Surface scattering of C60+: Recoil velocities and yield of C60

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Abstract
In the impact of a C60+ beam against solid surfaces, a substantial fraction of the beam is neutralized. By means of a pulsed photoionization experiment, we have detected the neutralized scatterers from a silicon (100) surface, and found that they are mainly intact C60, with little or no fragmentation at impact energies to 200 eV. The neutralization probability is found to increase monotonically with increasing impact energy. This reionization method has been used in a time-of-flight scheme to measure the recoil velocity distributions of scattered C60. Collisions at impact energies above 50 eV are found to be highly inelastic, and the normal recoil distribution changes very little with impact energy over the range from 50 to 200 eV. The peak in the velocity distribution is near 1150 m/s (∼5 eV), with a full width at half-maximum (FWHM) of 350 m/s. By comparison with molecular dynamics simulations, an interpretation of this speed is proposed in terms of a limit to reversible deformation of the fullerene cage. A model of the scattering kinematics, based on treating the fullerene cage as a deformable, hollow sphere, with a harmonic deformation limit (15 eV) as found by theory, predicts the observed speed quantitatively.