Suppression of Decoherence in Fast-Atom Diffraction at Surfaces

Scattering of fast neutral atoms with keV kinetic energies at alkali-halide surfaces under grazing angles displays intriguing diffraction patterns. The surprisingly strong persistence of quantum coherence despite the impulsive interaction with an environment at solid state density and elevated temperatures raises fundamental questions such as to the suppression of decoherence and of the quantum-to-classical crossover. We present an ab initio simulation of the quantum diffraction of fast helium beams at a LiF (100) surface in the $⟨110⟩$ direction and compare with recent experimental diffraction data. From the quantitative reconstruction of diffraction images the vertical LiF-surface reconstruction, or buckling, can be determined.

Figure 1

Helium atoms scattered at the LiF surface at a grazing angle of incidence (a). In 110 direction, the He atoms interact with longitudinal chains of either Li or F atoms (b). Averaged effective two-dimensional string potential $V_{\mathrm{str}}$ (c).

Figure 2

Probability density function of momentum transfer of a He atom hitting a LiF surface with a temperature of 620 K. $P(q_{\perp })$ and $P(q_{\parallel })$ are projections onto the transverse (left) and longitudinal (bottom) direction.

Figure 3

A delocalized wave packet hits the surface that is considerabely wider than the size of the unit cell. Different components of the wave packet [(indicated in (a) by narrower wave packets] are subject to different random momentum changes, which lead to a partial loss of coherence. The coherent width of the wave packet is therefore drastically reduced (b).

Figure 4

Experimental (from Ref. 9) (a) and theoretical (b),(c) diffraction patterns for scattering of 1 keV He from LiF under $\theta =1.1°$. In (b), with surface buckling $\delta z=0.053\AA$, (c) without surface buckling $\delta z=0$. Note the different number of interference maxima.