Time-dependent wavepacket calculations of atom scattering from surfaces with impurities

Exact quantum-mechanical calculations are reported on atom scattering from a crystalline surface with isolated impurities. The calculations are for He scattering from a one-dimensional model of a Cu surface with adsorbed Ar atoms. The difficulties of carrying out calculations on scattering from extended but non-periodic structures are overcome by using a time-dependent wavepacket approach. A recently developed method for solving the time-dependent Schrödinger equation is employed. Scattering intensities are given for several energies and incidence angles. Detailed insight is obtained on impurity effects on surface scattering. The main features are: (1) Broad intensity tails are superimposed on each diffraction spike. The width of the tails decreases with increasing diffraction order; (2) Shallow rainbow peaks arise, due to impurity induced local corrugation; (3) Weak intensity maxima arise due to interference between surface and impurity scattering. The intensities are somewhat sensitive to the position of the impurity within the surface unit cell. Physical interpretation of the effects is provided from exact calculations, and from a simple sudden approximation for the scattering intensities. It is argued that He scattering can be used to determine impurity locations on surfaces.