Sanja Korica

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.

Auger cascades versus direct double Auger: relaxation processes following photoionization of the Kr 3d and Xe 4d, 3d inner shells

The relaxation processes after non-resonant inner-shell photoionization are studied experimentally using electron?electron time-of-flight coincidence spectroscopy. Results for krypton 3d and xenon 4d as well as 3d photoionization are presented. The experimental data make it possible to disentangle sequential from simultaneous processes using the different electron emission characteristics as the differentiating property. For the population of final states having charges higher than 2, the measurements show a strong preference for sequential Auger cascade decay. Clear evidence for direct double Auger processes is found in the case of Xe 3d photoionization only.

Appearance of Plasmons in Fullerenes

The valence electrons of fullerenes may be regarded as spherical distributions with a finite width of a jellium-like potential giving rise to collective motions of this orange peel electron cloud. They cause strong enhancement of the photoionization cross section, a resonant behavior phenomenon know as plasmon excitations. The number and characteristic features of these excitations will be discussed.

Coherent localization exhibited by unequal Auger Doppler components.

If coherent superpositions such as the symmetry eigenstates gerade and ungerade are in turn coherently superimposed, localization occurs. This effect is studied by the intensities of the Doppler components of electrons emitted from dissociating fragments of superexcited O2. The measurements show clear evidence for such coherent localization.

Angular distributions and continuous intensity behavior in multi-photon processes

Multi-photon ionization for all rare gases by radiation from the Free Electron Laser FLASH in Hamburg is reported. The ionization events are analyzed by angle resolved electron Time-of-Flight spectroscopy. Photoelectrons belonging to different coupling schemes of the final ionic state are energetically resolved and their angular distributions could be determined separately for all three final ionic states for the first time. The behavior of the multi-photon specific term β4 shows different values for different final ionic states pointing to coupling scheme dependent anisotropic final state interactions.

Threshold behavior of quantum oscillations in mirror symmetric molecular systems

Cohen and Fano derived in their famous paper of 1966 a formula which describes the partial cross section oscillations of the ionization probability of mirror symmetric systems such as diatomic homonuclear molecules but also of larger system such as fullerenes. The general prediction of oscillating partial cross sections has been proven since that for different excitation schemes like ion, electron and photon impact. The specific threshold behavior of the Cohen-Fano formula, however, has been experimentally never proven. We present new measurements on this intriguing problem for N2 and C60.

Photoionization and Photofragmentation of Fullerenes

Due to their unique geometrical structure fullerenes have attracted much attention over the years since their discovery. However, a deeper understanding of their electronic structure and fragmentation behavior is still a challenge for both theoreticians and experimentalists. We have performed high resolution measurements of photoelectrons emitted from the valence shell of C60 and C70, for both gas phase and solid state samples, in order to obtain branching ratios, partial cross sections, and in the case of C 60 angular distribution anisotropy parameters of the two highest occupied molecular orbitals, HOMO and HOMO-1. The ratio between the corresponding HOMO and HOMO-1 levels, which consist of unresolved lines, exhibit the same oscillatory structure for both phases in the case of C60, but shows an nonoscillating offset of 0.5 for the gas phase measurements compared to solid state data in the case of C 70. Fourier transformation of the cross section rations displays the information about the geometrical properties of the molecules; their radius and the thickness of the electronic hull. The partial cross sections of the two outermost molecular orbitals exhibit in both cases oscillations with a frequency related to the diameter of the molecule superposed on the exponential decay curve. The overall agreement between different theoretical calculations and our experiment for branching ratios, partial cross sections and beta parameters is very good except of a striking disagreement with respect to the predicted discrete resonance structure in the partial cross sections. We assume that resonances in the partial cross sections are quenched by the vibrations of the molecule. During the same measurement we recorded a new series of the K-shell photoelectron spectra of C 60 with particular empahasis on the qualitative analysis of all ionization processes. We also studied valence shell and K-shell induced fragmentation dynamics of C 60 which proceeds via a subsequent emission of neutral C 2 particles. In the valence region cross sections of the C q+ 60 ions (q=1,2,3) and of the fragments {C + 60−2m(m≤ 3),C2+ 60−2m(m≤ 5)} are discussed. A possible excitation process for C 60 is the creation of a plasmon, a collective motion of the outer electrons. The so-called volume plasmon causes an enhancement of the ionization cross section of neutral C 60 accompanied by an increased production of C 2+ 60 . To learn more about the underlying excitation and relaxation processes, we performed corresponding electron-ion coincidence measurement. Here, fragments like C 2+ 58 or C 2+ 56 can be identified by their outgoing electrons, although the fragmentation takes place on a longer time scale than the ionization. Above the carbon K-shell of C 60 the main products are doubly and triply charged fullerenes. Corresponding electron-electron coincidence measurements were carried out to achive a deeper understanding about the fundamental processes causing the many-electron emission in this energy region. These results show that the main contributions to the triply charged ion yield is direct double photoionization of C60 followed by Auger decay. However, in contrast to most atoms and molecules, it is driven by the plasmon excitation associated with the K-shell photoionization of C 60. DISSERTATIONS. KORICA 5