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

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

Tandem mass spectrometry of helium nano droplets

In this thesis, helium nano droplets in highly charge states are analyzed. The helium nano droplets are formed in a free jet expansion source. Previous measurements showed a phenomenon, in which a narrow peak at relative small droplet sizes occurred in the size distribution of helium nano droplets, when high ionization power was applied. To test this, a tandem mass spectrometer of two identical 90 degree spherical capacitors with tunable voltage is realized. An ion source is mounted in front of each electrostatic sector. Behind the two sectors, the droplets are detected by a Channeltron Electron Multiplier (CEM). The first attempted explanation of the observed phenomenon as a fragmentation of the helium droplets via Coulomb explosion, is disproven. The insights gained by measurements using this setup suggest that the occurrence of the peaks in the size distribution can be explained by multiply charged helium droplets rather than their fragmentation. Furthermore, the minimum size of a helium droplet in a certain charge state is determined.Lukas TiefenthalerUniversität Innsbruck, Masterarbeit, 2018(VLID)280243

Dissociative attachment of low-energy electrons to acetonitrile

We experimentally probed the low-energy electron-induced dissociation of acetonitrile and acetonitrile-$$\hbox {d}_3$$ and performed density functional theory calculations to support the experimental results. The previous studies on electron attachment to acetonitrile presented a number of contradictory findings, which we aimed to resolve in the present study. We observed the formation of $$\hbox {H}^-$$, $$\hbox {CH}_2^-$$, $$\hbox {CH}_3^-$$, $$\hbox {CN}^-$$, $$\hbox {CCN}^-$$, $$\hbox {CHCN}^-$$ and $$\hbox {CH}_2 \hbox {CN}^-$$ anions and the corresponding deuterated fragments for acetonitrile-$$\hbox {d}_3$$ by dissociative electron attachment, and measured ion yields curves of each fragment. We saw no evidence for associative electron attachment to form the parent ion in these measurements. We also have measured the kinetic energy and angular distribution of selected fragments using a velocity map imaging (VMI) spectrometer

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

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

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

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

Solvation of Silver Ions in Noble Gases He, Ne, Ar, Kr, and Xe

We use a novel technique to solvate silver cations in small clusters of noble gases. The technique involves the formation of large, superfluid helium nanodroplets that are subsequently electron ionized, mass-selected by deflection in an electric field, and doped with silver atoms and noble gases (Ng) in pickup cells. Excess helium is then stripped from the doped nanodroplets by multiple collisions with helium gas at room temperature, producing cluster ions that contain no more than a few dozen noble gas atoms and just a few (or no) silver atoms. Under gentle stripping conditions, helium atoms remain attached to the cluster ions, demonstrating their low vibrational temperature. Under harsher stripping conditions, some of the heavier noble gas atoms will be evaporated as well, thus enriching stable clusters of NgnAgm+ at the expense of less stable ones. This results in local anomalies in the cluster ion abundance, which is measured in a high-resolution time-of-flight mass spectrometer. On the basis of these data, we identify specific “magic” sizes n of particularly stable ions. There is no evidence, however, for enhanced stability of Ng2Ag+, in contrast to the high stability of Ng2Au+ that derives from the covalent nature of the bond for heavy noble gases. “Magic” sizes are also identified for Ag2+ dimer ions complexed with He or Kr. Structural models will be tentatively proposed. A sequence of magic numbers n = 12, 32, and 44, indicative of three concentric solvation shells of icosahedral symmetry, is observed for HenH2O+