NEUTRALIZATION AND DELAYED IONIZATION IN FULLERENE SURFACE COLLISIONS- FRAGMENTATION AND IONIZATION RATES AS A ROUTE TO ACTIVATION-ENERGIES

The interaction of C-60(+) and C-70(+) ion beams with a surface of hig hly oriented pyrolitic graphite was investigated by probing the ioniza tion and fragmentation rates of scattered species within a time window of 20 mu s following impact. Neutralization/reionization and fragment ation behavior was observed and followed by a pulsed deflection field applied to the surface at variable delays after the collision event. A n almost complete collisional neutralization of the incident projectil e was found. For an impact energy of 140-180 eV, a significant part of the scattered species was found to reionize by delayed electron emiss ion within the experimental time window. The associated reionization a nd fragmentation kinetics were modeled with a system of differential e quations assuming a simple unimolecular reaction diagram. Rate constan ts for delayed ionization and fragmentation were calculated as functio ns of internal energy and respective activation energies with the ''fi nite heat bath'' model (Klots) and the Rice-Ramsperger-Kassel-Marcus e xpression, respectively. The calculated and measured (deflection field delay dependent) ion intensities were compared in a fit procedure. Th e best fit led to an activation energy for the fragmentation of C-60() (C-60(+)-->C58+C2) of 6.6 +/- 0.5 eV. This translates to an activati on energy of 7.1 +/- 0.5 eV for the fragmentation of neutral C-60 (usi ng the experimentally determined ionization potential of C-58) For C-7 0(+) we obtained an identical (within error) activation energy for fra gmentation (C-70(+)-->C-68(+)+C-2) of 6.6 +/- 0.5 eV. (C) 1996 America n Institute of Physics.