Thermally activated decay channels of superhot C 60−: delayed electron emission and dissociative attachment studied by hyperthermal negative surface ionization

Abstract

Superhot C 60 − ions were formed by attachment of low energy free electrons to superhot C 60 0 molecules in effusive beam. We have studied the thermally activated decay of the C 60 − ion as a function of its vibrational temperature. In addition to the main channel of delayed electron emission C 60 − → C 60 0 + e −, we give evidence for a secondary channel (only weakly competitive) of dissociative attachment C 60 0 + e − → C 58 0 + C 2 −. We have found no evidence for the nearly isoenergetic complementary channel C 60 0 + e − → C 58 − + C 2 0. In order to measure the initial C 60 − beam flux (which partially decays to C 60 0 during the flight time) independently for each nozzle temperature we have used a unique detection method that is completely insensitive to the initial charge state and therefore to the ratio of charge states in the primary beam. The method is based on collisional electron exchange and negative ion (C 60 −) formation in a hyperthermal collision with a surface. Analyzing the kinetics of the delayed electron emission we have obtained a straight Arrhenius plot with a slope (activation energy) of 2.64 ± 0.07 eV and intersection ( A value) of 1.3 × 10 11 s −1. This value of the pre-exponential factor is in good agreement with former measurements but is several orders of magnitude lower than current models’ predictions. A possible explanation for this difference is discussed.