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 heliu m atom scattering experiments with calculations based on a semi-empirical p otential. A (root 2 x root 2)R45 degrees structure is determined from diffr action measurements. The time of flight spectra reveal at least three well separated adsorbate layer phonon modes along the [110] direction, two of wh ich 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 f or this structure are used to assign the observed dispersion curves.