Adsorption, desorption, monolayer structure and dynamics of C2H4 on Cu(001)

High-resolution helium atom scattering has been used to characterize the adsorption and desorption, as well as the structure and low energy vibrational modes, of a monolayer of ethene (C2H4) on Cu(001). From the variation of the angular distributions during growth, a coverage independent sticking coefficient, S-0 = 0.93 is found. At coverages greater than Theta = 0.12 a strong repulsive interatomic interaction between adsorbed ethene molecules is observed. This is confirmed by the desorption energy which shows a large variation from 375 meV (8.6 kcal mol(-1)) at zero coverage to 225 meV (5.2 kcal mol(-1)) for a complete first layer. Helium diffraction measurements show that the ethene monolayer has a p(2 x 2) structure with a probable coverage of Theta = 0.5 relative to the Cu(001) substrate. Time-of-flight studies provide the first measurements of the frequencies of the external frustrated translational vibrations perpendicular to the surface (S-mode) at 6.8 meV and of the frustrated translational vibrations of ethene molecules parallel to the surface (T-mode) at 4.8 meV.