COINCIDENCE STUDY OF DOUBLE ELECTRON EMISSION ASSOCIATED WITH K-SHELL PHOTOIONIZATION OF C 60

The (multiple) photoionization and subsequent fragmentation of the C60 molecule was studied with the synchrotron radiation after removing electrons from the inner K-shell. Our intention was especially focused on the dynamics of the subsequent fragmentation. In addition to ’normal’ (non-coincident) electron and ion time-of-flight spectroscopy, we investigated this topic with the help of an electron–electron-coincidence measurement. Our experiment shows that in these processes C60 ions with charge states up to 3+ and several smaller C60-2m fragments are formed. In addition, the broad peak besides the C(1s) line, usually referred to as the 'plasmon' peak, has been observed.


Introduction
Since the discovery of C 60 molecule (Kroto et al, 1985, pp.162-163), (Krätschmer et al, 1990, pp.354-358) many studies were performed to investigate its fundamental properties.Due to its high symmetry, C 60 ACKNOWLEDGMENT: The authors are indebted to the Deutsche Forschungsgemainschaft (DFG) and to the Bundesministerium fűr Bildung und Forschung (BMBF) for the financial support.
Figure 3 shows contributions of singly, doubly and triply charged fullerene ions for several photon energies.These results indicate that, above the carbon K-shell of C 60 , the main products are doubly and triply charged fullerenes.C + 60 is the most abundant ion in the low energy region.The relative C 2+ 60 yield compared to the C + 60 yield first increases with increasing photon energy and stagnates above ≈350 eV at a nearly constant level.The yield of triply charged C 3+ 60 is similar to the yield of C 2+ 58 .Different doubly charged C 2+ 60−2m fragments appear step by step with increasing photon energy.The yields of singly charged fragments exhibit an enhancement in certain photon energy regions; at high photon energies, these yields decrease and are only slightly visible.
So the obtained ion yield spectroscopy of gas phase C 60 is corroborated by the corresponding photoelectron measurements (Korica et al, 2018).The continuous intensity distribution in the photoelectron spectra can be either the result of direct double photoionization or double-Auger decay.The quality of the former K-shell photoelectron measurements was insufficient to disentangle these two contributions experimentally (Aksela et al, 1995(Aksela et al, , pp.2112(Aksela et al, -2115)), (LeBrun et al, 1994, pp.3965-3968), (Brühwiler et al, 1993, pp.3721-3724), (Krummacher et al, 1993, pp.8424-8429).In general, the disentanglement of the two processes on the basis of normal ion or electron spectroscopy is not unambiguously possible.

Electron-electron coincidence spectroscopy of excited C 60
Electron-electron-coincidence measurements were carried out to get a deeper understanding about the fundamental processes causing the many-electron emission in C 60 .Here, a separation of different underlying processes can be better achieved.(Fig. 4).
Shake-off electrons are abundant at low-kinetic energies.Therefore, even at such high photon energies, shake-off processes are important to understand the yields of the multiply charged C 60 ions besides the Auger and double Auger processes.In the case that two shake-off electrons leave the C 60 molecule, the energy sharing is very asymmetric.The faster of the two shake-off electrons contributes signifcantly to the broad peak besides the C(1s) main line, usually referred to as the 'plasmon' peak (Hertel et al, 1992, pp.784-787), (Leiro et al, 2003, pp.205-213); this possibility has been unrevealed so far.

Conclusion
We have studied the photoionization of the C 60 molecule above the C(1s) threshold, in the photon energy range hν=(330-390)eV.A careful analysis of the spectra yielded a surprising and unexpected result.
Clear hints have been found that the major contribution to the triply charged ion yield is the direct double photoionization of C 60 .However, in contrast to most atoms and molecules, it is driven by the plasmon excitation associated with the K-shell photoionization of the fullerenes.Whereas the K-shell satellites are still bound core excited ionic states of the C 60 molecule, plasmon excitations at higher binding energies are already in the double electron emission continuum.This causes a specific intensity distribution and explains the origin of the broad resonance features in the continuum part of the spectrum and an unusual high amount of triply charged fullerenes of 40%.