Understanding the structure of high coverage CO adlayers

Abstract

Vibrational spectroscopy has indicated that the ordered overlayers formed by CO on metal surfaces at high coverage are not “compression” structures, or “floating” phases, but rather coincident site lattices in which the molecules remain adsorbed on high symmetry sites. In an equivalent description such layers may be regarded as a mixture of phase and anti-phase domains of a lower coverage structure separated by regularly spaced domain walls (solitons), at which the local CO density is higher. Due to the flat potential energy surface the molecules at the domain walls can adjust to the repulsive CO-CO interaction by moving off the high symmetry sites. The soliton model has the advantage of providing a convenient description of the formation of such phases and of the transitions from one phase to another. In the present paper low energy electron diffraction (LEED) and infrared spectroscopy are used to illustrate these phenomena in the adsorption systems Pt{111}-CO and Pd{111}-CO.