Adsorption and structure of N-2 on Pt(111)

The adsorption and structure of monolayer N-2 films on Pr(111) have been in vestigated in a combined experimental and theoretical study. Using specular helium scattering, the heat of adsorption for the unconstrained N-2 monola yer was found to be q(1) = 158+/-5 meV per molecule, and the 2D hear of con densation was found to be q(2D) = 21+/-3 meV. At low temperature (20 K), th e nitrogen molecules arrange in a (3 x 3) high-order commensurate phase. Th is structure irreversibly transforms into a (4 x 4) structure after anneali ng at 40 K or if N-2 is directly adsorbed at this higher temperature. Incre asing the coverage beyond completion of the unconstrained monolayer, the (3 x 3) phase experiences a Novaco-McTague rotation of +/-4.2 degrees out of the [110] high-symmetry direction and, in some cases, a secondary rotation by similar to 19.8 degrees. Based on realistic interaction potentials, the structure of the N-2 monolayer is investigated by molecular dynamics simula tions. The (3 x 3) and (4 x 4) structures are obtained:as respective equili brium structures for slightly different coverages with the same total bindi ng energy per molecule in both cases. With increasing temperature. the (4 x 4) structure becomes the most stable phase. In the (3 x 3) phase, each uni t cell contains four molecules, and the adlayer is aligned with the close p acked direction of the Pt(111) substrate. The unit cell of the (4 x 4) phas e contains seven molecules, and the dose packed direction of the adlayer is rotated by 19.1 degrees with respect to the substrate [110] direction. Wit hin the two structures, the molecules exhibit a different in-plane herringb one orientational order. Furthermore, the molecular dynamics simulations pr ovide evidence for an orientational transition within the (4 x 4) phase at about 30 K. (C) 2000 Elsevier Science B.V. All rights reserved.