Monolayers of saturated long-chain hydrocarbons (n-octane and n-nonane) wer
e adsorbed on a Cu(111) surface and subsequently studied by high-resolution
elastic and inelastic He-atom scattering as well as by low energy electron
diffraction. At low surface temperatures (<160 K) the alkane-chains form o
rdered well-defined two-dimensional lattices. We find a herringbone structu
re with a p(5 root 3 x 2) unit cell for octane-Cu(111) and a domain wall la
ttice with a c(root 3 x 11) unit cell for nonane-Cu(111). At elevated surfa
ce temperatures the systems undergo phase transitions to disordered, liquid
-like states. Unexpectedly, the different structure observed for the nonane
monolayer also results in a different type of phase transition. For nonane
, an anomalously large Debye-Waller factor is observed, providing indirect
evidence for the presence of low-energy excitations related to the presence
of domain-walls. (C) 1999 Elsevier Science B.V. All rights reserved.