Experimental and theoretical studies of the monolayer structure of OCS adsorbed on NaCl(001): Coexistence of orientationally inequivalent phases

Abstract

The structure of a physisorbed carbonyl sulfide (OCS) monolayer on a well-defined NaCl single crystal surface has been studied by helium atom scattering (HAS), low energy electron diffraction (LEED) and polarized Fourier-transform infrared spectroscopy, in the temperature range 30–100 K. The monolayer growth proceeds via large islands and both HAS and LEED indicate a (2×1) commensurate structure with two molecules per unit cell. The binding energy is estimated to be about 0.26 eV from desorption experiments. By contrast, the occurrence of three intense and three weaker absorption peaks in the infrared spectrum of the asymmetric stretching vibration ν3 is difficult to reconcile with the conclusions drawn from HAS and LEED. Therefore semi-empirical potential calculations are performed to determine the monolayer structure. Energy minimization rules out a high molecular density for the layer, and shows the existence of two isoenergetical (2×1) and (2×2) phases with respectively two and four molecules per unit cell, which differ only by the orientations of the molecules. Because of their similar geometry, these two structures cannot be discriminated by HAS and LEED, but the mixing of these two phases with different vibrational couplings between the admolecules provides a reasonable interpretation of the main features of the infrared spectrum.

Helium Atom Scattering Studies of the Structures and Vibrations of the H2, HD, and D2Monolayers on NaCl(001)

“…In Figure , angular distributions along the [100] direction for the clean surface (left) and a monolayer of n-H 2 (right) are shown, measured with different incident wave vectors between 4.55 and 8.13 Å -1 corresponding to incident energies between 10.8 and 34.5 meV. As already expected from the adsorption curves in Figure , the intensities of the monolayer angular distributions are comparable to those for the clean surface, and these relative intensities are considerably higher than for other molecular adsorbates. ,− All of the monolayer diffraction peaks are at precisely the same positions as and of comparable widths to those of the bare NaCl substrate. Also, along the other azimuthal direction, [110] (Figure ), no additional peaks were found…”

“…As already expected from the adsorption curves in Figure 1, the intensities of the monolayer angular distributions are comparable to those for the clean surface, and these relative intensities are considerably higher than for other molecular adsorbates. 69,[71][72][73] All of the monolayer diffraction peaks are at precisely the same positions as and of comparable widths to those of the bare NaCl substrate. Also, along the other azimuthal direction, [110] (Figure 3), no additional peaks were found.…”

Comparative Study of the Primary Photochemical Mechanisms of Nitric Oxide and Carbonyl Sulfide on Ag(111)

“…Unfortunately, the orientation of the adsorbate transition dipole relative to the surface normal is not known exactly in either case. For OCS on NaCl, adsorption is with the molecular axis parallel to the surface in a zigzag structure, while on LiF(001) OCS is adsorbed perpendicular to the surface with either S or O down in equal parts. This leads to the observation of both S and CO in the gas phase following photolysis at 222 nm, into the dissociative electronic transition of OCS .…”

Phonon-libron dynamics of acetylene adsorbed onNaCl(001)

“…[16][17][18][19][20][21][22][23] These results reveal that the bonding is governed by rather complex force fields that depend strongly on both the electrostatic properties of the admolecules as well as on the electric fields and field gradients produced by the ions of the substrate. 24 Ab initio potential energy calculations have provided additional insight into the adsorption characteristics of such systems.…”