HIGH-RESOLUTION HELIUM ATOM SCATTERING FROM AN EPITAXIALLY GROWN ONE MONOLAYER FILM OF KCN ON KBR(001)

High resolution helium atom diffraction and single phonon inelastic sc attering experiments were performed on a single monolayer KCN film epi taxially grown onto a KBr(001) substrate. Although the cleaved surface of single crystal KCN appears disordered, the one monolayer film was stabilized by the substrate. The measurements from this film in the [1 00] direction gave dispersion curves with a Rayleigh mode, a dispersio nless mode near 12 meV, a possible crossing mode, and a suggestion of a weak interface mode near 2 meV. The Debye temperature of the film wa s found to be 127 K, which was very close to that found for the cleave d single crystal KCN. A kink in the Debye-Waller factor near 150-160 K may be associated with a surface phase transition.