Direct evidence for the two-phonon bound states on the H/Ni(111) surface -art. no. 233404

We have investigated multiple losses and overtones of H vibrations at Ni(11 1) by using electron-energy-loss spectroscopy. We mapped the energies along the <(<Gamma>)over bar>(M) over bar direction of the surface Brillouin zon e, and found two branches outside the two-phonon continua. This gives direc t evidence of the existence of two-phonon bound states. Applying a one-dime nsional model of coupled anharmonic oscillators, we derive the anharmonicit y and discuss the localized property of the bound states.