Ordering and Phase Transitions in Adsorbed Monolayers of Diatomic Molecules. In: Prigogine I, Rice SA, editors. Advances in Chemical Physics. Hoboken: John Wiley & Sons, Inc.; 1996. pp. 213-394. [DOI: 10.1002/9780470141540.ch4]

IR spectroscopic study of the displacement of an SF6 monolayer on graphite by Xe

We report a study of displacement by xenon of a monolayer of sulphur hexafluoride initially condensed on a graphite surface. Earlier work showed that, below 112 K, Xe displaces SF₆ almost completely in a first-order transition. Working at higher temperatures, we show that this system has a simple eutectic-like phase diagram, at least for SF₆ not too dilute. In our experiment, both adsorbates are in equilibrium with their respective vapors in a cold cell. In our infrared reflection-absorption spectroscopy measurements, the SF₆ coverage on the surface is monitored by the frequency shift due to dynamic dipole coupling of the collective mode of the strong SF₆ ν₃ vibrational resonance. Simulations relate this frequency shift to the SF₆ areal density. Below T ≈ 134 K, with increasing Xe pressure, a small amount Xe dissolves in the solid SF₆ monolayer preceding its displacement by a solid predominantly Xe monolayer in a first-order transition. Above 134 K, there is a weaker first-order transition to a mixed liquid monolayer, followed by continuous increase in Xe concentration. If the initial SF₆ monolayer is near its melting line, the melting transition on adding Xe appears to become continuous.

Structures and thermal properties of the N2 monolayer on Pb(1 1 1)

The physisorption of N2 molecules has long been a model system of molecular adsorption. We present a low-energy electron diffraction (LEED) study of the adsorption structures and thermodynamics of monolayer N2 on Pb(1 1 1). The results indicate that the monolayer structure has a triangular incommensurate center-of-mass lattice, and that the N2-substrate interaction is weaker than that observed on other metal surfaces. The N2 monolayer undergoes a phase transition between an orientationally ordered phase (low-temperature) and an orientationally disordered phase at a temperature of 20 K. Potential energy and quasiharmonic calculations indicate that the weak N2-Pb(1 1 1) interaction is the main contributing factor for the difference in orientational order of incommensurate N2 monolayers on Pb(1 1 1) and other similar metal surfaces.

Novel behavior of monolayer quantum gases on graphene, graphane and fluorographene

This article discusses the behavior of submonolayer quantum films (He and H2) on graphene and newly discovered surfaces that are derived from graphene. Among these substrates are graphane (abbreviated GH), which has an H atom bonded to each C atom, and fluorographene (GF). The subject is introduced by describing the related problem of monolayer films on graphite. For that case, extensive experimental and theoretical investigations have revealed that the phase diagrams of the Bose gases (4)He and para-H2 are qualitatively similar, differing primarily in a higher characteristic temperature scale for H2 than for He. The phase behavior of these films on one side of pristine graphene, or both sides of free-standing graphene, is expected to be similar to that on graphite. We point out the possibility of novel phenomena in adsorption on graphene related to the large flexibility of the graphene sheet, to the non-negligible interaction between atoms adsorbed on opposite sides of the sheet and to the perturbation effect of the adsorbed layer on the Dirac electrons. In contrast, the behavior predicted on GF and GH surfaces is very different from that on graphite, a result of the different corrugation, i.e., the lateral variation of the potential experienced by these gases. This arises because on GF, for example, half of the F atoms are located above the C plane while the other half are below this plane. Hence, the He and H2 gases experience very different potentials from those on graphite or graphene. As a result of this novel geometry and potential, distinct properties are observed. For example, the (4)He film's ground state on graphite is a two-dimensional (2D) crystal commensurate with the substrate, the famous [Formula: see text] phase; on GF and GH, instead, it is predicted to be an anisotropic superfluid. On GF the anisotropy is so extreme that the roton excitations are very anisotropic, as if the bosons are moving in a multiconnected space along the bonds of a honeycomb lattice. Such a novel phase has not been predicted or observed previously on any substrate. Also, in the case of (3)He the film's ground state is a fluid, thus offering the possibility of studying an anisotropic Fermi fluid with a tunable density. The exotic properties expected for these films are discussed along with proposed experimental tests.

Weak molecular chemisorption of N(2)/Pt(111)

The ordering in a higher-order-commensurate monolayer solid of Pt(111)- (3 × 3)-4  N(2), which has coexisting physisorbed and weakly chemisorbed N(2) species, is analyzed with model calculations. Density functional theory calculations are also used to evaluate properties of chemisorbed N(2) in a (2 × 2) unit cell on Pt(111). The relation of these results to the orientational ordering of N(2) on other metal surfaces is discussed.