Radiationless deexcitation of excited helium atoms at surfaces

Impact of 20-30-keV ${\mathrm{He}}^{+}$ ions on polycrystalline niobium and copper surfaces causes some projectiles to be backscattered in a neutral excited state. These projectiles subsequently decay, radiatively emitting Doppler-broadened spectral lines; the broadening is characteristic of the distribution in speeds and direction of the scattered excited projectiles. Analysis of the line shape shows that slow reflected particles have a high probability of losing the excited electron by a radiationless transition while they are close to the surface. The spectral line shape has been predicted using the backscattering theory of McCracken and Freeman with the inclusion of a radiationless deexcitation term. By suitable choice of the radiationless deexcitation coefficients the theory and experiment may be brought into acceptable agreement. The coefficients so derived are in surprising agreement with calculated values of Cobas and Lamb for an Auger deexcitation mechanism.