Investigation of 1D chain formation of CO on Pd(1 1 0) by low temperature scanning tunneling microscope

In order to investigate intermolecular interaction between adsorbed molecules, we directly observed adsorbed structures of low coverage CO on Pd(1 1 0) using a low-temperature (4.7 K) scanning tunneling microscope. We found that the CO molecules dosed at room temperature prefer to form a one-dimensional (1D) chain along [1 1̄ 0] during the cooling down, in contrast to a random distribution of CO dosed at near liquid He temperature. These results show the existence of an attractive force between CO molecules chemisorbed on Pd(1 1 0) along [1 1̄ 0]. Here, we discuss effective intermolecular interactions for the 1D chain formation with energy estimations of individual interaction elements between chemisorbed CO molecules.