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

Trihydrogen Cation Helium Clusters: A New Potential Energy Surface

We present a new analytical potential energy surface (PES) for the interaction between the trihydrogen cation and a He atom,H 3 + - H e ${{H}_{3}^{+}-He}$ , in its electronic ground state. The proposed PES has been built as a sum of two contributions: a polarization energy term due to the electric field generated by the molecular cation at the position of the polarizable He atom, and an exchange-repulsion and dispersion interactions represented by a sum of "atom-bond" potentials between the three bonds ofH 3 + ${{H}_{3}^{+}}$ and the He atom. All parameters of this new PES have been chosen and fitted from data obtained from high-level ab-initio calculations. Using this new PES plus the Aziz-Slaman potential for the interaction between Helium atoms and assuming pair-wise interactions, we carry out classical Basin-Hopping (BH) global optimization, semiclassical BH with Zero Point Energy corrections, and quantum Diffusion Monte Carlo simulations. We have found the minimum energy configurations of small He clusters doped withH 3 + ${{H}_{3}^{+}}$ ,H 3 + H e N ${{H}_{3}^{+}{\left(He\right)}_{N}}$ , with N=1-16. The study of the energies of these clusters allows us to find a pronounced anomaly for N=12, in perfect agreement with previous experimental findings, which we relate to a greater relative stability of this aggregate.

Observation of Slow Eigen-Zundel Interconversion in H+(H2O)6 Clusters upon Isomer-Selective Vibrational Excitation and Buffer Gas Cooling in a Cryogenic Ion Trap

The formation of isomers when trapping floppy cluster ions in a temperature-controlled ion trap is a generally observed phenomenon. This involves collisional quenching of the ions initially formed at high temperature by buffer gas cooling until their internal energies fall below the barriers in the potential energy surface that separate them. Here we explore the kinetics at play in the case of the two isomers adopted by the H⁺(H₂O)₆ cluster ion that differ in the proton accommodation motif. One of these is most like the Eigen cation with a tricoordinated hydronium motif (denoted E), and the other is most like the Zundel ion with the proton equally shared between two water molecules (denoted Z). After initial cooling to about 20 K in the radiofrequency (Paul) trap, the relative populations of these two spectroscopically distinct isomers are abruptly changed through isomer-selective photoexcitation of bands in the OH stretching region with a pulsed (∼6 ns) infrared laser while the ions are in the trap. We then monitor the relaxation of the vibrationally excited clusters and reformation of the two cold isomers by recording infrared photodissociation spectra with a second IR laser as a function of delay time from the initial excitation. The latter spectra are obtained after ejecting the trapped ions into a time-of-flight photofragmentation mass spectrometer, thus enabling long (∼0.1 s) delay times. Excitation of the Z isomer is observed to display long-lived vibrationally excited states that are collisionally cooled on a ms time scale, some of which quench into the E isomer. These excited E species then display spontaneous interconversion to the Z form on a ∼10 ms time scale. These qualitative observations set the stage for a series of experimental measurements that can provide quantitative benchmarks for theoretical simulations of cluster dynamics and the potential energy surfaces that underlie them.

Chemistry and physics of dopants embedded in helium droplets

Helium droplets represent a cold inert matrix, free of walls with outstanding properties to grow complexes and clusters at conditions that are perfect to simulate cold and dense regions of the interstellar medium. At sub-Kelvin temperatures, barrierless reactions triggered by radicals or ions have been observed and studied by optical spectroscopy and mass spectrometry. The present review summarizes developments of experimental techniques and methods and recent results they enabled.

Structure, energetics, and spectroscopy of the chromophores of HHe+n, H2He+n, and He+n clusters and their deuterated isotopologues

The linear molecular ions H₂He⁺, HHe+2, and He+3 are the central units (chromophores) of certain He-solvated complexes of the H₂He+n, HHe+n, and He+n families, respectively. These are complexes which do exist, according to mass-spectrometry studies, up to very high n values. Apparently, for some of the H₂He+n and He+n complexes, the linear symmetric tetratomic H₂He+2 and the diatomic He+2 cations, respectively, may also be the central units. In this study, definitive structures, relative energies, zero-point vibrational energies, and (an)harmonic vibrational fundamentals, and, in some cases, overtones and combination bands, are established mostly for the triatomic chromophores. The study is also extended to the deuterated isotopologues D₂He⁺, DHe+2, and D₂He+2. To facilitate and improve the electronic-structure computations performed, new atom-centered, fixed-exponent, Gaussian-type basis sets called MAX, with X = T(3), Q(4), P(5), and H(6), are designed for the H and He atoms. The focal-point-analysis (FPA) technique is employed to determine definitive relative energies with tight uncertainties for reactions involving the molecular ions. The FPA results determined include the 0 K proton and deuteron affinities of the ⁴He atom, 14 875(9) cm^-1 [177.95(11) kJ mol^-1] and 15 229(8) cm^-1 [182.18(10) kJ mol^-1], respectively, the dissociation energies of the He+2 → He⁺ + He, HHe+2 → HHe⁺ + He, and He+3 → He+2 + He reactions, 19 099(13) cm^-1 [228.48(16) kJ mol^-1], 3948(7) cm^-1 [47.23(8) kJ mol^-1], and 1401(12) cm^-1 [16.76(14) kJ mol^-1], respectively, the dissociation energy of the DHe+2 → DHe⁺ + He reaction, 4033(6) cm^-1 [48.25(7) kJ mol^-1], the isomerization energy between the two linear isomers of the [H, He, He]⁺ system, 3828(40) cm^-1 [45.79(48) kJ mol^-1], and the dissociation energies of the H₂He⁺ → H+2 + He and the H₂He+2 → H₂He⁺ + He reactions, 1789(4) cm^-1 [21.40(5) kJ mol^-1] and 435(6) cm^-1 [5.20(7) kJ mol^-1], respectively. The FPA estimates of the first dissociation energy of D₂He⁺ and D₂He+2 are 1986(4) cm^-1 [23.76(5) kJ mol^-1] and 474(5) cm^-1 [5.67(6) kJ mol^-1], respectively. Determining the vibrational fundamentals of the triatomic chromophores with second-order vibrational perturbation theory (VPT2) and vibrational configuration interaction (VCI) techniques, both built around the Eckart-Watson Hamiltonian, proved unusually challenging. For the species studied, VPT2 has difficulties yielding dependable results, in some cases even for the fundamentals of the H-containing molecular cations, while carefully executed VCI computations yield considerably improved spectroscopic results. In a few cases unusually large anharmonic corrections to the fundamentals, on the order of 15% of the harmonic value, have been observed.

Microsolvation of H2O+, H3O+, and CH3OH2+ by He in a cryogenic ion trap: structure of solvation shells

Due to the weak interactions of He atoms with neutral molecules and ions, the preparation of size-selected clusters for the spectroscopic characterization of their structures, energies, and large amplitude motions is a challenging task. Herein, we generate H₂O⁺He_n (n ≤ 9) and H₃O⁺He_n (n ≤ 5) clusters by stepwise addition of He atoms to mass-selected ions stored in a cryogenic 22-pole ion trap held at 5 K. The population of the clusters as a function of n provides insight into the structure of the first He solvation shell around these ions given by the anisotropy of the cation-He interaction potential. To rationalize the observed cluster size distributions, the structural, energetic, and vibrational properties of the clusters are characterized by ab initio calculations up to the CCSD(T)/aug-cc-pVTZ level. The cluster growth around both the open-shell H₂O⁺ and closed-shell H₃O⁺ ions begins by forming nearly linear and equivalent OH⋯He hydrogen bonds (H-bonds) leading to symmetric structures. The strength of these H-bonds decreases slightly with n due to noncooperative three-body induction forces and is weaker for H₃O⁺ than for H₂O⁺ due to both enhanced charge delocalization and reduced acidity of the OH protons. After filling all available H-bonded sites, addition of further He ligands around H₂O⁺ (n = 3-4) occurs at the electrophilic singly occupied 2p_z orbital of O leading to O⋯He p-bonds stabilized by induction and small charge transfer from H₂O⁺ to He. As this orbital is filled for H₃O⁺, He atoms occupy in the n = 4-6 clusters positions between the H-bonded He atoms, leading to a slightly distorted regular hexagon ring for n = 6. Comparison between H₃O⁺He_n and CH₃OH₂⁺He_n illustrates that CH₃ substitution substantially reduces the acidity of the OH protons, so that only clusters up to n = 2 can be observed. The structure of the solvation sub-shells is visible in both the binding energies and the predicted vibrational OH stretch and bend frequencies.

The He–H3+ complex. I. Vibration-rotation-tunneling states and transition probabilities

With a He–[Formula: see text] interaction potential obtained from advanced electronic structure calculations, we computed the vibration-rotation-tunneling (VRT) states of this complex for total angular momenta J from 0 to 9, both for the vibrational ground state and for the twofold degenerate v2 = 1 excited state of [Formula: see text]. The potential has three equivalent global minima with depth D e = 455.3 cm−1 for He in the plane of [Formula: see text], three equatorial saddle points that separate these minima with barriers of 159.5 cm−1, and two axial saddle points with energies of 243.1 cm−1 above the minima. The dissociation energies calculated for the complexes of He with ortho-[Formula: see text] (o[Formula: see text]) and para-[Formula: see text] (p[Formula: see text]) are D0 = 234.5 and 236.3 cm−1, respectively. Wave function plots of the VRT states show that they may be characterized as weakly hindered internal rotor states, delocalized over the three minima in the potential and with considerable amplitude at the barriers. Most of them are dominated by the j k = 10 and 11 rotational ground states of o[Formula: see text] and p[Formula: see text], with the intermolecular stretching mode excited up to v = 4 inclusive. However, we also found excited internal rotor states: 33 in He–o[Formula: see text], and 22 and 21 in He–p[Formula: see text]. The VRT levels and wave functions were used to calculate the frequencies and line strengths of all allowed v2 = 0 → 1 rovibrational transitions in the complex. Theoretical spectra generated with these results are compared with the experimental spectra in Paper II [Salomon et al., J. Chem. Phys. 156, 144308 (2022)] and are extremely helpful in assigning these spectra. This comparison shows that the theoretical energy levels and spectra agree very well with the measured ones, which confirms the high accuracy of our ab initio He–[Formula: see text] interaction potential and of the ensuing calculations of the VRT states.

The He–H3+ complex. II. Infrared predissociation spectrum and energy term diagram

The rotationally resolved infrared (IR) spectrum of the He–[Formula: see text] complex has been measured in a cryogenic ion trap experiment at a nominal temperature of 4 K. Predissociation of the stored complex has been invoked by excitation of the degenerate ν2 mode of the [Formula: see text] sub-unit using a pulsed optical parametric oscillator system. An assignment of the experimental spectrum became possible through one-to-one correlations with bands of the spectrum theoretically predicted in Paper I [Harding et al., J. Chem. Phys. 156, 144307 (2022)]. 19 bands have been assigned and analyzed, and the energy term diagram of the lower states of this floppy molecular complex has been derived from combination differences (CDs) in the experimental spectrum. Ground state combination differences (GSCDs) reveal a large part of the energy term diagram for the He–[Formula: see text] complex in its vibrational ground state, v = 0. Experimental and theoretical term energies agree within experimental accuracy for the rotational fine structure associated with the total angular momentum quantum number J and the parity e/ f as well as for the coarse spacing of the lowest K states of the complex. This favorable comparison shows that the potential energy surface (PES) calculated in Paper I is accurate. The barriers between the three equivalent global minima in this PES are relatively low and the He–[Formula: see text] complex is extremely floppy, with nearly unhindered internal rotation of the [Formula: see text] sub-unit. The resulting Coriolis interactions couple the internal and end-over-end rotation of the complex and contribute significantly to the energy terms. They are observed both in experiment and theory and are, e.g., the origin of different rotational constants for states of e and f parity. Also in this respect, experiment and theory agree very well. Despite the assignment and analysis of many bands of the extremely rich IR spectrum of He–[Formula: see text], higher levels of excitation, including the complex stretching mode, need further attention.

Rotational action spectroscopy of trapped molecular ions

Rotational action spectroscopy is an experimental method in which rotational spectra of molecules, typically in the microwave to sub-mm-wave domain of the electromagnetic spectrum (∼1-1000 GHz), are recorded by action spectroscopy. Action spectroscopy means that the spectrum is recorded not by detecting the absorption of light by the molecules, but by the action of the light on the molecules, e.g., photon-induced dissociation of a chemical bond, a photon-triggered reaction, or photodetachment of an electron. Typically, such experiments are performed on molecular ions, which can be well controlled and mass-selected by guiding and storage techniques. Though coming with many advantages, the application of action schemes to rotational spectroscopy was hampered for a long time by the small energy content of a corresponding photon. Therefore, the first rotational action spectroscopic methods emerged only about one decade ago. Today, there exists a toolbox full of different rotational action spectroscopic schemes which are summarized in this review.

Rovibrational spectroscopy of the CH+-He and CH+-He4 complexes

A cryogenic 22-pole ion trap apparatus is used in combination with a table-top pulsed IR source to probe weakly bound CH⁺-He and CH⁺-He₄ complexes by predissociation spectroscopy at 4 K. The infrared photodissociation spectra of the C-H stretching vibrations are recorded in the range of 2720-2800 cm^-1. The spectrum of CH⁺-He exhibits perpendicular transitions of a near prolate top with a band origin at 2745.9 cm^-1, and thus confirms it to have a T-shaped structure. For CH⁺-He₄, the C-H stretch along the symmetry axis of this oblate top results in parallel transitions.

High-resolution infrared action spectroscopy of the fundamental vibrational band of CN. JOURNAL OF MOLECULAR SPECTROSCOPY 2020;374:111375. [PMID: 33162609 PMCID: PMC7116308 DOI: 10.1016/j.jms.2020.111375]

Rotational-vibrational transitions of the fundamental vibrational modes of the ¹²C¹⁴N⁺ and ¹²C¹⁵N⁺ cations have been observed for the first time using a cryogenic ion trap apparatus with an action spectroscopy scheme. The lines P(3) to R(3) of ¹²C¹⁴N⁺ and R(1) to R(3) of ¹²C¹⁵N⁺ have been measured, limited by the trap temperature of approximately 4 K and the restricted tuning range of the infrared laser. Spectroscopic parameters are presented for both isotopologues, with band origins at 2000.7587(1) and 1970.321(1) cm^-1, respectively, as well as an isotope independent fit combining the new and the literature data.

Formation of H3 + in Collisions of H2 + with H2 Studied in a Guided Ion Beam Instrument

In order to study collisions between ions and neutrals, a new Guided Ion Beam (GIB) apparatus, called NOVion, has been assembled and tested. The primary purpose of this instrument is to measure absolute cross sections at energies relevant for technical or inter- and circumstellar plasmas. New and improved results are presented for forming H₃ ⁺ in collisions of H₂ ⁺ with H₂ . Between 0.1 eV and 2 eV, our measured effective cross sections are in good overall agreement with most previous measurements. However, at higher energies, our results do not show the steep decline, recommended in the standard literature. After critical evaluation of all experimental and theoretical data, a new analytical function is proposed, describing properly the dependence of the title reaction on the collision energy up to 10 eV.

Cationic Noble-Gas Hydrides: From Ion Sources to Outer Space

Cationic species with noble gas (Ng)-hydrogen bonds play a major role in the gas-phase ion chemistry of the group 18 elements. These species first emerged more than 90 years ago, when the simplest HeH+ and HeH2 + were detected from ionized He/H2 mixtures. Over the years, the family has considerably expanded and currently includes various bonding motifs that are investigated with intense experimental and theoretical interest. Quite recently, the results of these studies acquired new and fascinating implications. The diatomic ArH+ and HeH+ were, in fact, detected in various galactic and extragalactic regions, and this stimulates intriguing questions concerning the actual role in the outer space of the Ng-H cations observed in the laboratory. The aim of this review is to briefly summarize the most relevant information currently available on the structure, stability, and routes of formation of these fascinating systems.

Metal Clusters on Semiconductor Surfaces and Application in Catalysis with a Focus on Au and Ru

Metal clusters typically consist of two to a few hundred atoms and have unique properties that change with the type and number of atoms that form the cluster. Metal clusters can be generated with a precise number of atoms, and therefore have specific size, shape, and electronic structures. When metal clusters are deposited onto a substrate, their shape and electronic structure depend on the interaction with the substrate surface and thus depend on the properties of both the clusters and those of the substrate. Deposited metal clusters have discrete, individual electron energy levels that differ from the electron energy levels in the constituting individual atoms, isolated clusters, and the respective bulk material. The properties of clusters with a focus on Au and Ru, the methods to generate metal clusters, and the methods of deposition of clusters onto substrate surfaces are covered. The properties of cluster-modified surfaces are important for their application. The main application covered here is catalysis, and the methods for characterization of the cluster-modified surfaces are described.

Spectroscopic signatures of HHe2+ and HHe3. Phys Chem Chem Phys 2020;22:22885-22888. [PMID: 33034329 DOI: 10.1039/d0cp04649c]

Using two different action spectroscopic techniques, a high-resolution quantum cascade laser operating around 1300 cm-1 and a cryogenic ion trap machine, the proton shuttle motion of the cations HHe2+ and HHe3+ has been probed at a nominal temperature of 4 K. For HHe3+, the loosely bound character of this complex allowed predissociation spectroscopy to be used, and the observed broad features point to a lifetime of a few ps in the vibrationally excited state. For He-H+-He, a fundamental linear molecule consisting of only three nuclei and four electrons, the method of laser-induced inhibition of complex growth (LIICG) enabled the measurement of three accurate rovibrational transitions, pinning down its molecular parameters for the first time.

Hydrogen molecular ions: H3+, H5+ and beyond

Three decades after the spectroscopic detection of H3+ in space, the inspiring developments in physics, chemistry and astronomy of Hn+ (n = 3, 5, 7) systems, which led to this Royal Society Discussion Meeting, are reviewed, the present state of the art as represented by the meeting surveyed and future lines of research considered. This article is part of a discussion meeting issue 'Advances in hydrogen molecular ions: H3+, H5+ and beyond'.

Double resonance rotational spectroscopy of He-HCO. Phys Chem Chem Phys 2019;21:3440-3445. [PMID: 30191208 DOI: 10.1039/c8cp04532a]

The ground state of He-HCO+ is investigated using a recently developed double resonance technique, consisting of a rotational transition followed by a vibrational transition into a dissociative state. In order to derive precise predictions for the rotational states, the high resolution infrared predissociation spectroscopy of the v1 C-H stretching mode is revisited. Eleven pure rotational transitions are measured via the double resonance method. A least squares fit of these transitions to a standard linear rotor Hamiltonian reveals that the semirigid rotor model cannot fully describe the loosely bound He-HCO+ complex. The novel double resonance technique is compared with other action spectroscopic schemes, and some potential future applications are presented.

The FELion cryogenic ion trap beam line at the FELIX free-electron laser laboratory: infrared signatures of primary alcohol cations

The FELion beamline – a cryogenic 22-pole trap for vibrational spectroscopy of molecular ions at the FELIX Laboratory.

Double Resonance Rotational Spectroscopy of Weakly Bound Ionic Complexes: The Case of Floppy CH_{3}^{+}-He

A novel rotational spectroscopy method applicable to ions stored in cold traps is presented. In a double resonance scheme, rotational excitation is followed by vibrational excitation into a dissociative resonance. Its general applicability is demonstrated for the CH_{3}^{+}-He complex, which undergoes predissociation through its C-H stretching modes ν_{1} and ν_{3}. High resolution rotational transitions are recorded for this symmetric top, and small unexpected splittings are resolved for K=1. Advantages and potential future applications of this new approach are discussed.

High-resolution infrared spectroscopy of O2H+ in a cryogenic ion trap

The protonated oxygen molecule, O₂H⁺, and its helium complex, He-O₂H⁺, have been investigated by vibrational action spectroscopy in a cryogenic 22-pole ion trap. For the He-O₂H⁺ complex, the frequencies of three vibrational bands have been determined by predissociation spectroscopy. The elusive O₂H⁺ has been characterized for the first time by high-resolution rovibrational spectroscopy via its ν₁ OH-stretching band. Thirty-eight rovibrational fine structure transitions with partly resolved hyperfine satellites were measured (56 resolved lines in total). Spectroscopic parameters were determined by fitting the observed lines with an effective Hamiltonian for an asymmetric rotor in a triplet electronic ground state, X̃³A^'', yielding a band origin at 3016.73 cm^-1. Based on these spectroscopic parameters, the rotational spectrum is predicted, but not yet detected.

Perspective: C60+ and laboratory spectroscopy related to diffuse interstellar bands

In the last 30 years, our research has focused on laboratory measurements of the electronic spectra of organic radicals and ions. Many of the species investigated were selected based on their potential astrophysical relevance, particularly in connection with the identification of appealing candidate molecules for the diffuse interstellar absorptions. Notably, carbon chains and derivatives containing hydrogen and nitrogen atoms in their neutral and ionic forms were studied. These data could be obtained after developing appropriate techniques to record spectra at low temperatures relevant to the interstellar medium. The measurement of gas phase laboratory spectra has enabled direct comparisons with astronomical data to be made and though many species were found to have electronic transitions in the visible where the majority of diffuse bands are observed, none of the absorptions matched the prominent interstellar features. In 2015, however, the first carrier molecule was identified: C₆₀⁺. This was achieved after the measurement of the electronic spectrum of C₆₀⁺-He at 6K in a radiofrequency ion trap.

First Laboratory Detection of Vibration-Rotation Transitions of 12CH+ and 13CH+ and Improved Measurement of their Rotational Transition Frequencies

The long-searched C-H stretches of the fundamental ions CH+ and 13CH+ have been observed for the first time in the laboratory. For this, the state-dependent attachment of He atoms to these ions at cryogenic temperatures has been exploited to obtain high-resolution rovibrational data. In addition, the lowest rotational transitions of CH+, 13CH+ and CD+ have been revisited and their rest frequency values improved substantially.

Rovibrational Resonances in H2He. J Chem Theory Comput 2018;14:1523-1533. [PMID: 29390185 DOI: 10.1021/acs.jctc.7b01136]

The nuclear dynamics of the metastable H₂He⁺ complex is explored by symmetry considerations and angular momentum addition rules as well as by accurate quantum chemical computations with complex coordinate scaling, complex absorbing potential, and stabilization techniques. About 200 long-lived rovibrational resonance states of the complex are characterized and selected long-lived states are analyzed in detail. The stabilization mechanism of these long-lived resonance states is discussed on the basis of probability density plots of the wave functions. Overlaps of wave functions derived by a reduced-dimensional model with the full-dimensional wave functions reveal dissociation pathways for the long-lived resonance states and allow the calculation of their branching ratios.