AN EXPERIMENTAL AND THEORETICAL-STUDY OF THE TRANSITION FROM SINGLE PHONON TO MULTIPHONON ATOM-SURFACE SCATTERING

The angular distributions and time-of-flight spectra of a monoenergeti c 100 meV He atom beam have been measured after scattering from a clea n Cu(001) surface at temperatures between 100 K and 1000 K. With incre asing temperatures sharp diffraction and phonon features of the quantu m regime become broad as expected for the multiphonon classical regime . Good agreement with theory is achieved over the entire range of temp eratures by assuming that the atoms are reflected by a smooth vibratin g potential barrier rather than by a lattice of repulsive atom cores a s previously frequently postulated.