Björn Zimmermann

Isotope-induced partial localization of core electrons in the homonuclear molecule N2

Because of inversion symmetry and particle exchange, all constituents of homonuclear diatomic molecules are in a quantum mechanically non-local coherent state; this includes the nuclei and deep-lying core electrons. Hence, the molecular photoemission can be regarded as a natural double-slit experiment: coherent electron emission originates from two identical sites, and should give rise to characteristic interference patterns. However, the quantum coherence is obscured if the two possible symmetry states of the electronic wavefunction (‘gerade’ and ‘ungerade’) are degenerate; the sum of the two exactly resembles the distinguishable, incoherent emission from two localized core sites. Here we observe the coherence of core electrons in N2 through a direct measurement of the interference exhibited in their emission. We also explore the gradual transition to a symmetry-broken system of localized electrons by comparing different isotope-substituted species—a phenomenon analogous to the acquisition of partial ‘which-way’ information in macroscopic double-slit experiments.

Molecular frame photoelectron angular distribution for oxygen 1s photoemission from CO2 molecules

We have measured photoelectron angular distributions in the molecular frame (MF-PADs) for O 1s photoemission from CO2, using photoelectron-O+–CO+ coincidence momentum imaging. Results for the molecular axis at 0, 45 and 90° to the electric vector of the light are reported. The major features of the MF-PADs are fairly well reproduced by calculations employing a relaxed-core Hartree–Fock approach. Weak asymmetric features are seen through a plane perpendicular to the molecular axis and attributed to symmetry lowering by anti-symmetric stretching motion.

Photoelectron scattering effects in molecular photoionization

Abstract An angle-resolved photoelectron–photoion coincidence experiment has been performed on CO at a photon energy of 321 eV. Complete, three-dimensional fragment ion angular distributions for several C(1s) photoelectron emission directions have been measured. For the case of photoelectron emission along the light polarization vector, the ionic fragment angular distribution has been compared to model calculations to gain information about the relative phase shift between σ→σ and σ→π transition matrix elements.

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.

Absorption of circularly polarized VUV radiation in polarized iron vapor.

The effect of magnetic dichroism on the total ion yield of iron, i.e. in the absorption of polarized iron vapour, was observed for the first time far away from the threshold or autoionizing states. These observations can be explained by the orientation of orbital angular momentum of the 3d-electrons. This effect has not yet been observed in the absorption measurements of magnetized iron films due to the smallness of the expectation value of the orbital angular momentum in the solid targets. The details of the dynamical behaviour go beyond single-electron approximations.

Coherence analysis and tensor polarization parameters of (gamma,egamma) photoionisation processes in atomic coincidence measurements

Publisher Summary This chapter discusses coherence analysis and tensor polarization parameters of (γ, eγ) photoionization processes in atomic coincidence measurements. Photoelectron spectroscopy refers to measurements of energies and intensities often including their angular distributions. So-called complete or perfect atomic collision and photoionization experiments were based on analysis of particle-photon-, particle-particle-coincident correlations and electron- and atomic spin and orientation effects. Photoionization and atomic collision processes can quantum mechanically is completely described by a limited number of amplitudes and their phase differences and thus the experiment from which the relevant amplitudes and phase differences can be extracted has been referred to as a complete or perfect experiment. A further approach to a complete photoionization analysis has been reported. The chapter summarizes previous and recent developments related to experimental approaches to complete scattering experiments in connection with (γ, eγ) processes and reports of some recent experimental investigations of (γ, eγ) processes for calcium and strontium atoms. Synchrotron radiation of fixed energy and polarization state simultaneously photoionized the atom and excites the ion into a higher state from which fluorescence photons are emitted. The experimental coincidence analysis of the photoelectron and the emitted fluorescence photon can lead to results for relevant amplitudes and phase differences which describe such processes.