Abstract Time-of-flight measurement of the inelastic scattering of Na + ions from CF 4 reveal two energy-loss mechanisms which have been observed at center-of-mass collison energies between 4 and 24 eV. At small angles θ 1ab≲20° a single energy transfer peak is observed and attributed to normal mode excitation. At larger angles an additional sharp peak is observed out to θ lab = 70° with nearly double the energy transfer. the final velocities and angular dependence agree with an impulsive interaction involving only one of the F atoms with an effective mass which approaches that of the F atoms with increasing collison energy. This is the first evidence for an impulsive mechanism in inelastic collisons at energies below the threshold for electonic excitation.