Adsorption and structure of N2 on Pt(111)

The adsorption and structure of monolayer N2 films on Pt(111) have been investigated in a combined experimental and theoretical study. Using specular helium scattering, the heat of adsorption for the unconstrained N2 monolayer was found to be q1=158±5 meV per molecule, and the 2D heat of condensation was found to be q2D=21±3 meV. At low temperature (20 K), the nitrogen molecules arrange in a (3×3) high-order commensurate phase. This structure irreversibly transforms into a (4×4) structure after annealing at 40 K or if N2 is directly adsorbed at this higher temperature. Increasing the coverage beyond completion of the unconstrained monolayer, the (3×3) phase experiences a Novaco–McTague rotation of±4.2° out of the [11̄0] high-symmetry direction and, in some cases, a secondary rotation by ∼19.8°. Based on realistic interaction potentials, the structure of the N2 monolayer is investigated by molecular dynamics simulations. The (3×3) and (4×4) structures are obtained as respective equilibrium structures for slightly different coverages with the same total binding energy per molecule in both cases. With increasing temperature, the (4×4) structure becomes the most stable phase. In the (3×3) phase, each unit cell contains four molecules, and the adlayer is aligned with the close packed direction of the Pt(111) substrate. The unit cell of the (4×4) phase contains seven molecules, and the close packed direction of the adlayer is rotated by 19.1° with respect to the substrate [11̄0] direction. Within the two structures, the molecules exhibit a different in-plane herringbone orientational order. Furthermore, the molecular dynamics simulations provide evidence for an orientational transition within the (4×4) phase at about 30 K.