We present theoretical and experimental evidence of an anomalous surface corrugation behavior in He-KCl(001) for incidence along $⟨110⟩$. When the He normal energy decreases below 100 meV, i.e., He-surface distances $Z>2\AA$, the corrugation unexpectedly increases up to an impressive $\gtrsim 85\%$. This is not due to van der Waals interactions but to the combination of soft potential effects and the evolution of He-cation and He-anion interactions with $Z$. This feature, not previously analyzed on alkali-halide surfaces, may favor the alignment properties of weakly interacting overlayers.

• Received 20 December 2019
• Accepted 31 July 2020

DOI:https://doi.org/10.1103/PhysRevLett.125.096101