Velocity correlated multifragmentation in C-60-surface impact: Energy distributions and incidence angle dependences

We have studied surface impact multifragmentation of C-60(-) following subkiloelectron volt, near-grazing collision with a nickel target. Field free, mass resolved kinetic energy distributions and incidence angle dependences were measured for the outgoing C-n(-) (n=2-15) fragments. All energy distributions could be described within a scenario where C-n groups are statistically (thermally) emitted off an outgoing precursor away from the surface, as manifested by shifted Maxwellian flux distributions with only two parameters: kinetic energy epsilon (per carbon atom) associated with the center-of-mass velocity of the moving precursor and its temperature T. The best fitted parameters were found to be epsilon=2.60 eV and kT=0.86 eV. A distinct narrowing effect was observed for the incidence angle dependences of the C-n(-) fragments yield, going from C-4(-) to C-15(-). The gradually decreasing angular width as a function of cluster size n was analyzed based on the assumption of isotropic, statistical fragments emission off an outgoing multifragmenting source (the superhot precursor). The predicted 1/root n narrowing law was found to be in good agreement with the observed width effect and the two parameters epsilon, kT extracted from the independently measured kinetic-energy distributions. We conclude that both energy and angle distributions clearly demonstrate a statistical expansionlike, precursor-mediated multifragmentation mechanism.