THE OBSERVATION OF WELL-DEFINED 2-PHONON AND 3-PHONON BANDS IN TIME-OF-FLIGHT SPECTRA OF HELIUM-ATOMS SCATTERED FROM THE C(2 X-2) STRUCTURE OF CO ON CU(001)

Well defined two and three phonon excitation bands are observed in time of flight spectra of helium atoms scattered from the Co-Cu(001) surface. The detailed intensity structure of the bands enables multiphonon interaction theories to be tested in a uniquely sensitive way for the first time. The Co-Cu(001) scattering system is well characterised, enabling density of states calculations to be performed for the two phonon excitation process. These can be used to show that phonons polarised out of plane, perpendicular to the scattering wavevector, are being created in contradiction to the current theoretical understanding of the process. A possible explanation is proposed in terms of scattering out of plane by the two phonon process, followed by diffraction from the highly corrugated surface back to the scattering plane.