germanium atoms. In the range of exposures betw...

Double-slit experiment with a polyatomic molecule: vibrationally resolved C 1s photoelectron spectra of acetylene

We report the first evidence for double-slit interferences in a polyatomic molecule, which we have observed in the experimental carbon 1s photoelectron spectra of acetylene (or ethyne). The spectra have been measured over the photon energy range of 310-930eV and show prominent oscillations in the intensity ratios g()/ u() for the vibrational quantum numbers = 0,1 and for the ratios s( = 1)/ s( = 0) for the symmetry s = g,u. The experimental findings are in very good agreement with ab initio density

S 2p photoexcitation spectra of SO2 at high resolution

The S 2p photoabsorption spectrum of gas-phase SO2 was studied at high resolution (ΔE≅30 meV) using synchrotron radiation. The spectrum contains excitations to both valence orbitals and Rydberg states. The lowest core-to-valence excitation, S 2p−13b1, was found to be fivefold split, and vibrational subbands originating from the symmetric-stretching mode could be resolved. A Franck–Condon analysis of these five states reveals different geometries depending on the symmetry of the total electronic wave function in the final state. In the Rydberg region, both the spin–orbit splitting (≅1.20 eV) and the ligand-field splitting (≅95 meV) were resolved.

Atomic Auger Doppler effects upon emission of fast photoelectrons

Studies of photoemission processes induced by hard X-rays including production of energetic electrons have become feasible due to recent substantial improvement of instrumentation. Novel dynamical phenomena have become possible to investigate in this new regime. Here we show a significant change in Auger emission following 1s photoionization of neon, which we attribute to the recoil of the Ne ion induced by the emission of a fast photoelectron. Because of the preferential motion of the ionized Ne atoms along two opposite directions, an Auger Doppler shift is revealed, which manifests itself as a gradual broadening and doubling of the Auger spectral features. This Auger Doppler effect should be a general phenomenon in high-energy photoemission of both isolated atoms and molecules, which will have to be taken into account in studies of other recoil effects such as vibrational or rotational recoil in molecules, and may also have consequences in measurements in solids.

State-dependent gerade/ungerade intensity ratios in the Auger spectrum of N2

We have applied both high-resolution photoelectron and Auger spectroscopy to study the N 1s core ionization of N2. From the respective spectra we are able to show that the Auger decay involves delocalized description of core-hole states. Specifically, the Auger spectra of the N 1sσg,u core holes to the quasi-stable final states X1Σ+g and D1Σ+u are presented. They exhibit the gerade/ungerade splitting as well as vibrational progressions. The 1sσg/1sσu Auger intensity ratios are, in agreement with ab initio calculations, 2.1 and 0.8 for the X and the D states respectively. The large value for the X state can qualitatively be related to the gerade or ungerade symmetry of the Auger electron.

K-shell photoionization of Be-like carbon ions : experiment and theory for C2+

Absolute cross-section measurements for K-shell photoionization of Be-like C2+ ions have been performed in the photon energy range 292 325 eV. These measurements have been made using the photon ion merged-beam endstation at the Advanced Light Source, Lawrence Berkeley National Laboratory. Absolute measurements compared with theoretical results from the R-matrix method indicate that the primary C2+ ion beam consisted of 62 percent ground-state (1s22s2 1S) and 38 percent metastable state (1s22s2p 3Po) ions. Reasonable agreement is seen between theory and experiment for absolute photoionization cross sections, resonance energies and autoionization line widths of K-shell-vacancy Auger states.

Vibrationally resolved Si 2p photoabsorption spectrum of SiF4

Abstract A high-resolution study of the Si 2p photoabsorption spectrum of gas-phase SiF 4 is reported. In the excitation region of the Rydberg orbitals, vibrational substates from symmetric stretching vibrational modes were resolved, which were described on the basis of a Franck-Condon analysis resulting in vibrational energies and equilibrium interatomic distances of the core-excited molecule. The natural lifetime width of the Si 2p core excitation amounts to 85 meV, a value that is substantially larger than the theoretically predicted one.