A density functional study of the structure of small OCS@3HeN clusters

Abstract

Kohn-Sham density functional calculations are reported for the structures of clusters consisting of a carbonyl sulfide (OCS) molecule with N = 1, 8, 18, and 40 attached (3)He atoms. The N = 1 cluster ground state is highly localized at the molecular waist (donut ring position), but for higher levels of excitation becomes increasingly delocalized. The first magic cluster with 8 atoms has a significant density at both ends of the molecule in addition to the donut ring. With N = 18 (3)He atoms the molecule is enclosed by a magic number closed shell. Another magic stable structure consisting of two nearly isotropically spherical closed shells is found at N = 40. A comparison with calculations for the same sized (4)He clusters show some important similarities, e.g., pile up at the donut ring position but altogether a more diffuse, less anisotropic structure. These results are discussed in the light of the recently analyzed infrared spectra measured in large pure (3)He droplets (N ≈ 1.2 × 10(4)) [B. Sartakov, J. P. Toennies, and A. F. Vilesov, J. Chem. Phys. 136, 134316 (2012)]. The moments of inertia of the 11 atom spherical shell structure, which is consistent with the experimental spectrum, lies between the predicted moments of inertia for N = 8 and N = 18 clusters. Overall the calculations reveal that the structures and energies of small doped (3)He are only slightly more diffuse and less energetic than the same (4)He clusters.

The structure of mixed 3He-4He droplets doped with OCS: A density functional approach

“…It does not mean, however, that the dopant is not affected by the droplet. In particular, the analysis of the experimental rovibrational spectrum for heavy molecules in helium droplets is consistent with an increase of their effective moment of inertia explained by the appearance of a ring of 4 He atoms around the waist of the molecule that participate in its end-over-end rotation.…”

“…8 Such a small temperature (T) would hardly affect the boson component of the droplet nor its fermionic component for configurations corresponding to a magic number of 3 He atoms yielding marked shell closures in the Kohn-Sham framework. 4 For other numbers of 3 He atoms it may have some effect, especially if the level spacing of the single-particle (s.p.) energy levels, in K, is of the order of T. Since we want to address mixed droplets of arbitrary number of 3 He and 4 He atoms, N 3 and N 4 respectively, we have adopted a scheme in which 4 He is described at zero temperature whereas 3 He is described within a finite temperature KS approach.…”

“…It is well established that molecules embedded in superfluid 4 He droplets rotate inside them, as inferred from the appearance of well resolved rotational lines in the molecular spectrum, see, e.g., Ref. 1 and references therein.…”

“…Many experimental and theoretical studies of isotopically pure 4 He droplets doped with different atomic and molecular impurities are available and have been reviewed in a series of recent papers. 2 More scarce are those on doped 3 He droplets, see, for instance, Refs.…”

Raman scattering signatures of the unusual vibronic interaction of molecules in liquid helium-3

“…ðk 0 ; ' 0 m 0 Þ, where k, k 0 are radial wave vectors and 'm; ' 0 m 0 angular quantum numbers. Since in the 3 He Fermi liquid the CM rotations are completely damped and orientation is random [10,11] it can be assumed, as previously done for adsorption processes [1], that no angular momentum is transferred to Fermi excitations and, moreover, that only spherical (' ¼ m ¼ 0) excitations couple to the electronic transition, so that V ij ! V k00;k 0 00 .…”