Evidence for threshold effects in positron diffraction from NaF and LiF.

Abstract

We have measured the energy dependence of the intensity of a positron beam specularly reflected from the (100) surfaces of NaF and LiF. A 1-eV-wide peak located at an energy that decreases with increasing angle of incidence \ensuremath{\theta} is qualitatively identified as a beam threshold effect. Given the known small positron affinity for the solids, the narrowness of the peak is consistent with its being due to a true surface resonance. However, the dispersion appears to depart from the expected energy versus \ensuremath{\theta} trajectory parallel to the kinematic threshold. Additionally, a search in 50-meV steps did not reveal any fine structure in the peaks, possibly due to our 0.3-eV effective instrumental resolution. Further experiments to map the dispersion with greater precision, to measure the binding energies of the resonances, and to search for possible fine structure are proposed.