Spatial dimensionality and the binding of small clusters.

The effect of spatial dimensionality D on the near-threshold binding of small clusters of identical particles is shown. Estimates of the threshold coupling constants for 2 and N bosons are given for D equal to 1 to 5 and the relation to conditions for the Efimov effect is discussed. Variational trial functions for 4 identical spin-½ fermions in D = 2 are given.

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.

Atomic scattering from an adsorbed monolayer solid with a helium beam that penetrates to the substrate.

Diffraction and one-phonon inelastic scattering of a thermal energy helium atomic beam are evaluated in the situation that the target monolayer lattice is so dilated that the atomic beam penetrates to the interlayer region between the monolayer and the substrate. The scattering is simulated by propagating a wavepacket and including the effect of a feedback of the inelastic wave onto the diffracted wave, which represents a coherent re-absorption of the created phonons. Parameters are chosen to be representative of an observed p(1 × 1) commensurate monolayer solid of H2/NaCl(001) and a conjectured p(1 × 1) commensurate monolayer solid of H2/KCl(001). For the latter, there are cases where part of the incident beam is trapped in the interlayer region for times exceeding 50 ps, depending on the spacing between the monolayer and the substrate and on the angle of incidence. The feedback effect is large for cases of strong transient trapping.

Calculation of inelastic helium atom scattering from H2/NaCl(001).

The one-phonon inelastic low energy helium atom scattering theory is adapted to cases where the target monolayer is a p(1 × 1) commensurate square lattice. Experimental data for para-H(2)/NaCl(001) are re-analyzed and the relative intensities of energy loss peaks in the range 6 to 9 meV are determined. The case of the H(2)/NaCl(001) monolayer for 26 meV scattering energy is computationally challenging and difficult because it has a much more corrugated surface than those in the previous applications for triangular lattices. This requires a large number of coupled channels for convergence in the wave-packet-scattering calculation and a long series of Fourier amplitudes to represent the helium-target potential energy surface. A modified series is constructed in which a truncated Fourier expansion of the potential is constrained to give the exact value of the potential at some key points and which mimics the potential with fewer Fourier amplitudes. The shear horizontal phonon mode is again accessed by the helium scattering for small misalignment of the scattering plane relative to symmetry axes of the monolayer. For 1° misalignment, the calculated intensity of the longitudinal acoustic phonon mode frequently is higher than that of the shear horizontal phonon mode in contrast to what was found at scattering energies near 10 meV for triangular lattices of Ar, Kr, and Xe on Pt(111).

Properties of a suggested commensurate monolayer solid of Kr/NaCl(001).

It is shown that a commensurate square monolayer solid of Kr/NaCl(001) can be stabilized with a model incorporating a rather large energy corrugation amplitude. Then a square bilayer is formed under a compression and preempts the formation of an incommensurate triangular monolayer lattice. The lattice dynamics of the commensurate monolayer may be complex, because the modeling admits the possibility that it has a (2 × 2) unit cell with four Kr atoms.

Transitions of gases physisorbed on graphene.

The behavior of gases physically adsorbed on graphene might be expected to be similar to that of ones adsorbed on graphite. Here, three kinds of phase transitions are examined for gases adsorbed on suspended, free-standing graphene. In one case, the quasi-two-dimensional condensation of a van der Waals fluid is evaluated, using perturbation theory. In a second case, changes are discussed for contributions to the ground state energies of monolayer solid and liquid phases of quantum adsorbates (especially He) on graphene. This includes a determination of the leading perturbation terms in the adsorption-mediated (McLachlan) dispersion energy for two adatoms on a graphene sheet. The third problem is the wetting transition of water and other fluids on graphene. In each case, the relevant energies are somewhat different from those for adsorption on graphite.

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.

Enhanced creation of dispersive monolayer phonons in XePt(111) by inelastic helium atom scattering at low energies.

Conditions likely to lead to enhanced inelastic atomic scattering that creates shear horizontal (SH) and longitudinal acoustic (LA) monolayer phonons are identified, specifically examining the inelastic scattering of (4)He atoms by a monolayer solid of XePt(111) at incident energies of 2-25 meV. There is strong inelastic scattering for both dispersive phonon branches (SH and LA) of the monolayer at incident energies below 8 meV. Several improvements enable more complete wave packet calculations of the inelastic scattering than in previous work. Long propagation times are made feasible by adding an absorbing potential at large distance. The times now extend to beyond 100 ps and enable a clarification of processes involving transient trapping of the He atoms. The wave packet is made more monochromatic by significantly increasing the spatial width of the initial Gaussian shape. The narrower energy distribution in the incident beam then enables a demonstration of strong energy dependence of the scattering over a scale of less than 0.3 meV.

Excitation of the shear horizontal mode in a monolayer by inelastic helium atom scattering.

Inelastic scattering of a low-energy atomic helium beam (HAS) by a physisorbed monolayer is treated in the one-phonon approximation using a time-dependent wave packet formulation. The calculations show that modes with shear horizontal polarization can be excited near high symmetry azimuths of the monolayer, in agreement with recent experiments. The parameters of the calculations are chosen to match the conditions of HAS experiments for triangular incommensurate monolayer solids of xenon, krypton, and argon adsorbed on the (111) face of platinum, and the results show many of the systematic experimental trends for relative excitation probability of the shear horizontal and longitudinal acoustic phonon branches. The inelastic scattering at beam energies near 8 meV is exceedingly sensitive to small misalignment between the scattering plane and the high symmetry directions of the monolayer solid. The diffraction and inelastic processes arise from a strong coupling of the incident atom to the target and the calculated results show large departures from expectations based on analogies to inelastic thermal neutron scattering.

Kinetic temperature effects on 4He dimers in jets.

The formation of dimers in a free jet cryogenic expansion of 4He gas has been studied by measuring mole fractions as a function of source temperature and pressure using diffraction from a nanostructured transmission grating. The data sets are limited to low source pressures for which dimers and trimers are the only appreciable cluster populations in the beam. The final cluster mole fractions are corrected for residual gas attenuation in the source chamber by an extrapolation over several residual gas pressures. A set of rate equations used to model the cluster formation in a free jet expansion has been extended to include the departure of the ambient translational temperature from the isentropic-equilibrium values as the density decreases with increasing distance. The effect of collisions in restoring the equilibrium temperature is treated with a relaxation time approximation. There are distinct distance ranges where the dimer and trimer mole fractions and the ambient temperature near their asymptotic values. The present modeling reproduces the apparent threshold observed at low source pressures for the survival of dimers in the asymptotic beam. Except for these low source pressures, there are only small changes relative to results based on the isentropic temperature.

Corrugation energy for octane on Cu(111).

The energy barrier for sliding of octane on Cu(111) is estimated from an experimental datum for the Brillouin-zone-center gap for translation of a monolayer solid of the octane.