Axel Reinköster

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.

Multi-fragmentation of C60 after collisions with Arz+ ions

We studied the fragmentation of C60 molecules in collisions with slow Ar+ ions (v ≈ 0.04–0.55 au) and with Arz+ ions (z = 2, 3) at somewhat higher velocities (v ≈ 0.3–0.95 au). The measured mass spectra show C60q+ ions (q ≤ 5), smaller C60−2mq+ fullerenes and C60−2mq+ + C n+ fission products. Special attention is devoted to the C60 multi-fragmentation process, i.e. the breakup of the C60 cage into several small Cn+ fragments (n = 1–29). Up to seven correlated small Cn+ fragments (n = 1–5) have been observed. The multi-fragmentation pattern depends on the projectile charge. Furthermore, it varies strongly with the projectile velocity for slow ions, whereas it becomes nearly constant for faster ions. This is in accordance with recent non-adiabatic quantum molecular dynamic calculations.

Ion-impact induced excitation and fragmentation of C60

Abstract We studied the multiple ionization and fragmentation of C60 in collisions with 50– 300 keV H + , He+, Ne+, and 6.5– 300 keV ·z Ar z+ (z=1–3) ions using a time- and position-sensitive multi-particle detector. Several aspects of C2 evaporations, fullerene fission and multi-fragmentation processes are discussed. For Ar++C60 the multi-fragmentation patterns depend on the projectile velocity and especially for slow ions strong variations occur. This behaviour is in accordance with recent non-adiabatic quantum molecular dynamic (NA-QMD) studies (Phys. Rev. Lett. 86 (2001) 5258).

Upper limits for the circular dichroism for the C 1s and O 1s core excitation of methyl oxirane

The circular dichroism in the total and partial ion yields of methyl-oxirane C3H6O was measured at the C 1s and O 1s edges. The dierence of the response of the chiral molecule to circularly polarized light with opposite handedness was found to be less than 0.2% for the total ion yield and less than 0.5% for the partial ion yield. Additionally we tried to nd a dipole allowed molecular orientation CD eect by analyzing the fragmentation in the forward and backward direction. For this eect we found an upper limit of 1-2% for all abundant ionic fragments.

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.

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.