Quasielastic Helium Atom Scattering from a Two-Dimensional Gas of Xe Atoms on Pt(111)

Abstract

Quasielastic helium atom scattering provides direct evidence for a fully mobile two-dimensional gas of Xe atoms on Pt(111) at a coverage of Q Xe 0.017 and a temperature of T s 105 K. Molecular dynamics simulations of the experimental data show that the frictional coupling rate between the Xe atoms and substrate is less than 0.25 ps 21 and the lateral potential corrugation for single Xe atoms is less than 9.6 meV, which is only 40% of the value previously determined at high coverage.[S0031-9007 (99)09358-8] PACS numbers: 68.35.Fx, 05.10.Gg, 61.18.BnThe phase diagrams of adsorbate species on single crystal surfaces have been extensively studied for many systems [1]. The adsorbates can form solid, liquid, or gas phase analogs. Whereas the solid phase is accessible to many surface science techniques, these same techniques are largely insensitive to the motion that characterizes the liquid and gas phases. In the 1950's, Van Hove and Vineyard developed a formalism to describe the "quasielastic" energetic broadening of the elastic signal induced when neutrons are scattered from moving target species in crystals, bulk liquids, and gases [2][3][4]. They identified a number of classes of dynamical behavior, each with a characteristic dependence of the quasielastic peak width on the scattering momentum transfer. Helium atom scattering has been shown to be well suited for analogous experiments on single crystal surfaces because of its unique sensitivity, even to single atoms [5]. Using quasielastic helium atom scattering (QHAS), the microscopic jump diffusion of isolated atoms and molecules has been studied on metal surfaces [5,6] and recently even under conditions approaching a liquid phase [7]. The present paper reports the first experimental QHAS observation of the linear dependence of the peak width on the parallel momentum transfer for a two-dimensional ideal gas. While adsorbate "gas" phases have been observed before [8], it has not been possible to distinguish between a lattice gas, in which adatoms occupy specific adsorption sites and move by jump diffusion, and a fully mobile two-dimensional gas, where atoms run virtually unimpeded across the surface.In the dilute adsorbate phases, the helium atoms are scattered from isolated particles which all have the same atomic form factor F͑DK, DE͒ (where DK and DE are the surface parallel momentum and energy transfers of the helium atoms, respectively), so the kinematic scattering approximation can be used. Therefore, the scattered amplitude is given as the product of a form factor and a structure factor S͑DK, DE͒ [9]: I͑DK, DE͒ jF͑DK, DE͒j 2 jS͑DK, DE͒j 2 .(1)If F can be assumed to be a slowly varying function of DK, the quasielastic peak shape for a particular momen-tum transfer is given by jS͑DK, DE͒j 2 around DE 0 [9]. Moreover, at low coverages, the Vineyard approximation [3] may be used, jS͑DK, DE͒j 2~G s ͑DK, DE͒, where G s is the Fourier transform of the self-paircorrelation function G s ͑R, t͒ [the probability of finding a particle at ͑R, t͒ that was at R 0 at t 0 [9...

Quasi-elastic peak lineshapes in adsorbate diffusion on nearly flat surfaces at low coverages: the motional narrowing effect in Xe on Pt(111)

“…These results are in excellent agreement with those obtained by Ellis et al [4] by means of Langevin molecular dynamics simulations, i.e. considering interacting adsorbates, though the interaction among them is relatively weak because the coverage used in the experiment was relatively low (θ = 0.017).…”

“…Both the dynamic structure factor and the intermediate scattering function can be readily obtained from Langevin numerical simulations as follows. Within a low coverage regime (as happens in Ellis et al experiment [4], where θ = 0.017), adsorbate-adsorbate interactions can be neglected, and diffusion is fully characterized by only studying the dynamics of an isolated adsorbate on a metal surface. This is the so-called single adsorbate approximation.…”

Step effects and coverage dependence of hydrogen atom diffusion onPt(111)surfaces

“…It will be important to see whether future QHAS experiments with improved resolution 57 give the same higher value of D and lower value of E d for the present H on Pt͑111͒ system, and in many other cases when comparisons from other techniques are available. 36,[58][59][60][61][62][63][64][65] We also note the different temperature range used in the two experiments. As shown by TDS of H / Pt͑111͒, 12 the take-up desorption temperature of H is about 170 K at high coverage and 200 K at low coverage, thus, the H atoms will partially desorb from the Pt͑111͒ over the QHAS measurement temperature range from 140 to 250 K. The partial desorption of H will not only affect the coverage of H, but also the H-H lateral interaction.…”

Molecular diffusion in monolayer and submonolayer nitrogen

“…Although most of the data for monolayers of molecular nitrogen [1][2][3][4][5] are for solids, the recent extensions of quasielastic neutron [6][7][8] and helium atom 9 scattering experiments raise the prospect that data on the dynamics of the fluid phases may become available. Here, we characterize the diffusive motions in monolayer N 2 fluids using molecular dynamics simulations and explore how these motions are reflected in the correlation functions for inelastic scattering.…”

“…The substrate is taken to be static. Ellis et al, 9 in a QHAS study of low-density Xe/Pt͑111͒, modeled the damping of adatom motions with a Brownian friction arising from the substrate and found scarcely any effect in the linewidth for a damping coefficient of 0.25 ps Ϫ1 . Since the damping coefficient for N 2 /graphite is estimated 16 to be in the range 2 -4 ns Ϫ1 , their simulations give support to the use of the static-substrate approximation in the present work.…”