A Theoretical Study of 1‐D and 2‐D Helium Lattices

Abstract

AbstractOne‐ and two‐dimensional (1‐D and 2‐D) helium lattices have been studied using ab initio RHF/6–31G** computations. Structural, physical and thermochemical properties have been calculated and analyzed for the 1‐D and 2‐D HeN lattices respectively up to N = 50 and N = 36. Asymptotic properties of the 1‐D HeN lattices are obtained by extrapolating N‐dependence properties to large values of N. Analysis of the results show that the bulk per‐atom interaction (binding) energies increase while the optimized interatomic distances (bond lengths) slightly decrease with the increase in size of the 1‐D HeN lattices and both reach their asymptotic values of 0.352 cm−1 and 3.18775 Å, respectively. Between the square and hexagonal (packed) structures of the 2‐D HeN lattices, the latter is more favored. Extrapolated values of the calculated properties, including lattice parameter, binding and zero point energies, heat capacity, and entropy have also been calculated for both 1‐D and 2‐D HeN lattices. The surface densities for monolayer films of helium atoms with square and hexagonal configurations have been calculated to be respectively 9.84 × 1018 and 1.04 × 1019 helium atoms/cm2 which are comparable to the experimental value of 2.4 × 1019 helium atom/m2 well within the typical large and directional error bars of the experiments. Surface effects have been investigated by comparing the packed HeN2‐D lattices with the same value of N but with different geometries (arrangements). This comparison showed that the HeN lattices prefer arrangements with the smallest surface area.