EXPERIMENTAL AND THEORETICAL-STUDIES OF THE MONOLAYER STRUCTURE OF OCS ADSORBED ON NACL(001) - COEXISTENCE OF ORIENTATIONALLY INEQUIVALENT PHASES

The structure of a physisorbed carbonyl sulfide (OCS) monolayer on a w ell-defined NaCl single crystal surface has been studied by helium ato m scattering (HAS), low energy electron diffraction (LEED) and polariz ed Fourier-transform infrared spectroscopy, in the temperature range 3 0-100 K. The monolayer growth proceeds via large islands and both HAS and LEED indicate a (2 x 1) commensurate structure with two molecules per unit cell. The binding energy is estimated to be about 0.26 eV fro m desorption experiments. By contrast, the occurrence of three intense and three weaker absorption peaks in the infrared spectrum of the asy mmetric stretching vibration v(3) is difficult to reconcile with the c onclusions drawn from HAS and LEED. Therefore semi-empirical potential calculations are performed to determine the monolayer structure. Ener gy minimization rules out a high molecular density for the layer, and shows the existence of two isoenergetical (2 x 1) and (2 x 2) phases w ith respectively two and four molecules per unit cell, which differ on ly by the orientations of the molecules. Because of their similar geom etry, these two structures cannot be discriminated by HAS and LEED, bu t the mixing of these two phases with different vibrational couplings between the admolecules provides a reasonable interpretation of the ma in features of the infrared spectrum. (C) 1997 American Institute of P hysics.