Hydrogen Cluster Ions Research Articles

Cluster ion source for external injection and high capacity filling of light elements into the relativistic heavy ion collider electron beam ion source.

An advanced Electron Beam Ion Source (EBIS) is the primary ion source to supply highly charged ion beams of different elements to the Relativistic Heavy Ion Collider (RHIC) and to the NASA Space Radiation Laboratory (NSRL). Intense beams of highly charged ions of various elements of the periodic table, ranging from helium to uranium, have been demonstrated since EBIS became operational in 2010. EBIS routinely provides ion beams to RHIC and NSRL quasisimultaneously with about 1 s switching time between different ion species. Such unique flexibility and rapid switching between ion species are based on external injection of singly charged ions into the EBIS trap either in "fast" or "slow" injection modes. At present, a Laser Ion Source (LIS) provides most of the ion species of solid materials using the "fast" injection mode into the EBIS trap and a Hollow Cathode Ion Source (HCIS) provides most of the ion species of gaseous elements using the "slow" injection mode into the EBIS trap. Gas injection into the EBIS trap is also possible and has been used but imposes some restrictions for the simultaneous generation of highly charged ions such as Au32+ ions for RHIC and ions of gaseous species for NSRL. Because light ions have relatively high velocity inside the EBIS trap, efficient injection of hydrogen and helium ions and filling of the EBIS trap to high capacity is difficult from either LIS or HCIS. To overcome this restriction and enhance EBIS operational capability, we suggest injecting beams of hydrogen and helium cluster ions into the EBIS trap. Required parameters of cluster ion beam injection into the EBIS trap are estimated, and advantages of such an injection are highlighted. A cluster ion source with required high intensity is visible and will be designed, built, optimized, and tested.

Cs+ Solvated in Hydrogen—Evidence for Several Distinct Solvation Shells

Helium nanodroplets are doped with cesium and molecular hydrogen and subsequently ionized by electrons. Mass spectra reveal H$_x$Cs$^{+}$ ions that contain as many as 130 hydrogen atoms. Two features in the spectra are striking: First, the abundance of ions with an odd number of hydrogen atoms is very low; the abundance of HCs$^+$ is only 1 % that of H$_2$Cs$^+$. The dominance of even-numbered species is in stark contrast to previous studies of pure or doped hydrogen cluster ions. Second, the abundance of (H$_2$)$_n$Cs$^+$ features anomalies at n = 8, 12, 32, 44, and 52. Guided by previous work on ions solvated in hydrogen and helium we assign the anomalies at n = 12, 32, 44 to the formation of three concentric, solid-like solvation shells of icosahedral symmetry around Cs$^+$. Preliminary density functional theory calculations for n $\le$ 14 are reported as well.

Anionic Hydrogen Cluster Ions as a New Form of Condensed Hydrogen.

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.

Do coordinating and solvating effects of H2 explain high concentrations of H3 + in interstellar dense clouds? A molecular orbital study of hydrogen cluster ions H3 + to H21 +

Discovered in glow discharge in 1980, molecular ion H3+ presented a quantum mechanical enigma that took scientists a decade and a half to solve. Its assigned spectrum helped spot the first signs of its presence among dense interstellar clouds. H3+ was also thought to be a reactive protonating agent in space. Later, it was also discovered in diffuse clouds. H3+ owes its ubiquity to the reaction of cosmic rays with the ever-present element in the interstellar space, the hydrogen molecule. Through a description of molecular orbital diagrams, not only the high concentrations of H3+, but also high D/H ratios observed in cold interstellar dense clouds can be justified. A series of complexation/solvation mechanisms were used to study ion clusters H2n+1+ (n = 1–10). Electrostatic potential charge analysis and typical intrinsic reaction coordinate computations show a Lennard-Jones tailing effect, characteristic of liquid phase behavior, which suggest a solvation mechanism for H11+ to H23+ that needs further molecular dynamic computations to get more insight on the kinetics of solvation.

Adsorption of hydrogen on neutral and charged fullerene: Experiment and theory

Helium droplets are doped with fullerenes (either C60 or C70) and hydrogen (H2 or D2) and investigated by high-resolution mass spectrometry. In addition to pure helium and hydrogen cluster ions, hydrogen-fullerene complexes are observed upon electron ionization. The composition of the main ion series is (H2)(n)HC(m)(+) where m = 60 or 70. Another series of even-numbered ions, (H2)(n)C(m)(+), is slightly weaker in stark contrast to pure hydrogen cluster ions for which the even-numbered series (H2)(n)(+) is barely detectable. The ion series (H2)(n)HC(m)(+) and (H2)(n)C(m)(+) exhibit abrupt drops in ion abundance at n = 32 for C60 and 37 for C70, indicating formation of an energetically favorable commensurate phase, with each face of the fullerene ion being covered by one adsorbate molecule. However, the first solvation layer is not complete until a total of 49 H2 are adsorbed on C60(+); the corresponding value for C70(+) is 51. Surprisingly, these values do not exhibit a hydrogen-deuterium isotope effect even though the isotope effect for H2/D2 adsorbates on graphite exceeds 6%. We also observe doubly charged fullerene-deuterium clusters; they, too, exhibit abrupt drops in ion abundance at n = 32 and 37 for C60 and C70, respectively. The findings imply that the charge is localized on the fullerene, stabilizing the system against charge separation. Density functional calculations for C60-hydrogen complexes with up to five hydrogen atoms provide insight into the experimental findings and the structure of the ions. The binding energy of physisorbed H2 is 57 meV for H2C60(+) and (H2)2C60(+), and slightly above 70 meV for H2HC60(+) and (H2)2HC60(+). The lone hydrogen in the odd-numbered complexes is covalently bound atop a carbon atom but a large barrier of 1.69 eV impedes chemisorption of the H2 molecules. Calculations for neutral and doubly charged complexes are presented as well.

Photoionization and Vibrational Spectroscopy of the Aniline−Methanol Clusters

Aniline-methanol mixed clusters are ionized by single photon vacuum ultraviolet (VUV, 118 nm) radiation with which absorption to an excited intermediate S(1) state is not required. Aniline ion (An(+)), a series of (An)(n)(+)-(CH(3)OH)(m) (n = 1, 2) cluster ions, and their hydrogenated cluster ions, (An)(n)(+)-(CH(3)OH)(m)H (n = 1, 2) are observed by mass spectrometry. Infrared (IR) absorption spectra of aniline-methanol cluster cations and neutrals are measured through IR and VUV (118 nm) "ion dip" spectroscopy in the range 2500-4000 cm(-1). The observed mid-IR spectrum of the An(+)-CH(3)OH has two sharp absorption bands, at 3438 and 3668 cm(-1), which are assigned to the free NH stretch vibration of the aniline cation and the free OH stretch vibration of methanol, respectively. Calculations demonstrated that a change in the charges on the nitrogen atom of the amine group upon ionization of the neutral to the cluster cation alters the role of aniline from hydrogen acceptor to hydrogen donor in its interaction with methanol. Theoretical and experimental results suggest that a hydrogen bond forms between one of the H atoms of the aniline amine group and the lone pair of electrons of the methanol oxygen atom in the aniline-methanol cluster cation. Measured IR spectra and theoretical results for neutral clusters suggest that the H atom of the methanol OH moiety is bonded to the aniline amine group lone pair electrons for the neutral ground state aniline-methanol cluster.

Formation of even-numbered hydrogen cluster cations in ultracold helium droplets

Neutral hydrogen clusters are grown in ultracold helium nanodroplets by successive pickup of hydrogen molecules. Even-numbered hydrogen cluster cations are observed upon electron-impact ionization with and without attached helium atoms and in addition to the familiar odd-numbered H(n)(+). The helium matrix affects the fragmentation dynamics that usually lead to the formation of overwhelmingly odd-numbered H(n)(+). The use of high-resolution mass spectrometry allows the unambiguous identification of even-numbered H(n)(+) up to n approximately = 120 by their mass excess that distinguishes them from He(n)(+), mixed He(m)H(n)(+), and background ions. The large range in size of these hydrogen cluster ions is unprecedented, as is the accuracy of their definition. Apart from the previously observed magic number n=6, pronounced drops in the abundance of even-numbered cluster ions are seen at n=30 and 114, which suggest icosahedral shell closures at H(6)(+)(H(2))(12) and H(6)(+)(H(2))(54). Possible isomers of H(6)(+) are identified at the quadratic configuration interaction with inclusion of single and double excitations (QCISD)/aug-cc-pVTZ level of theory.

Scanning atom probe study of graphite nanofibers

Field emission characteristics of graphite nanofibers (GNFs) are studied by operating a scanning atom probe as a field emission microscope. The Fowler-Nordheim plot indicates that the work function of GNF is larger than that of carbon nanotubes and is comparable to tungsten. Field emission patterns indicate that electrons are emitted from the edges of graphene sheets in the direction perpendicular to the GNF axis. The ratio of the field emission voltage and that of the field evaporation voltage implies that the binding between the graphene sheets is weak. The field evaporated cluster ions of carbon and hydrogen indicate that hydrogen is not distributed randomly but is forming the characteristic carbon-hydrogen cluster, C23H2. The proposed structure of the cluster is the triangularly arranged six hexagonal cells with hydrogen terminated two carbon atoms and an extra carbon atom.

Absolute cross sections for the dissociation of hydrogen cluster ions in high-energy collisions with helium atoms

Absolute dissociation cross sections are reported for $\mathrm{H}_{n}{}^{+}$ clusters of varied mass $(n=3,5,\dots{},35)$ following collisions with He atoms at $60\phantom{\rule{0.3em}{0ex}}\mathrm{keV}∕\mathrm{amu}$. Initial results have been published previously for a smaller range of cluster sizes [Ouaskit et al., Phys. Rev. A 49, 1484 (1994)]. The present extended study includes further experimental results, reducing the statistical errors associated with the absolute cross sections. The previously suggested quasilinear dependence of the $\mathrm{H}_{n}{}^{+}$ dissociation cross sections upon $n$ is developed with reference to expected series of geometrical shells of ${\mathrm{H}}_{2}$ molecules surrounding a $\mathrm{H}_{3}{}^{+}$ core. Recent calculations identify $n=9$ as corresponding to the first closed ${\mathrm{H}}_{2}$ shell [e.g., \ifmmode \check{S}\else \v{S}\fi{}tich et al., J. Chem. Phys. 107, 9482 (1997)]. Recurrence of the distinct characteristics observed in the dissociation-cross-section dependence upon cluster size around $n=9$ provides the basis for the presently proposed subsequent closed shells at $n=15$, 21, 27, and 33, in agreement with the calculations of Nagashima et al. [J. Phys. Chem. 96, 4294 (1992)].

Evidence for double-electron capture in the H9+–He collision

The double-electron capture process in the H9+–He collision has been evidenced using the accelerator mass spectrometry method associated to a multi-coincidence detection. An absolute value for the cross-section is measured. The double-electron capture of a hydrogen cluster ion from a helium atom can be thought as a very localised process involving the H3+ core ion of the molecular cluster.

Collision induced cluster fragmentation: From fragment size distributions to the caloric curve

We report on a cluster fragmentation study involving collisions of high-energy (60 keV/amu) H 3 +(H 2) m hydrogen cluster ions ( m=9, 11) with atomic helium or fullerenes. The experimental characterisation of the cluster fragmentation not only by the average fragment size distribution but also by a statistical analysis of the fragmentation events has become possible owing to a recently developed multi-coincidence technique in which all the fragments of all collisions occurring in the experiment are mass analysed on an event-by-event basis. By selecting specific decay reactions we can start after the energizing collision with a microcanonical cluster ion ensemble of fixed excitation energy. From the respective fragment distributions for these selected decay reactions we derive corresponding temperatures of the decaying cluster ions. The relation between this temperature and the excitation energy (caloric curve) exhibits the typical prerequisites of a first order phase transition in a finite system, in the present case signalling the transition from a bound cluster type situation to the free gas phase.

Direct experimental evidence for a negative heat capacity in the liquid-to-gas phase transition in hydrogen cluster ions: backbending of the caloric curve.

By selecting specific decay reactions in high-energy collisions (60 keV/amu) of hydrogen cluster ions with a helium target (utilizing event-by-event data of a recently developed multicoincidence experiment) and by deriving corresponding temperatures for these microcanonical cluster ensembles (analyzing respective fragment distributions), we are able to construct caloric curves for H+3(H2)(m) cluster ions (6<or=m<or=14). All individual curves and the mean of these curves show a backbending in the plateau region, thus constituting direct evidence for a negative microcanonical heat capacity in the liquid-to-gas transition of these finite systems.

Caloric curve for the liquid-to-gas transition of the H 25+ hydrogen cluster ion

High energy collisions (60 keV/amu) of the H 25 + hydrogen cluster ions with an helium target have been completely analyzed on an event-by-event basis in a recently developed multi-coincidence experiment. By selecting specific decay reactions we can start after the energizing collision with a microcanonical cluster ion ensemble of fixed excitation energy. From the respective fragment distributions for these selected decay reactions we derive corresponding temperatures of the decaying cluster ions. The relation between this temperature and the excitation energy (caloric curve) exhibits the typical prerequisites (plateau region) of a first order phase transition in a finite system, in the present case signaling the transition from a bound cluster to the free gas phase type situation.

Probing the liquid-to-gas phase transition in a cluster via a caloric curve.

High-energy collisions ( 60 keV/amu) of hydrogen cluster ions with a helium target have been completely analyzed on an event-by-event basis. By selecting specific decay reactions we can start after the energizing collision with a microcanonical cluster ion ensemble of fixed excitation energy and we derive corresponding temperatures of the decaying cluster ions. The relation between the temperature and the excitation energy (caloric curve) exhibits the typical prerequisites of a first-order phase transition in a finite system, in the present case signaling the transition from a bound cluster to the gas phase.

Cluster multifragmentation and percolation transition: A quantitative comparison for two systems of the same size

Using a recently developed multicoincidence technique, fragmentation resulting from collisions between high energy $(60 \mathrm{k}\mathrm{e}\mathrm{V}/\mathrm{u})$ ${\mathrm{H}}_{27}^{+}$ ions with an He target are investigated on an event by event basis. The data obtained are analyzed in terms of fluctuations in the fragment size distribution. A comparison with results from two different three-dimensional (3D) lattice bond percolation models of the same size (27 constituents) indicates the presence of a critical behavior in the decaying finite system and the importance of accounting in the percolation model for the specific binding situation in the hydrogen cluster ion.

Infrared Spectrum ofp-Hydrogen Crystals Ionized by 3 MeV Electrons: Cluster Ions of Hydrogen in Condensed Phase

High purity p-H_2 crystals have been ionized by 3 MeV electron beams from a van de Graaff accelerator, and their infrared spectra have been studied from 1800 to 8000 cm^(-1) using a Fourier transform infrared spectrometer. Out of 16 spectral lines induced by the electron bombardment, 5 lines have been assigned to p-H_2 spectral lines that are induced by the electric field of charges deposited in the ionized crystal. The stability of such spectral lines several hours after electron bombardment demonstrates the stability of charges in the crystal. A mechanism of stabilization of positive and negative charges as cation clusters H_3^+(H_2)_n and anion clusters H^-(H_2)_n, respectively, is proposed.

Multifragmentation after multi-ionization of hydrogen clusters in high energy cluster–atom and cluster–cluster collisions

We report on a cluster fragmentation study involving collisions of high-energy hydrogen cluster ions with atomic helium or fullerenes. The experimental characterization of the cluster fragmentation, not only by the average fragment size distribution but also by a statistical analysis of the fragmentation events, has become possible because of a recently developed multicoincidence technique in which all the fragments of all the collisions occurring in the experiment are mass analyzed on an event by event basis. From the breakup in two fragments to the complete disintegration of the cluster, the fragmentation phenomenon exhibits a transition with an increase of the fluctuations. The fragmentation events with more than one H 3 + fragment evidence the cluster multifragmentation process. An important aspect of these results is that high-energy cluster collision can induce a reaction in the cluster.

Collisions of ionized hydrogen clusters with helium atoms: electron capture as a function of the cluster size

Electron capture cross sections of hydrogen cluster ions Hn+ colliding with atomic helium have been measured in a large range of cluster size (5⩽n⩽35) for the same velocity (1.5v0, 60 keV/u). While the electron capture cross section decreases from the H+ ion to the H3+ one, the cluster electron capture cross section is found to be independent of the cluster size and nearly equal to the capture cross section of the H3+ ion. Electron capture by hydrogen clusters on a helium atom is a process involving only the H3+ core of the cluster where the positive charge is localized. It appears that this very localized electron capture is not disturbed by the presence of molecules, up to sixteen, around the H3+ core.

Atomic level analysis of electron emitter surfaces by the scanning atom probe

Fine grains of chemical vapor deposition (CVD) diamonds and silicon tip apexes of a microtip array are mass-analyzed at atomic dimension by a scanning atom probe (SAP). The study revealed that the CVD diamonds contain a large amount of hydrogen, which is field-evaporated as carbon–hydrogen cluster ions. Some hydrogen atoms are weakly bound with the carbon–hydrogen clusters by hydrogen bonding and are released from the clusters while flying from the diamond surface to the detector. The depth profile of the hydrogen distribution indicates that hydrogen concentration decreases logarithmically with depth. The analysis of the silicon tip apex also revealed that the tip apex contains a significant amount of carbon and hydrogen. The silicon–carbon ratio of the uppermost surface layer is as high as 1:1 and rapidly decreases with depth and approaches nearly 2% at a depth of 20 nm. This suggests that the carbon might be intermixed or absorbed during the fabrication and/or dry etching process of the microtip array.

Model for hydrogen-atom production from the dissociation of fastHnf+cluster ions by thin foils

We present a quantitative model to describe the formation of hydrogen atoms (1s and 2p states) following the dissociation of fast (30--120 keV/u) hydrogen cluster ions ${\mathrm{H}}_{\mathrm{n}}^{+}$ (n=2 and n= 3 -- 21, odd) induced by a thin carbon foil. The model includes Coulomb explosion, multiple scattering, energy loss, and a simplified description of charge-exchange processes inside the solid. This description takes into account the dynamic screening of the moving protons by the target electrons and the relative position of the protons during the charge-exchange process. The results of the calculations performed with a Monte Carlo computer code reproduce remarkably well the experimental data.