Structure and infrared absorption of the first layer C2H2 on the NaCl(100) single-crystal surface

The adsorbate structure of the first layer acetylene on the NaCl(100) single-crystal surface is investigated using polarization infrared spectroscopy in combination with low-energy electron diffraction (LEED) experiments, and potential calculations on the basis of pair potentials. In agreement with a previous study, a triplet of infrared absorptions in the region of the asymmetric stretch vibration of C2H2 was observed and assigned to an adsorbate phase with (3 root 2 x root 2)R45 degrees translational symmetry determined in the LEED experiment. The polarization dependence of the infrared spectra is consistent with a parallel orientation of the molecules with respect to the surface. The number of molecules per unit cell is four to six as determined by photometric considerations. Total energy minimizations support a new structure model which contains five inequivalent molecules per unit cell in a herringbone arrangement. The application of a vibrational exciton approach demonstrates that this new structure model can reproduce the triplet spectrum observed in the infrared experiments.