Cationic Complexes of Hydrogen with Helium

Bartl, Peter; Leidlmair, Christian; Denifl, Stephan; Scheier, P.; Echt, Olof

doi:10.1002/cphc.201200664

Cited by 29 publications

(43 citation statements)

References 68 publications

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“…The computed evaporation energies show a steady decreasing tendency, changing from 0.7 to 0.4 kcal mol −1 when going from n = 3 to n = 6. This computational result is in contrast to the results of experimental MS studies [35,39] showing that n = 6 is a 'magic number' and HHe + 6 has pronounced stability (see also our own MS study in Section 2 supporting this statement). Classical and quantum dynamical studies have also been attempted on these systems [73].…”

Section: Introductioncontrasting

confidence: 95%

“…Despite the fact that the H x He + n molecular ions and their He-solvated complexes are formed by the two most abundant elements of our known universe, surprisingly little is known about the structure, energetics, and especially the nuclear dynamics of H x He + n species. Most of the experimental investigations, especially for larger x and (in particular) larger n values, are mass spectrometry (MS) studies [33][34][35][36][37][38][39]. MS does provide important information about the existence of H x He + n species (vide infra), and even some limited information about their relative stabilities.…”

Section: Introductionmentioning

confidence: 99%

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Fingerprints of microscopic superfluidity in HHen+ clusters

2019

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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.

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Section: Introductioncontrasting

confidence: 95%

Section: Introductionmentioning

confidence: 99%

Fingerprints of microscopic superfluidity in HHen+ clusters

2019

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“…A few years later, using polarised Gaussian orbitals, it has been confirmed that it is stable with a binding energy of about 100 cm −1 and it was predicted that this complex ion should be detectable below 140 K [60]. As already mentioned above, stable He n -H + x complexes have been found, first in a 4.4 K drift tube [45] and recently using He droplets [46]. But there are no spectroscopic studies at all and no state of the art calculations.…”

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confidence: 83%

“…Superfluid helium droplets provide a very cold environment (0.37 K) in which complexes of ions with He can be embedded. Using a highresolution mass spectrometer, a whole zoo of He n H + x and He n D + x ions has been identified recently [46]. However, neither of these experiments is capable to follow the dynamics of the growth of such ions.…”

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confidence: 99%

Controlled synthesis and analysis of He–H⁺₃in a 3.7 K ion trap

et al. 2015

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Complexes of the triatomic hydrogen ion with helium were synthesised in a low-temperature 22-pole rf ion trap at He number densities of up to 10 16 cm −3 . Absolute ternary rate coefficients for sequentially attaching He atoms have been determined from the growth of complexes with increasing storage time. The number of helium-tagged ions is significantly reduced when increasing the nominal temperature from 4 to 25 K. Competition between attachment and dissociation via collisions leads to stationary He n -H + 3 (n up to 9) distributions. State-specific excitation of the trapped H + 3 ions via IR transitions significantly reduces the formation of complexes. Tuning the laser to v 2 = 1 transitions in the range of 2726 cm −1 leads to LIICG lines, i.e., to spectra caused by laser-induced inhibition of complex growth. In addition, almost 100 lines have been found between 2700 and 2765 cm −1 , which are attributed to laser-induced dissociation of the in situ formed He-H + 3 complex ions. These lines are not yet assigned; however, their absorption strength, statistics and predissociation lifetimes provide interesting information on both the stable complexes as well as on scattering resonances in low-energy H + 3 + He collisions. New calculations of the potential energy surface will help to analyse the dissociation spectrum. There are some indications that para-H + 3 is enriched under the conditions of the present experiment. IntroductionStudies of reactions between ions and neutrals under conditions relevant for astrochemistry are important for understanding different processes in the interstellar medium (ISM). Reactions involving helium and hydrogen significantly influence the overall evolution of our universe including the early universe chemistry and especially the 'Dark Age'. For modelling such environments, one needs reliable rate coefficients. A detailed discussion of primordial chemistry can be found in [1] and references therein. According to [2], helium became neutral at a red shift of z ∼ 2500. At those times, the density was already very low and the only way to form molecules was via radiative association. Because hydrogen was still ionised, models predict that the first molecule formed in space was HeH + . The coexistence of He and He + also lead to the formation of some He + 2 . Molecules were needed as coolants during the formation of the first galaxies. Also after these early times, helium and hydrogen play an important role. In all models of interstellar ion chemistry, the first steps are ionisation of He and H 2 by cosmic rays.

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“…15,23 Here, n = 12 is a frequently observed anomaly, often interpreted to indicate formation of a solvation shell with icosahedral symmetry, 22,24 but the number of helium atoms in the first solvation shell may be much larger. For example, 18–20 atoms are needed to complete the first solvation shell for Mg + , 20,25 20 for I 2 + , 21 60 for C 60 + , and 62 for C 70 + .…”

Section: Introductionmentioning

confidence: 99%

On the Size and Structure of Helium Snowballs Formed around Charged Atoms and Clusters of Noble Gases

et al. 2013

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Helium nanodroplets doped with argon, krypton, or xenon are ionized by electrons and analyzed in a mass spectrometer. HenNgx+ ions containing up to seven noble gas (Ng) atoms and dozens of helium atoms are identified; the high resolution of the mass spectrometer combined with advanced data analysis make it possible to unscramble contributions from isotopologues that have the same nominal mass but different numbers of helium or Ng atoms, such as the magic He2084Kr2+ and the isobaric, nonmagic He4184Kr+. Anomalies in these ion abundances reveal particularly stable ions; several intriguing patterns emerge. Perhaps most astounding are the results for HenAr+, which show evidence for three distinct, solid-like solvation shells containing 12, 20, and 12 helium atoms. This observation runs counter to the common notion that only the first solvation shell is solid-like but agrees with calculations by Galli et al. for HenNa+ [21568337J. Phys. Chem. A20111157300] that reveal three shells of icosahedral symmetry. HenArx+ (2 ≤ x ≤ 7) ions appear to be especially stable if they contain a total of n + x = 19 atoms. A sequence of anomalies in the abundance distribution of HenKrx+ suggests that rings of six helium atoms are inserted into the solvation shell each time a krypton atom is added to the ionic core, from Kr+ to Kr3+. Previously reported strong anomalies at He12Kr2+ and He12Kr3+ [KimJ. H.16774401J. Chem. Phys.2006124214301] are attributed to a contamination. Only minor local anomalies appear in the distributions of HenXex+ (x ≤ 3). The distributions of HenKr+ and HenXe+ show strikingly similar, broad features that are absent from the distribution of HenAr+; differences are tentatively ascribed to the very different fragmentation dynamics of these ions.

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Cationic Complexes of Hydrogen with Helium

Cited by 29 publications

References 68 publications

Fingerprints of microscopic superfluidity in HHen+ clusters

Fingerprints of microscopic superfluidity in HHen+ clusters

Controlled synthesis and analysis of He–H⁺₃in a 3.7 K ion trap

On the Size and Structure of Helium Snowballs Formed around Charged Atoms and Clusters of Noble Gases

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Cationic Complexes of Hydrogen with Helium

Cited by 29 publications

References 68 publications

Fingerprints of microscopic superfluidity in HHen+ clusters

Fingerprints of microscopic superfluidity in HHen+ clusters

Controlled synthesis and analysis of He–H+3in a 3.7 K ion trap

On the Size and Structure of Helium Snowballs Formed around Charged Atoms and Clusters of Noble Gases

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Controlled synthesis and analysis of He–H⁺₃in a 3.7 K ion trap