Adsorption and structure of N-2 on copper(110)

The adsorption and structure of ultrathin physisorbed N-2 films on Cu(110) has been investigated in a combined experimental and theoretical study. Using specular helium scattering the heat of adsorption for the unconstrained N-2 monolayer was determined to q(st) = 88 meV per molecule and the 2D heat of condensation to q(2D) = 5.5 meV. In the coverage range up to one monolayer the nitrogen molecules form a complex ((1)(1 3)) high-order commensurate phase with an oblique, quasi-hexagonal unit cell. Increasing the coverage beyond completion of the unconstrained monolayer, the adlayer first undergoes a phase transition into a weakly compressed phase before the condensation of the bilayer sets in. In the bilayer the N-2 molecules are arranged in a slightly distorted hexagonal structure. Using realistic interaction potentials, the structure of the N-2 monolayer is investigated by detailed total energy calculations. As a result, the most stable structure is found to be the ((1 3) (4 1)) phase in agreement with experiment. In addition, the arrangement of the nitrogen molecules has been determined: the unit cell contains seven N-2 molecules arranged in a novel type of 7-sublattice pinwheel structure. Molecular dynamics simulations corroborate this structure and further demonstrate its remarkable thermal stability. (C) 1997 Elsevier Science B.V.