Structure and phase transitions of ultrathin films of alkanes adsorbed on Cu(111)

Monolayers of saturated long-chain hydrocarbons (n-octane and n-nonane) were 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 ordered well-defined two-dimensional lattices. We find a herringbone structure with a p(5 root 3 x 2) unit cell for octane-Cu(111) and a domain wall lattice with a c(root 3 x 11) unit cell for nonane-Cu(111). At elevated surface 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.