Temperature effects in compression structures: carbon monoxide on Pd(110) by rairs

Abstract

High resolution FT-RAIRS and LEED data have been obtained for CO on Pd(110) at 140 and 300 K. At 300 K the following behaviour is deduced. At low coverages, a single infrared band is seen corresponding to the formation of a c(2 × 2) structure. As θ is increased beyond 0.5, there is a striking transformation of the infrared spectrum where a broad absorption band with distinctive features over a 300 cm-1 frequency region is seen, attributed to small domains of p(4 × 2) structure separated by high density domain boundaries. As θ approaches 0.75, the p(4 × 2) domains increase in size and the infrared spectrum shows only two bands which are attributed to this structure. Finally at ambient pressures, islands of a (2 × l)plgl structure coexist with islands of p(4 × 2) structure. At 140 K, immobility of the adlayer leads to different behaviour. At low coverages, islands of CO molecules in B2 sites exist, surrounded by a disordered sea. With increasing coverage a “frozen-in” amorphous-like state is formed in which a number of domains with varying local densities (0.5 < θ < 1) coexist. At saturation a single, sharp, infrared band at 2007 cm-1 arises as the only ordered LEED structure (the (2 × l)plgl) observed at this temperature is formed.