Andy Rüdel

A comprehensive photoabsorption, photoionization, and shake-up excitation study of the C 1s cross section of benzene

The absolute photoabsorption cross section of benzene (C6H6), encompassing the C 1s−1 π*e2u resonance, the C 1s threshold, the satellite thresholds, and extending up to 800 eV, has been measured using synchrotron radiation. Measurements of the discrete absorption structure from below the C 1s ionization threshold have been performed at high resolution. In order to unambiguously assign all structure present in the photoabsorption cross section, C 1s photoelectron spectra were measured from the C 1s threshold region up to 350 eV along with satellite spectra. The C 1s−1 single-hole and the satellite cross sections have been derived in absolute units, and their angular distributions have been determined. Resonant and normal Auger spectra were taken on the main features of the photoabsorption and single-hole cross sections. From the best resolved photoelectron spectra the underlying structure in the asymmetric benzene photoelectron peak can be partly disentangled. The experimental data show that at least two v...

Resonant photofragmentation of methanol at the carbon and oxygen K-edge by high-resolution ion-yield spectroscopy

Resonant photofragmentation of core-excited methanol has been studied with high-resolution partial ion-yield spectroscopy near both the carbon and oxygen K-edge. Non-coincident and coincident fragmentation channels were monitored simultaneously. A steep increase in the coincidence yield curves above the ionization threshold, where the normal Auger decay produces doubly charged or multiply charged species which yield two or more charged fragments, has been observed. A relevant difference in resonant structure has been observed while comparing the fragmentation at the carbon K-edge and the oxygen K-edge. Evidence for inter-atomic Auger decay is presented for one particular single channel around the oxygen K-edge.

The resonant Auger spectra of formic acid, acetaldehyde, acetic acid and methyl formate

Abstract We have measured the resonant Auger spectra of four simple organic molecules containing a carbonyl (CO) group, following excitation of an oxygen 1s electron to an antibonding valence orbital. The full range of final state energies is presented. The spectra are analysed for similarities arising from similar chemical structures within the molecules. An ab initio calculation of the resonant Auger spectrum of the simplest molecule, formic acid, is shown to aid in the interpretation.

Photoelectron scattering in molecules and fullerenes

Abstract New coincidence techniques in photoelectron spectroscopy of molecules and clusters made it possible to approach complete experiments in molecular photoionization for the first time. This method — angle-resolved photoelectron–photoion coincidence spectroscopy — a versatile tool for the study of electron–ion momentum vector correlations reveals the intermolecular scattering and associated interference of the photoelectron on its way out. This effect is illustrated by selected showcase examples of molecular core level and valence photoionization. The application of this ‘photoelectron interference’ concept on the valence photoionization of fullerenes, in particular C 60 , is discussed.

A core-level photoionization study of furan

We have measured the vibrationally resolved C 1s photoelectron spectrum of the aromatic heterocycle furan (C4H4O). The vibrational profile appears to be dominated by a low-frequency mode, which shows an unusually high degree of excitation for ionization of the C atoms furthest from the oxygen. Our experimental results are compared with calculations, which reveal that several different vibrational modes participate in the vibrational pattern. A well resolved C 1s satellite spectrum at a photon energy of 335 eV has also been measured, and is compared to previous experimental and theoretical results. In a separate experiment the partial electron yield (“absorption”) spectrum has been obtained in the region from just below the C 1s π* resonance up to the respective ionization thresholds. A new high intensity feature has been identified on the high-energy flank of the C 1s−1π(3b1)* resonance. With the aid of resonant Auger spectroscopy we have tentatively assigned this feature to a Rydberg resonance. We also p...

Angle-resolved electronic decay of the π*, σ* and Rydberg resonances below the O K-edge in O2

Abstract Angle-resolved electronic decay spectra of the O 2 molecule have been measured at photon energies corresponding to the excitation of core electrons into the π*, σ* and Rydberg levels. Asymmetry parameters have been determined for resonant Auger transitions and compared with previous data where available. The π* spectrum has also been studied with a narrow photon bandwidth in different regions of the vibrationally unresolved absorption feature.

N2 valence photoionization below and above the 1s-1 core ionization threshold

The intensity of N2 valence photoionization into the three lowest ionic states is given on an absolute scale for photon energies spanning the 1s resonances and the 1s continuum. All three channels, i.e. 3σ−1g X, 1π−1u A and 2σ−1u B, show a coupling to the N 1s–π* resonance and their partial cross sections are characterized by asymmetric Fano profiles. Results for the photoionization of the 2σ−1u B state are compared with measurements of the B–X fluorescence yield. A coupling of the valence photoionization processes to the σ* shape resonance cannot be verified.

Vibrational and shake-up excitations in the C 1s photoionization of ethane and deuterated ethane

The absolute photoabsorption spectra of ethane (C2H6), encompassing the C 1s-1 3p excitation, the C 1s threshold and the satellite thresholds is measured. Further, photoelectron spectra are taken in the region from the C 1s threshold region up to 380 eV, and the vibrational fine structure and C 1s-1 partial cross section studied. The complex vibrational fine structure for both ethane and deuterated ethane (C2D6) is analysed using a coupled least-squares fit procedure. While we cannot exclude the presence of the C-C stretching mode (3), the experimental data show that the C 1s photoelectron spectrum is dominated by two vibrational modes, the C-H stretch 1 and the C-H deformation mode 2. Compared with their values in the neutral ground state there is a significant increase in the vibrational energies to 407±5 meV and 176±5 meV for ethane and 291±10 meV and 130±10 meV for deuterated ethane, respectively. The lifetime broadening is determined to be 98±10 meV, which is in good agreement with calculations. The investigated shake-up structure closely resembles that of methane. The lowest-lying satellite band gains intensity towards threshold which may be attributed to conjugate shake-up processes.

Energy and angle resolved studies of double photo-ionization of helium by electron time-of-flight coincidence spectroscopy

Helium double photo-ionization is studied by a novel coincidence technique which employs time-of-flight spectrometers. Using this technique it is possible to collect simultaneously all the electron pairs, with different energy sharing, emitted by the absorption of a single energetic incident photon. The measurements, in a configuration where the two electrons emerge at relative angle, provide the more complete information on the contribution of the ungerade amplitude to the triple differential cross section and allow the establishment of a relative scale for the full coincident angular distribution measured by other experiments at the same photon energies, but only for a few selected energy-sharing conditions.

Exchange interaction effects in NO core-level photoionization cross-sections

The effect of the exchange interaction in the photoionization continuum is investigated, using N 1s photoionization of NO into the 1s −1 2 π( 1 �) and ( 3 �) final states as an example. The separation in energy of these two final states is 1.41 eV. Significant differences in their partial photoionization cross- sections are observed over a wide range of energies and cannot be accounted for by the different multiplicity of the states. We suggest that the deviation of the 3 �/ 1 � cross-section ratio from the statistical weighting at intermediate energies is dominated by the difference in the final-state potential experienced by the photoelectron and at asymptotically high energies by the multiplet-dependent amount of intensity going into multi-electron (shake-up) processes. Calculations underpinning this point are presented. We also show supporting measurements of the 3 �/ 1 � cross-section ratio for O 1s ionization and the absolute photoabsorption cross-section for NO over a wide energy range covering the core level region.