ENHANCED SURFACE ANHARMONICITY OBSERVED IN VIBRATIONS ON CU(110)

Surface anharmonicity has been measured on Cu(110) by examining the temperature dependence of surface phonons with atomic motion normal to the surface using high-resolution electron-energy-loss spectroscopy. At the Brillouin zone center GAMMA-BAR the energy of the MS7 resonance decreases by 3.2 meV and the intrinsic width increases by 8.6 meV as the temperature increases from 21 to 766 K. Energy decrease and width increase are also observed at the Brillouin zone edge YBAR in the S3 phonon. The percentage energy decrease is the same in both phonons. Direct comparison with bulk-phonon measurements shows that the anharmonicity at the surface is 4.1-4.8 times greater than in the bulk.