Thermally activated decay channels of superhot C_(60)~-: delayed electron emission and dissociative attachment studied by hyperthermal negative surface ionization

摘要

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.