Berlin 2001 – wissenschaftliches Programm
Bereiche | Tage | Auswahl | Suche | Downloads | Hilfe
AMPD: EPS AMPD
AMPD 4: Sitzung 4
AMPD 4.3: Vortrag
Dienstag, 3. April 2001, 11:30–11:55, H104
Determination of the C60 binding energy by analysing spontaneous and electron induced dissociation reactions — •S. Matt, O. Echt, C. Lifshitz, P. Scheier, and T.D. Maerk — Institut für Ionenphysik, Universität Innsbruck, Technikerstr.25, A-6020 Innsbruck, Austria
Since the discovery of the C60 numerous experiments have been carried out and many intriguing properties have been observed. One of these features which distinguishes C60 from other molecules is its huge binding energy. Not only the absolute value of the binding energy (BE) is important, but also the fact that it is even higher than the ionization energy. In collaboration with the group of Martin Quack of the ETH Zürich we discovered in 1996 that it is possible to heat neutral C60 with infrared photons to a degree that it ionizes without fragmenting. This is only possible when the BE is at least equal to the ionization energy. The ionization energy is known to be 7.6 eV from electron impact experiments as well as photo ionization studies, whereas the exact number for the C60 BE was still not known at this time, despite the fact that in 1996 about 25 values had been published. The experimental results lay between 2 eV and 12 eV, whereas the calculations favoured a value close to 14 eV. This unsatisfying situation arises because due to the complexity of C60 it is not possible to measure the BE directly. As a consequence of the large number of degrees of freedom a large amount of energy can be stored in the vibrational modes (without leading to immediate dissociation) and thus C60 shows an enormous kinetic shift, which makes a direct determination of the BE by simple subtracting the parent ion ionization energy from the appearance energy of the fragment ion impossible. To solve this problem new experiments have been designed, but also in more sophisticated attempts it turned out that one has to deal with unexpected difficulties. In contrast to other molecules hot C60 not only exhibits on the *s time scale cooling by evaporation of C2 units but also by evaporation of electrons and emission of photons. Moreover it turned out that the transition state of the decaying C60 seems to be extremely loose. The C2 unit leaving the cage is not fixed in position, it is rather free rotating on the surface. These properties of the dissociating C60 are quite unusual and thus have been overlooked or misinterpreted in many of the earlier experiments. In the present study we measured all quantities needed for the determination of the BE directly in our experimental set up. By using the MIKE scan technique we measured metastable fractions and the kinetic energy release distribution (KERD) for C2 evaporation of C60. The data analysis needed to extract the BE from the KERD is based on the finite heat bath theory developed by Eph Klots. In addition, we studied the electron-induced dissociation of singly and multiply charged C60 ions. In contrast to the analysis of spontaneous (time window 50–60 µs) decay reactions we sampled here the decay reactions which happen within 3 µs after the excitation. This difference in time sheds on the one hand more light onto the decay mechanism (transition state) and on the other hand allows also the determination of the BE. The results we obtained for the BE in these two independent experiments are not only in excellent agreement with each other and other recently published experimentally determined values but also with the most recent theoretical result.