On the Stability of Neon Cluster Ions – Evidence for Isomeric Structures

We have adopted the newly developed technique of growing cationic clusters in size-to-charge selected helium nanodroplets (HNDs), with subsequent removal of helium in a collision cell, to record high-resolution mass spectra of Nen+. Growth in singly charged HNDs leads to mass spectra that feature the same anomalies in the cluster ion abundance as in previous work, namely maxima at n = 14, 21, 55/56, 75. Several other, weaker but statistically significant anomalies are observed at n = 9, 26, 29, 33, 35, 69, 82, 89. However, when neon clusters are grown in larger HNDs, which are likely to be multiply charged, we observe a different set of magic numbers, at n = 7, 13, 19, 26, 29, 34, 55, 71, 81, plus many other numbers for larger clusters, up to n = 197. A transition from the first to the second set is observed in a limited size range if the collision pressure is increased. The most likely reason for the existence of two different sets of magic numbers appears to be the existence of two distinct structural families

Phenanthrene: Establishing Lower and Upper Bounds to the Binding Energy of a Very Weakly Bound Anion

Quite a few molecules do not form stable anions that survive the time needed for their detection; their electron affinities (EA) are either very small or negative. How does one measure the EA if the anion cannot be observed? Or, at least, can one establish lower and upper bounds to their EA? We propose two approaches that provide lower and upper bounds. We choose the phenanthrene (Ph) molecule whose EA is controversial. Through competition between helium evaporation and electron detachment in HenPh- clusters, formed in helium nanodroplets, we estimate the lower bound of the vertical detachment energy (VDE) of Ph- as about – 3 meV. In the second step, Ph is complexed with calcium whose electron affinity is just 24.55 meV. When CaPh- ions are collided with a thermal gas of argon, one observes Ca- product ions but no Ph-, suggesting that the EA of Ph is below that of Ca

Stabilization of Phenanthrene Anions in Helium Nanodroplets

It has been debated for years if the polycyclic aromatic hydrocarbon phenanthrene exists in its anionic form, or, in other words, if its electron affinity (EA) is positive or negative. In this contribution we confirm that the bare phenanthrene anion Ph- created in a binary collision with an electron at room temperature has a lifetime shorter than microseconds. However, the embedding of neutral phenanthrene molecules in negatively charged helium nanodroplets enables the formation of phenanthrene anions by charge transfer processes and the stabilization of the latter in the ultracold environment. Gentle shrinking of the helium matrix of phenanthrene-doped HNDs by collisions with helium gas makes the bare Ph- visible by high-resolution mass spectrometry. From these and previous measurements we conclude, that the EA of phenanthrene is positive and smaller than 24.55 meV

Helium nanodroplets as an efficient tool to investigate hydrogen attachment to alkali cations

9 pags., 10 figs. -- This article is part of the themed collection: Stability and properties of new-generation metal and metal-oxide clusters down to subnanometer scaleWe report a novel method to reversibly attach and detach hydrogen molecules to positively charged sodium clusters formed inside a helium nanodroplet host matrix. It is based on the controlled production of multiply charged helium droplets which, after picking up sodium atoms and exposure to H2 vapor, lead to the formation of Nam+(H2)n clusters, whose population was accurately measured using a time-of-flight mass spectrometer. The mass spectra reveal particularly favorable Na+(H2)n and Na2+(H2)n clusters for specific “magic” numbers of attached hydrogen molecules. The energies and structures of these clusters have been investigated by means of quantum-mechanical calculations employing analytical interaction potentials based on ab initio electronic structure calculations. A good agreement is found between the experimental and the theoretical magic numbers.This work was supported by the Austrian Science Fund, FWF (project number P31149) and the Spanish MICINN with Grants FIS2016-79596-P and PID2019-105225GB-I00 (JB, JHR); PID2020- 114654GB-I00/AEI/10.13039/501100011033, 2021-2024 (TGL,MB) and PID2020-114957GB-I00/AEI/10.13039/501100011033 (JCM, MIH). Collaboration has also been supported by the CSIC under i-Link+ program LINKB20041. Allocation of computing time by CESGA (Spain) is also acknowledged.Peer reviewe

Proton Transfer at Subkelvin Temperatures

We demonstrate a novel method to ionize molecules or molecular clusters by proton transfer at temperatures below 1 K. The method yields nascent ions and largely eliminates secondary reactions, even for notoriously ‘delicate’ molecules. Protonation is achieved inside liquid helium nanodroplets (HNDs) and begins with the formation of (H2)mH+ ions as the proton donors. In a separate and subsequent step the HNDs are doped with a proton acceptor molecule, X. Proton transfer occurs between X and the cold proton donor ions inside a helium droplet, an approach that avoids the large excess energy that is released if HNDs are first doped and then ionized. Mass spectra, recorded after stripping excess helium and hydrogen in a collision cell, show that this method offers a new way to determine proton affinities of molecules and clusters by proton-transfer bracketing, to investigate astrochemically relevant ion–molecule reactions at sub-kelvin temperatures, and to prepare XH+ ions that are suitable for messenger-tagging action spectroscopy

Efficient Formation of Size-Selected Clusters upon Pickup of Dopants into Multiply Charged Helium Droplets

Properties of clusters often depend critically on the exact number of atomic or molecular building blocks, however, most methods of cluster formation lead to a broad, size distribution and cluster intensity anomalies that are often designated as magic numbers. Here we present a novel approach of breeding size-selected clusters via pickup of dopants into multiply charged helium nanodroplets. The size and charge state of the initially undoped droplets and the vapor pressure of the dopant in the pickup region, determines the size of the dopant cluster ions that are extracted from the host droplets, via evaporation of the helium matrix in a collision cell filled with room temperature helium or via surface collisions. Size distributions of the selected dopant cluster ions are determined utilizing a high-resolution time of flight mass spectrometer. The comparison of the experimental data, with simulations taking into consideration the pickup probability into a shrinking He droplet due to evaporation during the pickup process, provides a simple explanation for the emergence of size distributions that are narrower than Poisson

Isotope Enrichment in Neon Clusters Grown in Helium Nanodroplets

Neon cluster ions Nes+ grown in pre-ionized, mass-to-charge selected helium nanodroplets (HNDs) reveal a strong enrichment of the heavy isotope 22Ne that depends on cluster size s and the experimental conditions. For small sizes, the enrichment is much larger than previously reported for bare neon clusters grown in nozzle expansions and subsequently ionized. The enrichment is traced to the massive evaporation of neon atoms in a collision cell that is used to strip helium from the HNDs. We derive a relation between the enrichment of 22Ne in the cluster ion and its corresponding depletion factor F in the vapor phase. The value thus found for F is in excellent agreement with a theoretical expression that relates isotopic fractionation in two-phase equilibria of atomic gases to the Debye temperature. Furthermore, the difference in zero-point energies between the two isotopes computed from F agrees reasonably well with theoretical studies of neon cluster ions that include nuclear quantum effects in the harmonic approximation. Another fitting parameter provides an estimate for the size si of the precursor of the observed Nes+. The value is in satisfactory agreement with the size estimated by modeling the growth of Nes+ and with lower and upper limits deduced from other experimental data. On the other hand, neon clusters grown in neutral HNDs that are subsequently ionized by electron bombardment exhibit no statistically significant isotope enrichment at all. The finding suggests that the extent of ionization-induced dissociation of clusters embedded in HNDs is considerably smaller than that for bare clusters

Efficient Formation of Size-Selected Clusters upon Pickup of Dopants into Multiply Charged Helium Droplets

Properties of clusters often depend critically on the exact number of atomic or molecular building blocks, however, most methods of cluster formation lead to a broad, size distribution and cluster intensity anomalies that are often designated as magic numbers. Here we present a novel approach of breeding size-selected clusters via pickup of dopants into multiply charged helium nanodroplets. The size and charge state of the initially undoped droplets and the vapor pressure of the dopant in the pickup region, determines the size of the dopant cluster ions that are extracted from the host droplets, via evaporation of the helium matrix in a collision cell filled with room temperature helium or via surface collisions. Size distributions of the selected dopant cluster ions are determined utilizing a high-resolution time of flight mass spectrometer. The comparison of the experimental data, with simulations taking into consideration the pickup probability into a shrinking He droplet due to evaporation during the pickup process, provides a simple explanation for the emergence of size distributions that are narrower than Poisson

Adsorption of helium on a charged propeller molecule: Hexaphenylbenzene

International audiencePhysisorption on planar or curved graphitic surfaces or aromatic rings has been investigated by various research groups, but in these studies the substrate is usually strictly rigid. Here we report a combined experimental and theoretical study of helium adsorption on cationic hexaphenylbenzene (HPB), a propeller-shaped molecule. The orientation of its propeller blades is known to be sensitive to the environment, with substantial differences between the molecule in the gas phase and in the crystalline solid. Mass spectra of He n HPB + , synthesized in helium nanodroplets, indicate enhanced stability for ions containing n = 2, 4, 14, 28, 42, 44, or 46 helium atoms. Path-integral molecular dynamics simulations reveal a significant dependence of the dissociation energy on the details of the HPB geometry. Good agreement between the experimental data and calculated dissociation energies are obtained provided that the symmetry of HPB + is reduced from D 6 to D 2 , such a lower symmetry being suggested from quantum chemical calculations as arising upon electron removal

Adsorption of Helium and Hydrogen on Triphenylene and 1,3,5-Triphenylbenzene

International audienceThe adsorption of helium or hydrogen on cationic triphenylene (TPL, C18H12), a planar polycyclic aromatic hydrocarbon (PAH) molecule, and of helium on cationic 1,3,5-triphenylbenzene (TPB, C24H18), a propeller-shaped PAH, is studied by a combination of high-resolution mass spectrometry and classical and quantum computational methods. Mass spectra indicate that HenTPL+ complexes are particularly stable if n = 2 or 6, in good agreement with the quantum calculations that show that for these sizes, the helium atoms are strongly localized on either side of the central carbon ring for n = 2 and on either side of the three outer rings for n = 6. Theory suggests that He14TPL+ is also particularly stable, with the helium atoms strongly localized on either side of the central and outer rings plus the vacancies between the outer rings. For HenTPB+, the mass spectra hint at enhanced stability for n = 2, 4 and, possibly, 11. Here, the agreement with theory is less satisfactory, probably because TPB+ is a highly fluxional molecule. In the global energy minimum, the phenyl groups are rotated in the same direction, but when the zero-point harmonic correction is included, a structure with one phenyl group being rotated opposite to the other two becomes lower in energy. The energy barrier between the two isomers is very small, and TPB+ could be in a mixture of symmetric and antisymmetric states, or possibly even vibrationally delocalize