Experimental evidence for kinematical focusing in the inelastic scattering of helium from the NaF(001) surface

The kinematical focusing (KF) effect along the dispersion curve of Rayleigh waves has been observed in the inelastic scattering of ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ from a NaF(001) surface. From a direct inspection of the time-of-flight spectra, a number of spikes appearing in the angular distribution are discriminated from bound-state resonances and shown to correspond to KF. We show that, in principle, the dispersion relation of Rayleigh waves can be obtained directly from the position of such singularities, without having to resort to time-of-flight measurements.