Collision of alkali ions with C60/C70: Insertion, thermionic emission, and fragmentation

Abstract

Interactions of Li+, Na+, and K+ with C60 molecules have been studied over the collision energy range from 0 to 150 eV. For Li+ and Na+, insertion of the alkali to form the endohedral [Li@C60]+ and [Na@C60]+ species is observed, with appearance energies of ∼6 and ∼18 eV, respectively. At higher collision energies several processes are observed. For Na+, the hot [Na@C60]+ appears to relax mainly by loss of C2 units from the fullerene cage, yielding [Na@C60−2n ]+. For [Li@C60]+, escape of Li+ dominates over C2 loss. For K+, very little [K@C60]+ is observed, and the main endohedral species observed are fragments [K@C60−2n ]+ (n=1–7), which begin to appear at ∼40 eV. Pure fullerene fragment ions (C60−2n +) are also observed, with cross sections that increase sharply at ∼30 eV for all three projectile ions. These ions are believed to result primarily from thermionic emission of collisionally excited neutrals. For K+, we also examined collisions with C70, obtaining results similar to those for C60. Changes in the endohedral insertion mechanism with projectile size are discussed, as is the competition between C2 evaporation, thermionic emission, and IR emission.