Dynamic and static interactions in CO overlayers on sintered NaCl films

Abstract

Sublimation of NaCl onto a cold (77 K) NaCl plate followed by sintering at 300 K gives an IR transparent film formed by individual crystallites having cubic shape. CO adsorbed on {100} faces of the crystallites gives an IR peak at 2161-2156 cm −1, while CO adsorbed on edges (steps) gives a weaker peak at ∼2175 cm −1. The CO molecules are adsorbed on Na + ions and form a bidimensional array of parallel oscillators. Lateral adsorbate-adsorbate interactions (dynamic and static) are responsible for the shift of the IR frequency. The relative weight of the static and dynamic effects is estimated by using 12CO− 13CO mixtures. The dynamic effect is well described by the modified Hammaker equation, with α v =0.0235 A ̊ −3. The unusually low value of the dynamic polarizability agrees very well with literature data concerning the intensity of the stretching band of CO adsorbed on ions. The static effect, causing a consistent negative shift of the CO frequency, is related to through-solid interactions of inductive nature.