Reaction dynamics of the methoxy anion CHO with methyl iodide CHI.

Studying larger nucleophiles in bimolecular nucleophilic substitution (S2) reactions bridges the gap from simple model systems to those relevant to organic chemistry. Therefore, we investigated the reaction dynamics between the methoxy anion (CHO) and iodomethane (CHI) in our crossed-beam setup combined with velocity map imaging at the four collision energies 0.4, 0.

Size limits and fission channels of doubly charged noble gas clusters.

Small, highly charged liquid droplets are unstable with respect to spontaneous charge separation when their size drops below the Rayleigh limit or, in other words, their fissility parameter exceeds the value 1. The absence of small doubly charged atomic cluster ions in mass spectra below an element-specific appearance size has sometimes been attributed to the onset of barrierless fission at = 1. However, more realistic models suggest that marks the size below which the rate of fission surpasses that of competing dissociative channels, and the Rayleigh limit of doubly charged van der Waals clusters has remained unchartered.

Imaging the Ion-Molecule Reaction Dynamics of O + CD.

While neutral reactions involved in methane oxidation have been intensively studied, much less information is known about the reaction dynamics of the oxygen radical anion with methane. Here, we study the scattering dynamics of this anion-molecule reaction using crossed-beam velocity map imaging with deuterated methane. Differential scattering cross sections for the deuterium abstraction channel have been determined at relative collision energies between 0.

Spectroscopy of helium-tagged molecular ions-Development of a novel experimental setup.

In this contribution, we present an efficient and alternative method to the commonly used RF-multipole trap technique to produce He-tagged molecular ions at cryogenic temperatures, which are perfectly suitable for messenger spectroscopy. The seeding of dopant ions in multiply charged helium nanodroplets, in combination with a gentle extraction of the latter from the helium matrix, enables the efficient production of He-tagged ion species. With a quadrupole mass filter, a specific ion of interest is selected, merged with a laser beam, and the photoproducts are measured in a time-of-flight mass-spectrometer.

Observation of Multiple Ordered Solvation Shells in Doped Helium Droplets: The Case of HeCa.

In this Letter, we report the experimental detection of likely the largest ordered structure of helium atoms surrounding a monatomic impurity observed to date using a recently developed technique. The mass spectrometry investigation of HeCa clusters, formed in multiply charged helium nanodroplets, reveals magic numbers at = 12, 32, 44, and 74. Classical optimization and path integral Monte Carlo calculations suggest the existence of up to four shells surrounding the calcium dication which are closed with well-ordered Mozartkugel-like structures: HeCa with an icosahedron, the second at HeCa with a dodecahedron, the third at HeCa with a larger icosahedron, and finally for HeCa, we find that the outermost He atoms form an icosidodecahedron which contains the other inner shells.

Solvation of Large Polycyclic Aromatic Hydrocarbons in Helium: Cationic and Anionic Hexabenzocoronene.

The adsorption of helium on charged hexabenzocoronene (Hbc, CH), a planar polycyclic aromatic hydrocarbon (PAH) molecule of symmetry, was investigated by a combination of high-resolution mass spectrometry and classical and quantum computational methods. The ion abundance of HeHbc complexes versus size features prominent local anomalies at = 14, 38, 68, 82, and a weak one at 26, indicating that for these "magic" sizes, the helium evaporation energies are relatively large. Surprisingly, the mass spectra of anionic HeHbc complexes feature a different set of anomalies, namely at = 14, 26, 60, and 62, suggesting that the preferred arrangement of the adsorbate atoms depends on the charge of the substrate.

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

The adsorption of helium or hydrogen on cationic triphenylene (TPL, CH), a planar polycyclic aromatic hydrocarbon (PAH) molecule, and of helium on cationic 1,3,5-triphenylbenzene (TPB, CH), a propeller-shaped PAH, is studied by a combination of high-resolution mass spectrometry and classical and quantum computational methods. Mass spectra indicate that HeTPL complexes are particularly stable if = 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 = 2 and on either side of the three outer rings for = 6. Theory suggests that HeTPL 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.

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.

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.

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.

Splashing of Large Helium Nanodroplets upon Surface Collisions.

In the present work we observe that helium nanodroplets colliding with surfaces can exhibit splashing in a way that is analogous to classical liquids. We use transmission electron microscopy and mass spectrometry to demonstrate that neutral and ionic dopants embedded in the droplets are efficiently backscattered in such events. High abundances of weakly bound He-tagged ions of both polarities indicate a gentle extraction mechanism of these ions from the droplets upon collision with a solid surface.

Helium structures around SF and SF: novel intermolecular potential and mass spectrometry experiments.

Helium clusters around the recently experimentally observed sulphur hexafluoride SF and sulphur pentafluoride SF ions are investigated using a combined experimental and theoretical effort. Mass spectrometry ion yields are obtained and the energetics and structure of the corresponding He-SF and He-SF clusters are analyzed using path integral molecular dynamics calculations as a function of , the number of He atoms, employing a new intermolecular potential describing the interaction between the dopant and the surrounding helium. The new force field is optimized on benchmark potential energy calculations and represented by improved Lennard-Jonnes analytical expressions.

Electron ionization of clusters containing the formamide molecule.

Studies on electron interactions with formamide (FA) clusters promote scientific interest as a model system to understand phenomena relevant to astrophysical, prebiotic, and radiobiological processes. In this work, mass spectrometric detection of cationic species for both small bare and microhydrated formamide clusters was performed at an electron ionization of 70 eV. Furthermore, a comparative analysis of the cluster spectra with the literature-reported gas-phase spectra is presented and discussed, revealing different reaction channels affected by the cluster environment.

Adsorption of Helium on Small Cationic PAHs: Influence of Hydrocarbon Structure on the Microsolvation Pattern.

The adsorption of up to ∼100 helium atoms on cations of the planar polycyclic aromatic hydrocarbons (PAHs) anthracene, phenanthrene, fluoranthene, and pyrene was studied by combining helium nanodroplet mass spectrometry with classical and quantum computational methods. Recorded time-of-flight mass spectra reveal a unique set of structural features in the ion abundance as a function of the number of attached helium atoms for each of the investigated PAHs. Path-integral molecular dynamics simulations were used with a polarizable potential to determine the underlying adsorption patterns of helium around the studied PAH cations and in good general agreement with the experimental data.

Complexes with Atomic Gold Ions: Efficient Bis-Ligand Formation.

Complexes of atomic gold with a variety of ligands have been formed by passing helium nanodroplets (HNDs) through two pickup cells containing gold vapor and the vapor of another dopant, namely a rare gas, a diatomic molecule (H, N, O, I, P), or various polyatomic molecules (HO, CO, SF, CH, adamantane, imidazole, dicyclopentadiene, and fullerene). The doped HNDs were irradiated by electrons; ensuing cations were identified in a high-resolution mass spectrometer. Anions were detected for benzene, dicyclopentadiene, and fullerene.

Electron Driven Reactions in Tetrafluoroethane: Positive and Negative Ion Formation.

In the search for alternatives to chlorine-containing gases, tetrafluoroethane, CFCHF (R134a), a widely used refrigerant gas, has been recognized as a promising substitute for dichlorodifluoromethane, CClF (R12). When R12 is replaced by R134a, the global warming potential drops from 8100 to 1430, the ozone depletion potential changes from 1 to 0, and the atmospheric lifetime decreases from 100 to 14 years. Electron interactions in the gas phase play a fundamental role in the atmospheric sciences.

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.

SF: Stabilizing Transient Ions in Helium Nanodroplets.

There are myriad ions that are deemed too short-lived to be experimentally accessible. One of them is SF. It has never been observed, although not for lack of trying.

Multiply Charged Helium Droplet Anions.

The detection of multiply charged helium droplet anions is reported for the first time. By ionizing droplets of superfluid helium with low energy electrons (up to 25 eV), it was possible to produce droplets containing up to five negative charges, which remain intact on the timescale of the experiment. The appearance sizes for different charge states are determined and are found to be orders of magnitude larger than for the equivalent cationic droplets, starting at 4 million He atoms for dianions.

Highly Charged Droplets of Superfluid Helium.

We report on the production and study of stable, highly charged droplets of superfluid helium. Using a novel experimental setup we produce neutral beams of liquid helium nanodroplets containing millions of atoms or more that can be ionized by electron impact, mass-per-charge selected, and ionized a second time before being analyzed. Droplets containing up to 55 net positive charges are identified and the appearance sizes of multiply charge droplets are determined as a function of the charge state.

Electron attachment to CO2 embedded in superfluid He droplets.

Electron attachment to CO2 embedded in superfluid He droplets leads to ionic complexes of the form (CO2)n(-) and (CO2)nO(-) and, at much lower intensities, He containing ions of the form Hem(CO2)nO(-). At low energies (<5 eV), predominantly the non-decomposed complexes (CO2)n(-) are formed via two resonance contributions, similar to electron attachment to pristine CO2 clusters. The significantly different shapes and relative resonance positions, however, indicate particular quenching and mediation processes in CO2@He.

Electron-driven ionization of large methanol clusters in helium nanodroplets.

The electron-driven ionization of helium droplets doped with pure methanol and ethanol clusters has been investigated for the first time using high resolution mass spectrometry. Large clusters are readily accessible by this route, with up to 100 alcohol molecules seen in the present study. The mass spectra for the doped helium droplets show many similarities with previous gas phase mass spectrometric studies of methanol and ethanol clusters.