1,2-dichloroethane on Cu(100) and oxygen-covered Cu(100): Conformation, orientation, and transformation

Temperature-programmed reaction/desorption, reflection-absorption infrared spectroscopy, and density functional theory calculations have been employed to investigate the adsorption of ClCH₂CH₂Cl on Cu(100) and O/Cu(100) in the subjects of identification of the rotational isomers, adsorption energy, adsorption geometry, thermal stability of the layer structure, and transformation between the trans and gauche conformers. On Cu(100), both the trans and gauche forms of ClCH₂CH₂Cl coexist on the surface at a monolayer coverage, and their thermal desorption peak appears at 187 K. The trans molecules are likely adsorbed with the C-Cl bonds approximately parallel to the surface. On O/Cu(100), the gauche conformer is the predominant species and the desorption temperature is shifted to 199 K. It is found that the ClCH₂CH₂Cl layer structure on Cu(100), up to a ∼15 monolayer coverage studied, is stable between 120-140 K. However, O/Cu(100) shows an interesting contrast. The thermal stability only appears at a coverage below ~3 monolayers. The transformation from gauche to trans takes place for higher coverages. On the basis of the cluster-model calculations without consideration of the interaction between adsorbed molecules, it is shown that the activation energy for the transformation between trans and gauche ClCH₂CH₂Cl on O/Cu(100) is much lower than that on Cu(100), suggesting that the trans molecules are easier to be transformed into gauche ones on O/Cu(100), as they are adsorbed on the proximity of preadsorbed oxygen atoms.