FOCUSED INELASTIC RESONANCES IN THE SCATTERING OF HE ATOMS FROM NACL(001)

High-resolution inelastic He-atom surface scattering experiments have been carried out on an in situ cleaved NaCl(001) crystal surface for t he incident wave-vector range of 4.29-6.07 Angstrom(-1) in the [110] h igh-symmetry direction to detect the recently predicted effect, the fo cused inelastic resonance (FIR). The scattering angle for the resonanc e in the [110] direction is uniquely predicted based on the reciprocal lattice vector exchanged in the process for the bound state /epsilon( 1)/ =3.4+/-0.1 meV. This value is slightly lower than the 3.7+/-0.4 me V value found in the literature. The convincing evidence for the FIR e ffect comes from a systematic analysis of the angular distributions in combination with an energy analysis based on time-of-flight spectra.