IMPACT-INDUCED VIBRATIONAL-EXCITATION IN SURFACE SCATTERING OF HYPERTHERMAL NEUTRAL C-60 MOLECULE

A mass-spectrometry based method for measuring the average vibrational energy (vibrational thermometry) of large and hot polyatomic molecule s is presented. The method is applied to C-60 inelastically scattered off nickel with impact energies of 10-50 eV. Both the vibrational cool ing effect on the supersonically expanded C-60 and the collisional exc itation upon surface impact are measured within an experimental accura cy of +/-0.25 eV. Under nearly normal beam incidence conditions and im pact energy of 33.0 eV (out of which 30.6 eV are in the normal energy component), vibrational excitation was found to be below similar to 2% of the impact energy, showing that the recoiled C-60 is nearly nondef ormed at these collision energies. This value is much lower than estim ated before for higher energies C-60(+) ion surface scattering. The im plication of this result in relation with the low energy scattering dy namics of C-60 is discussed. (C) 1997 American Institute of Physics.