Fingerprints of microscopic superfluidity in HHen+ clusters

Abstract

The structures and the vibrational dynamics of the complexes HHe + n are investigated experimentally (via mass spectrometry (MS)) and at high levels of electronic-structure theory. The MS measurements reveal interesting trends about the stability of the starting members of the HHe + n family. The computations establish that the basically linear, strongly bound, symmetric triatomic molecular ion He(H +)He, with an equilibrium H-He distance of 0.925 Å and about 2/3 but at least 1/2 of the positive charge on H, is the molecular core of all of the n ≥ 3 complexes. Definitive quantumchemical results are obtained for HHe + and HHe + 2 , including the proton affinity of He (computed to be 14, 876 ± 12 cm −1 via the focal-point analysis (FPA) scheme), the FPA isomerisation energy between the two linear isomers of HHe + 2 (3826 ± 20 cm −1), and the dissociation energy of the HHe + 2 → HHe + + He reaction, with an FPA estimate of 3931 ± 20 cm −1. The structural isomers of the He-solvated complexes are discussed up to n = 18. A useful notation, [k−l−m]-HHe + n , is introduced to characterise qualitatively the three possible belts around the He-H +-He core in HHe + n (n ≥ 3), where l denotes the number of He atoms in the central belt and k ≥ m denote the number of He atoms in the top and bottom belts. Capping He atoms attached to the belts can be indicated by sub-and superscripts. Several possible indicators of microscopic superfluidity are investigated: He evaporation energies, rotational constants, and vibrational fundamentals.

Protonated and Cationic Helium Clusters

“…However, other magic sizes that are characteristic for protonated clusters of heavier rare gases, such as n = 7 and 19, and which also signify icosahedral structures, are missing for protonated He. Theoretical calculations of He n H + structures suggest that sizes of n = 6 and 13 are particularly stable, in good agreement with our findings, but surprisingly show no enhanced stability for n = 11 [35], which is the strongest abundance anomaly in our data. The binding energies determined by the calculations show that atoms in the first solvation layer around the proton (3 ≤ n ≤ 6) each have a binding energy of about 250 meV per atom [35].…”

“…Theoretical calculations of He n H + structures suggest that sizes of n = 6 and 13 are particularly stable, in good agreement with our findings, but surprisingly show no enhanced stability for n = 11 [35], which is the strongest abundance anomaly in our data. The binding energies determined by the calculations show that atoms in the first solvation layer around the proton (3 ≤ n ≤ 6) each have a binding energy of about 250 meV per atom [35]. The second layer atoms (up to n = 13) are bound by approximately 6 meV per He, while subsequent layers are bound even more weakly, with the binding energy eventually approaching that of neutral clusters (≈0.6 meV per atom) [35,47].…”

“…The binding energies determined by the calculations show that atoms in the first solvation layer around the proton (3 ≤ n ≤ 6) each have a binding energy of about 250 meV per atom [35]. The second layer atoms (up to n = 13) are bound by approximately 6 meV per He, while subsequent layers are bound even more weakly, with the binding energy eventually approaching that of neutral clusters (≈0.6 meV per atom) [35,47]. The calculations also show that the structures do not display icosahedral symmetry or any other higher order level symmetry for most sizes [35], unlike other protonated rare gas clusters [26,27].…”

“…Protonated He atoms and clusters are attractive systems from a theoretical standpoint, due to the relatively small number of electrons and the nuclear quantum effects present with light atoms, and have been the focus of numerous computational studies [28][29][30][31][32][33][34][35]. Experimentally, small protonated He clusters have, for example, been produced from He nanodroplets doped with molecular hydrogen for mass spectrometry and spectroscopic studies [35][36][37], and in low temperature drift tubes filled with He gas [38]. Consisting of the two most abundant elements in the universe, HeH + ions and complexes containing this ion as a chromophore are expected to be plentiful in astrophysical plasmas [39][40][41], but was only very recently discovered in spectra obtained by the SOFIA airborne observatory [42].…”

Digitális kémia

“…Szin tetikus és mé rési szem pontból is könnyebben kezel he tõk a kisebb komp lexek, bár a mikroszkópikus szuper flui ditás kísérleti bizonyításához a HHe n + komp lexek esetében a na gyobb, kö zel 20 He atomot tar talmazó komplexek spekt rosz kó piai vizs gálata lenne kí vánatos. 7 Különleges stabilitással rendelkezik a He evaporációja szem pontjából a HHe 6 + komplex, melynek egyensúlyi szer ke zete az 1. ábrán látható. Jól látható, hogy az elsõ He-szol va tációs szféra a legnagyobb pozitív parciális töltéssel ren del ke zõ proton körül alakul ki.…”