The sub-monolayer (Theta < 0.5) adsorption structures and vibrational dynamics of ethene molecules (C2H4) on copper(001) have been studied with high-resolution helium atom scattering at T-s = 50 K. Diffraction rings observed in the angular distributions indicate that the ethene molecules repel each other on the surface at intermediate exposures, thereby forming ordered, nearly hexagonal structures. The time of flight spectra at Theta = 0.038 reveal two peaks, one at 6.3 meV (51 cm(-1)), which is attributed to the perpendicular stretch vibrational mode (S) of the adsorbate, and another mode at a somewhat lower frequency, which is attributed to the frustrated translational vibration parallel to the surface (T-mode). The T-mode frequency increased significantly with increasing coverage from 4.2 meV (34 cm(-1)) at Theta = 0.014-5.3 meV (46 cm(-1)) for the monolayer (Theta = 0.5), whereas the S-mode frequency was virtually independent of ethene exposure. The shift in the T-mode frequency with ethene exposure is attributed to a long-range substrate mediated interaction between adsorbed ethene molecules.