Diffraction of fast heavy noble gas atoms, Ar, Kr and Xe on a LiF(001) surface Changing the tip of a 'perfect' AFM

TL;DR

This work investigates diffraction of fast Ar, Kr and Xe atoms on LiF(001) along [100] and [110] using GIFAD, revealing a transition from quantum-like to classical scattering as energy and mass increase. The authors combine elastic and inelastic diffraction analysis with a semi-classical HCW framework, incorporating Beeby-type refraction to account for attractive forces, and extract a consistent surface corrugation topology. A two-term screened-Coulomb PEL model is fitted to the data to obtain real-space iso-energy contours, showing a low-energy leveling of corrugation and providing a practical path to seed full quantum scattering calculations. The study highlights the link between GIFAD measurements and AFM-like surface topology, and demonstrates a robust, rapid front-end approach for quantifying surface corrugation in heavy-atom scattering regimes.

Abstract

We investigate experimentally the diffraction of fast atoms of noble gas on a LiF(100) crystal oriented along the [100] and [110] directions. The wavelengths are so short that the observed quantum features are qualitatively described by semi-classical models. With increasing mass and energy, the scattering profiles show an increasing number of diffraction peaks forming an increasing number of supernumerary rainbow peaks but progressively weakening in contrast. The innermost peaks corresponding to individual Bragg peaks disappear first. Along the [100] direction, only one type of atomic row contributes to the diffraction signal. After removing the contributions of the attractive forces, we present topological corrugation that should compare with those accessible with an atomic force microscope (AFM).

Figures (13)

• Figure 1: Raw camera capture of the scattered atoms on the detector for 500 eV Ar impinging LiF(001) along [110] direction at $\theta_i=1^{\circ}$, $\phi_i=0^{\circ}$. The dashed circle $\theta_f=\theta_i$ corresponds to energy conservation; $|k_f|=|k_i|$, the vertical line is the specular plane $k_{y_f}=k_{y_i}$. In the lab frame, the azimuthal (lateral) deflection angle is $\phi_f=\arctan(k_{y_f}/k_{x_f})\simeq k_{y_f}/|k_{i}|$, less than a degree while the angle in the $(y,z)$ detector plane is $\Theta_f=\arctan({k_{y_f}/k_{z_f}})\simeq \arctan(\phi_f/\theta_i)$ is equivalent to the angles measured in TEAS and can reach $\pm 90^\circ$.
• Figure 2: For 500 eV Ar impinging LiF along [100] at $\theta_i$=0.29$^\circ$, the diffracted intensity measured on the specular circle is fitted (red line) by quasi Lorentz line-shape (eq.\ref{['eq:LG']}) measured along the [Rnd] direction. The measured intensities are reported on the inset.
• Figure 3: Along the [100] direction, scattering profiles of 4 keV Kr and Xe atom at $\theta_i=0.45^\circ$ and $0.48^\circ$ respectively, have been fitted by a HCW formula giving comparable values of $\zeta$ close to 3.9 for which the intensity $I_{\pm 1}=J_1^2(\zeta)$ is close to zero (see text). Data for Xe have been symmetrized.
• Figure 4: Scattering profile of 700 eV Ar impinging LiF along [110] at $\theta_i=0.42^\circ$ ($E_\perp$=38 meV). The inelastic diffracted intensity is fitted (red line) by line-shape measured along a [Rnd] direction. The peaks labeled s$_i$ correspond essentially to even diffraction orders $|m|$=0,2,4 while $|m|$=7, 8, 9 contribute to the outer classical rainbow.
• Figure 5: Scattering profile of 1784 eV Ar impinging LiF along [110] at $\theta_i=0.5^\circ$. The red line is a fit with a line-shape width $\sigma_{lw}\approx$ 30% larger than the Bragg angle.