Density functional theory of the structure of magnesium-doped helium nanodroplets

Abstract

We have studied the structure of 4 He droplets doped with magnesium atoms using density functional theory.We have found that the solvation properties of this system strongly depend on the size of the 4 He droplet. For small drops, Mg resides in a deep surface state, whereas for large-size drops it is fully solvated but radially delocalized in their interior. We have studied the 3s3p 1 P 1 ← 3s 21 S 0 transition of the dopant, and have compared our results with experimental data from laser-induced fluorescence ͑LIF͒. Line-broadening effects due to the coupling of dynamical deformations of the surrounding helium with the dipole excitation of the impurity are explicitly taken into account. We show that the Mg radial delocalization inside large droplets may help reconcile the apparently contradictory solvation properties of magnesium as provided by LIF and electronimpact ionization experiments. The structure of 4 He drops doped with two magnesium atoms is also studied and used to interpret the results of resonant two-photon-ionization ͑R2PI͒ and LIF experiments. We have found that the two solvated Mg atoms do not easily merge into a dimer, but rather form a weakly bound state due to the presence of an energy barrier caused by the helium environment that keeps them some 9.5 Å apart, preventing the formation of the Mg 2 cluster. From this observation, we suggest that Mg atoms in 4 He drops may form, under suitable conditions, a soft "foamlike" aggregate rather than coalesce into a compact metallic cluster. Our findings are in qualitative agreement with recent R2PI experimental evidence. We predict that, contrarily, Mg atoms adsorbed in 3 He droplets do not form such metastable aggregates.

Submersion of potassium clusters in helium nanodroplets

“…These apparently contradictory findings were reconciled by density functional theory calculations, which show that although Mg becomes fully solvated in sufficiently large helium droplets (>200 helium atoms), the energy lowering delivered by full solvation is relatively modest. 19 As a consequence, Mg is highly delocalized within the helium droplet and therefore has the opportunity to occasionally explore the subsurface region. This dual nature of the Mg atoms, in which they show both interior and near-surface behavior, accounts for the experimental findings.…”

Ca impurity in small mixed H4e–H3e clusters

“…It makes use of the Franck-Condon principle within a semiclassical approach. It has been adapted and used by several authors to analyze the absorption spectrum of e.g., lithium in solid H 2 [40], and of different atoms in helium clusters [41,42,43,45]. The particular case of Ca atoms we are interested in has been addressed within DF in pure and mixed helium clusters [12,43].…”

Mg impurity in helium droplets

“…Due to the light mass of this alkaline earth atom and its weak interaction with helium, Mg impurities are found to be very delocalized inside droplets containing several thousand 4 He atoms, 13 as those of interest in recent experiments. 18,19 In the case of 3 He, we have found that Mg is always in the bulk of the 3 He droplet, as determined by previous DFT calculations.…”

“…A similar transition from surface to bulk location of Mg was also found within DFT. 13 This paper is organized as follows. In Sec.…”

“…Within Density Functional Theory (DFT), it has been found to solvate in 3 He droplets and to be very delocalized in 4 He ones. 10,13 Previous diffusion Monte Carlo calculations (DMC) 14 have yielded the result that Mg is fully solvated for a critical number of 4 He atoms of about N 4 = 30. A similar transition from surface to bulk location of Mg was also found within DFT.…”