Burkhard Langer

Competition between direct dissociation and resonant Auger decay: a quasi-classical model applied to the 2p−1 σ∗ states of HCl, DCl and Cl2

Resonant Auger spectra of HCl, DCl, and Cl2 following the (Cl 2p32 → σ∗ excitation were studied. Both ‘molecular’ and ‘atomic’ Auger transitions are observed and assigned. The branching ratio of ‘molecular’ versus ‘atomic’ Auger transitions increases with increasing reduced mass of the target molecules. Using a quasi-classical model, we are able to deduce the principal Auger lineshape in agreement with experiment. The simultaneous occurrence of ‘atomic’ and ‘molecular’ Auger transitions is shown to be the general case for repulsive intermediate states. The kinematic model successfully describes the mass and lifetime dependence of the competition between the Auger process and dissociation.

High-resolution angle-resolved measurements in atoms and molecules using advanced photoelectron spectroscopy at the ALS

Abstract A two-dimensional photoelectron imaging technique using time-of-flight spectrometry and a high-resolution SES-200 electron energy analyzer have been used with photons from the atomic and molecular undulator beamline of the Advanced Light Source to study the structure and dynamics of atoms and molecules. The analyzers are rotatable around the direction of the photon beam, allowing angular-dependent studies to be performed. Results on the photoexcitation and decay mechanisms of doubly excited resonances in neon illustrate the importance of spin-orbit effects for such a light atom, since significant breakdown of LS-coupling was measured for such a light atom . Also, fully vibrationally resolved Auger electron spectra measured under Resonant Raman conditions following the C 1s→π* excitation in a diatomic molecule, CO, is presented which allows us to probe the core excited states, their excitation and de-excitation dynamics with unprecedented resolution.

Core level photoionization on free sub-10-nm nanoparticles using synchrotron radiation

A novel instrument is presented, which permits studies on singly charged free nanoparticles in the diameter range from 1 to 30 nm using synchrotron radiation in the soft x-ray regime. It consists of a high pressure nanoparticle source, a high efficiency nanoparticle beam inlet, and an electron time-of-flight spectrometer suitable for probing surface and bulk properties of free, levitated nanoparticles. We show results from x-ray photoelectron spectroscopy study near the Si L(3,2)-edge on 8.2 nm SiO(2) particles prepared in a nanoparticle beam. The possible use of this apparatus regarding chemical reactions on the surface of nanometer-sized particles is highlighted. This approach has the potential to be exploited for process studies on heterogeneous atmospheric chemistry.

Photon energy dependence of ionization-excitation in helium at medium energies

We have measured the photoionization-excitation-to-photoionization ratio for (n = 2 - 6) at several photon energies from 90 to 900 eV. By extrapolating these values we could determine the asymptotic high-energy ratios for (n = 2 - 5) which agree with theoretical predictions. We show that the satellite-to-main-line ratios are consistent with experimental double-to-single photoionization ratios and agree well with recent measurements.

Angular distribution parameters and R-matrix calculations of Ar 3s-1 -> np resonances

Angle-resolved photoelectron spectra have been measured with 6 meV photon resolution for the Ar (n = 4 - 16) autoionization resonances in the energy range between 26 and 29.3 eV, using synchrotron light from an undulator beamline of the Advanced Light Source. Photoelectron angular distribution parameters have been parametrized according to the prescription of Kabachnik and Sazhina and have been compared with R-matrix calculations similar to those of Burke and Taylor. The agreement was found to be very good. We have also analysed the cross section shape for the first six resonances, confirming previous measurements performed for the lower resonances, and compared them with various calculations.

Elastic light scattering from free sub-micron particles in the soft X-ray regime

We report the first experimental results on angle-resolved elastic light scattering in the soft X-ray regime, where free sub-micron particles in the size regime between 150 and 250 nm are studied in the gas phase by using a continuous particle beam. Two different types of studies are reported: (i) Angle-resolved elastic light scattering experiments provide specific information on the scattering patterns in the regime of element-selective inner-shell excitation near the Si 2p-edge (80-150 eV). In addition to intense forward scattering, we observe distinct features in the angle-resolved scattering patterns. These are modelled by using Mie theory as well as a model that includes contributions from diffuse and specular reflection. The results are primarily attributed to scattering from soft X-rays in the surface layer. (ii) Spectroscopic experiments are reported, where the photon detector is placed at a given scattering angle while scanning the photon energy near the Si 2p-absorption edge. These results are also analyzed by a Mie model, yielding accurate information of the size distribution.

Angular distributions of the C(1s) photoelectron satellites in CO

Angle-resolved high-resolution photoelectron spectra of CO C 1s satellite lines were taken at a photon energy of 330 eV. The observed angular distributions assist in identifying the correlation effects which lead to the formation of different satellite lines, confirming recent theoretical and experimental results on this topic. Four conjugate shake-up satellite lines with beta =0 were observed, leading to a strongly aligned core-excited state in the molecular ion. These states decay anisotropically via participator Auger transitions, which carry along the angular momentum of the ionized core to double-hole final states. We present the first clear identification of the initial states of these recently observed K-shell satellite Auger transitions with pronounced decay anisotropy.

Angle-integrated magnetic linear dichroism in valence photoionization of free oxygen atoms

The ion-counting rate was investigated in a photoionization experiment using polarized and aligned oxygen atoms and linearly polarized light. The cross section of the valence photoionization was found to depend on the angle between the polarization axis of the ionizing synchrotron radiation and the axis of atomic alignment. A significant asymmetry in the ion rates corresponding to and was observed close to the resonance. This asymmetry is caused by j-dependent resonant behaviour of the and continuum channels.

New setup to study trapped nano-particles using synchrotron radiation

A novel setup is presented that is suitable to investigate the change of the properties of a single trapped nanoparticle by charging it with monochromatic synchrotron radiation. The particles are stored for long time periods that reach up to days without any contact to a substrate in the center of a three dimensional electrodynamic quadrupole trap. The accurate Q/M value is derived from the oscillatory motion of the particle in the trap recorded via scattering of visible light.

Angular distribution of the Ne autoionization resonances: experimental and theoretical study

We have used angle-resolved photoelectron spectroscopy and improved R-matrix calculations to study the (n = 3 - 5) autoionizing resonances and the doubly excited resonance in the photon energy range between 44 and 48.5 eV. The photoelectrons were detected at different angles using three time-of-flight spectrometers. The measurements were obtained utilizing the 18 meV photon energy resolution provided by an undulator coupled with a spherical grating monochromator of the Advanced Light Source. Angular distribution parameters of these resonances have been parametrized using a model by Kabachnik and Sazhina and have been compared with our R-matrix calculations which are similar to those of Burke and Taylor. We find that the calculations agree quite well with the measurements. Our analysis of the cross section shape shows no strong n dependence of the q and parameter, while the width varies as the quantum defect theory predicts. Also, the differential cross section parameters agree well with previous experiments carried out for the lower resonances.