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Abstract:
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 C60 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 C70. 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 C60 with particular empahasis on the qualitative analysis of all ionization processes.
We also studied valence shell and K-shell induced fragmentation dynamics of C60 which proceeds via a subsequent emission of neutral C2 particles. In the valence region cross sections of the C60q+ ions (q=1,2,3) and of the fragments {C60-2m+(m≤3),C60-2m2+(m≤5)} are discussed. A possible excitation process for C60 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 C60 accompanied by an increased production of C602+. To learn more about the underlying excitation and relaxation processes, we performed corresponding electron-ion coincidence measurement. Here, fragments like C582+ or C562+ 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 C60 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 C60.
