Experimental and theoretical studies of acetylene layers adsorbed on KCl(001)

The structure and lattice dynamics of a physisorbed acetylene monolayer on a KCl(001) single crystal surface have been investigated by comparing helium atom scattering experiments with calculations based on a semi-empirical potential. A (root 2 x root 2)R45 degrees structure is determined from diffraction measurements. The time of flight spectra reveal at least three well separated adsorbate layer phonon modes along the [110] direction, two of which are only weakly dispersive, while the third is characterized by a very pronounced dispersion. The calculations also predict a(root 2 x root 2)R45 degrees structure containing two molecules per unit cell, which lie flat on the surface and which are mutually perpendicular. The phonons calculated for this structure are used to assign the observed dispersion curves.