^{1}, Michael A. Gleeson

^{1}and Aart W. Kleyn

^{1,2,a)}

### Abstract

Hyperthermal Ar atoms were scattered from a Ru(0001) surface held at temperatures of 180, 400 and 600 K, and from a Ru(0001)-(1×1)D surface held at 114 and 180 K. The resultant angular intensity and energy distributions are complex. The in-plane angular distributions have narrow (FWHM ≤ 10°) near-specular peaks and additional off-specular features. The energy distributions show an oscillatory behavior as a function of outgoing angle. In comparison, scattered Ar atoms from a Ag(111) surface exhibit a broad angular intensity distribution and an energy distribution that qualitatively tracks the binary collision model. The features observed for Ru, which are most evident when scattering from the clean surface at 180 K and from the Ru(0001)-(1×1)D surface, are consistent with rainbow scattering. The measured TOF profiles cannot be adequately described with a single shifted Maxwell-Boltzmann distribution. They can be fitted by two components that exhibit complex variations as a function of outgoing angle. This suggests at least two significantly different site and/or trajectory dependent energy loss processes at the surface. The results are interpreted in terms of the stiffness of the surface and highlight the anomalous nature of the apparently simple hcp(0001) ruthenium surface.

This work is part of the research programme of the Stichting voor Fundamenteel Onderzoek der Materie (FOM) and is supported financially by the Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO). It is supported by the European Communities under the contract of Association between EURATOM and FOM and carried out within the framework of the European Fusion Programme. The views and opinions expressed herein do not necessarily reflect those of the European Commission.

I. INTRODUCTION

II. EXPERIMENTAL

III. RESULTS

A. Angularly resolved intensity distributions

B. Angularly resolved energy distributions

C. Comparison with results from Ag(111)

D. Two-component TOF fitting

IV. DISCUSSION

A. Ar scattering from Ru(0001): Angular distributions

B. Ar scattering from Ru(0001): Energy distributions

V. CONCLUSIONS

### Key Topics

- Surface scattering
- 38.0
- Atom scattering
- 34.0
- Atom surface collisions
- 30.0
- Atom surface interactions
- 23.0
- Angular distribution
- 17.0

## Figures

Angle-resolved flux distributions of Ar atoms (〈*E* _{i}〉 ∼ 6.2 eV; *θ* _{i} = 40°) scattered from (a) Ru(0001) and (b) Ru(0001)-(1×1)D. The angular distribution from the bare surface at *T* _{S} = 180 K is replotted in the panel (b) as a dashed line. The scattered intensities are normalized to the intensity of the corresponding direct beam. The lines connecting the data points are intended to guide the eye.

Angle-resolved flux distributions of Ar atoms (〈*E* _{i}〉 ∼ 6.2 eV; *θ* _{i} = 40°) scattered from (a) Ru(0001) and (b) Ru(0001)-(1×1)D. The angular distribution from the bare surface at *T* _{S} = 180 K is replotted in the panel (b) as a dashed line. The scattered intensities are normalized to the intensity of the corresponding direct beam. The lines connecting the data points are intended to guide the eye.

Angle-resolved flux distributions of Ar atoms (〈*E* _{i}〉 ∼ 6.4 eV; *θ* _{i} = 60°) scattered from (a) Ru(0001) and (b) Ru(0001)-(1×1)D. The angular distribution from the bare surface at *T* _{S} = 180 K is replotted in the panel (b) as a dashed line. The scattered intensities are normalized to the intensity of the corresponding direct beam. The lines connecting the data points are intended to guide the eye.

Angle-resolved flux distributions of Ar atoms (〈*E* _{i}〉 ∼ 6.4 eV; *θ* _{i} = 60°) scattered from (a) Ru(0001) and (b) Ru(0001)-(1×1)D. The angular distribution from the bare surface at *T* _{S} = 180 K is replotted in the panel (b) as a dashed line. The scattered intensities are normalized to the intensity of the corresponding direct beam. The lines connecting the data points are intended to guide the eye.

Angle-resolved final-to-initial energy ratios (〈*E* _{f}〉/〈*E* _{i}〉) of Ar scattered from (a) Ru(0001) and (b) Ru(0001)-(1×1)D for *θ* _{i} = 40°, and from (c) Ru(0001) and (d) Ru(0001)-(1×1)D for *θ* _{i} = 60°. In (b) and (d) the results from the bare surface at *T* _{S} = 180 K are replotted as a dashed lines. The lines connecting the data points are intended to guide the eye. Two simple models are illustrated on the panels. Solid lines correspond to parallel momentum conservation and dashed-dotted lines represent the model of single-collision hard sphere scattering of Ar from an isolated ruthenium atom (mass ratio of 40/101).

Angle-resolved final-to-initial energy ratios (〈*E* _{f}〉/〈*E* _{i}〉) of Ar scattered from (a) Ru(0001) and (b) Ru(0001)-(1×1)D for *θ* _{i} = 40°, and from (c) Ru(0001) and (d) Ru(0001)-(1×1)D for *θ* _{i} = 60°. In (b) and (d) the results from the bare surface at *T* _{S} = 180 K are replotted as a dashed lines. The lines connecting the data points are intended to guide the eye. Two simple models are illustrated on the panels. Solid lines correspond to parallel momentum conservation and dashed-dotted lines represent the model of single-collision hard sphere scattering of Ar from an isolated ruthenium atom (mass ratio of 40/101).

Angle-resolved flux distributions of Ar (〈*E* _{i}〉 ∼ 6.5 eV) scattered from Ru(0001) and Ag(111) at (a) *θ* _{i} = 40° and (b) *θ* _{i} = 60°, and the corresponding angle-resolved 〈*E* _{f}〉/〈*E* _{i}〉 ratios from those surfaces at (c) *θ* _{i} = 40° and (d) *θ* _{i} = 60°. In panels (c) and (d), the dashed-dotted lines represent the models of single-collision hard sphere scattering of Ar from an isolated Ag atom (mass ratio of 40/108). *T* _{S} was 600 K in all cases.

Angle-resolved flux distributions of Ar (〈*E* _{i}〉 ∼ 6.5 eV) scattered from Ru(0001) and Ag(111) at (a) *θ* _{i} = 40° and (b) *θ* _{i} = 60°, and the corresponding angle-resolved 〈*E* _{f}〉/〈*E* _{i}〉 ratios from those surfaces at (c) *θ* _{i} = 40° and (d) *θ* _{i} = 60°. In panels (c) and (d), the dashed-dotted lines represent the models of single-collision hard sphere scattering of Ar from an isolated Ag atom (mass ratio of 40/108). *T* _{S} was 600 K in all cases.

Contour plots produced from Ar TOF spectra after normalization to a peak intensity of one: Ar scattered from (a) Ag(111) at *θ* _{i} = 40° and *T* _{S} = 600 K; (b) Ru(0001) at *θ* _{i} = 40° and *T* _{S} = 600 K; (c) from Ru(0001)-(1 × 1)D at *θ* _{i} = 40° and *T* _{S} = 114 K. The right-hand-side panels show some representative raw TOF spectra for the three surfaces and the associated two-component fits. The measurement for (a) and (b) were done at 400 Hz chopper frequency, and for (c) at 200 Hz chopper frequency. The times shown represent the flight time from the chopper to the ionizer (i.e., the trigger delay and QMS flight-time corrections have been applied).

Contour plots produced from Ar TOF spectra after normalization to a peak intensity of one: Ar scattered from (a) Ag(111) at *θ* _{i} = 40° and *T* _{S} = 600 K; (b) Ru(0001) at *θ* _{i} = 40° and *T* _{S} = 600 K; (c) from Ru(0001)-(1 × 1)D at *θ* _{i} = 40° and *T* _{S} = 114 K. The right-hand-side panels show some representative raw TOF spectra for the three surfaces and the associated two-component fits. The measurement for (a) and (b) were done at 400 Hz chopper frequency, and for (c) at 200 Hz chopper frequency. The times shown represent the flight time from the chopper to the ionizer (i.e., the trigger delay and QMS flight-time corrections have been applied).

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