Clustering of Cold Hydrogen Gas on Protons

Clampitt, R.; Gowland, L.

doi:10.1038/223815a0

Cited by 96 publications

(36 citation statements)

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“…From these multiple-steps reactions, H + n clusters (n odd) are formed, and clusters as large as H + 99 have been observed [1]. Depending on the temperature and pressure conditions, the H + 15 and H + 17 clusters are the more abundant.…”

Section: Introductionmentioning

confidence: 99%

On the formation mechanisms of hydrogen ionic clusters

Barbatti¹,

Nascimento²

2003

Braz. J. Phys.

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Structural and thermodynamic properties of hydrogen molecular clusters formed around an atomic or molecular cation are examined. The shell distribution of H2 molecules and the size of the clusters are discussed. The Bloom-Margenau model for predicting the number of neutral molecules that could bind to a cation core is investigated and its limitations are illustrated using the Li + (H2) k clusters as test case. Finally, results for the entropy of the H + n clusters (n = 5 − 27, odd) and for the Gibbs free energy variations associated to the cluster formation are presented and the spontaneity of the clustering process in different conditions is examined. I IntroductionThe presence of a cation in a molecular hydrogen environment leads to the formation of molecular clusters around the ion. In the case of an H 2 homogeneous atmosphere, an H + 2 ion is quickly converted to the H + 3 molecule, which becomes the core for the clustering processwhere the exceeding H 2 molecule carries away the excess of energy, stabilizing the cluster. From these multiple-steps reactions, H + n clusters (n odd) are formed, and clusters as large as H + 99 have been observed [1]. Depending on the temperature and pressure conditions, the H + 15 and H + 17 clusters are the more abundant.One of the most interesting features of these species is that in the cluster, each H 2 molecule is strongly bound by the coulombian field of the cation. This may be very useful when dealing with hydrogen storage problems.In the last decade, ab initio calculations have been performed to clusters as large as H + 35 [2][3][4][5][6][7][8][9][10][11][12]. Recently, more attention has been dedicated to the H + 5 cluster. Its potential energy surface has been studied at high levels of calculation [13,14] and its presence in some interstellar environments in concentrations up two times higher than that of the H + 3 molecule has been investigated [15].Besides the H + 3 molecular ion, a large variety of atomic and molecular cations has been considered as a core for H 2 clustering: the first-column Li + , Na + and K + ions [16][17][18][19][20][21][22] Table I summarizes some properties of these X + (H 2 ) k hydrogen clusters.The experimental work on the hydrogen clusters has been concentrated on enthalpy variation measurements (∆H) [16,24,25,26,31,33] and collisional induced dissociation (CID) studies [34,35,36,37]. The ∆H measurements for the H + n clusters and for the most general X + (H 2 ) k clusters have been an important source of information about the cluster energetic properties and have guided the theoretical studies. On the other hand, the CID studies have revealed patterns for the cluster dissociation and their dependence on the shell structure of H 2 distribution. Hitherto, very few theoretical and experimental attention has been payed to the vibrational properties and the infrared spectra of the hydrogen clusters, these studies being basically restricted to the H + 5 species [38,39]. For clusters larger than the H + 5 one, only a few experimental results are...

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Section: Introductionmentioning

confidence: 99%

On the formation mechanisms of hydrogen ionic clusters

Barbatti¹,

Nascimento²

2003

Braz. J. Phys.

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“…This is in marked contrast to helium droplet experiments focusing on the cationic clusters, H þ n , formed by electron ionization. Although odd n ions overwhelmingly dominate in the gas phase [20], even n ions become significant positively charged products when experiments are carried out in helium nanodroplets [21]. This is presumably because of partial quenching of the initially "hot" ions by the surrounding helium, which thereby stabilizes the even n ions.…”

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confidence: 99%

Anionic Hydrogen Cluster Ions as a New Form of Condensed Hydrogen

et al. 2016

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We report the first experimental observation of negatively charged hydrogen and deuterium cluster ions, H − n and D − n , where n ≥ 5. These anions are formed by an electron addition to liquid helium nanodroplets doped with molecular hydrogen or deuterium. The ions are stable for at least the lifetime of the experiment, which is several tens of microseconds. Only anions with odd values of n are detected, and some specific ions show anomalously high abundances. The sizes of these "magic number" ions suggest an icosahedral framework of H 2 (D 2 ) molecules in solvent shells around a central H − (D − ) ion. The first three shells, which contain a total of 44 H 2 or D 2 molecules, appear to be solidlike, but thereafter a more liquidlike arrangement of the H 2 (D 2 ) molecules is adopted. to be strongly bent rather than linear. Consequently, there are real doubts about the basic structures of H − n ions that need to be resolved, and given that there are no reliable estimates of the actual dissociation energies (D 0 ) of these clusters, it is not even clear if anions with n > 3 are stable.Here we report the first experimental detection of anionic hydrogen and deuterium clusters larger than H − 3 =D − 3 and have done so for a wide range of cluster sizes. The experimental procedure involved the formation of the corresponding neutral ðH 2 Þ N and ðD 2 Þ N clusters by adding H 2 or D 2 gas to liquid helium nanodroplets. The neutral clusters were cooled to the ambient temperature of the helium nanodroplets, 0.38 K [13], prior to an impact by a beam of electrons with a controlled energy. H 2 and D 2 are heliophilic (have a negative chemical potential when immersed in liquid helium [13]) and so will reside inside the helium droplets rather than on the surface. The helium droplets were then exposed to a beam of electrons, which generated anionic products in the gas phase that were detected by mass spectrometry. The transfer of negative charge to the ðH 2 Þ n and ðD 2 Þ N clusters occurs via a mobile electron bubble, whose formation has a threshold energy in excess of 1 eV in order to inject the electron into the helium conduction band [14]. The droplets used in the present work were relatively large (∼10 6 helium atoms), and for droplets of this size the electron bubble, although

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“…All of the literature cited in § §2.1,2.2 implicitly assumes that molecular hydrogen ion chemistry in the condensed phase resembles that of the gas phase, for which it is well established that clusters H + n with odd-n occur in much greater abundance than those with evenn (Clampitt and Gowland 1969;Hiraoka 1987;Ekinci, Knuth and Toennies 2006). The subsequent discovery of H + 6 in the ESR experiments on irradiated solid hydrogens ( §2.3) challenges that assumption.…”

Section: Molecular Hydrogen Ion Clustersmentioning

confidence: 99%

Interstellar Solid Hydrogen

Lin¹,

Gilbert²,

Walker³

2011

ApJ

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We consider the possibility that solid molecular hydrogen is present in interstellar space. If so cosmic-rays and energetic photons cause ionisation in the solid leading to the formation of H + 6 . This ion is not produced by gas-phase reactions and its radiative transitions therefore provide a signature of solid H 2 in the astrophysical context. The vibrational transitions of H + 6 are yet to be observed in the laboratory, but we have characterised them in a quantum-theoretical treatment of the molecule; our calculations include anharmonic corrections, which are large. Here we report on those calculations and compare our results with astronomical data. In addition to the H + 6 isotopomer, we focus on the deuterated species (HD) + 3 which is expected to dominate at low ionisation rates as a result of isotopic condensation reactions. We can reliably predict the frequencies of the fundamental bands for five modes of vibration. For (HD) + 3 all of these are found to lie close to some of the strongest of the pervasive mid-infrared astronomical emission bands, making it difficult to exclude hydrogen precipitates on observational grounds. By the same token these results suggest that (HD) + 3 could be the carrier of the observed bands. We consider this possibility within the broader picture of ISM photo-processes and we conclude that solid hydrogen may indeed be abundant in astrophysical environments.

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Clustering of Cold Hydrogen Gas on Protons

Cited by 96 publications

References 4 publications

On the formation mechanisms of hydrogen ionic clusters

On the formation mechanisms of hydrogen ionic clusters

Anionic Hydrogen Cluster Ions as a New Form of Condensed Hydrogen

Interstellar Solid Hydrogen

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