Electron Attachment to Formamide Clusters in Helium Nanodroplets

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.

London dispersion dominating diamantane packing in helium nanodroplets

Diamantane clusters formed inside superfluid helium nanodroplets were analyzed by time-of-flight mass spectrometry. Distinct cluster sizes were identified as "magic numbers" and the corresponding feasible structures for clusters consisting of up to 19 diamantane molecules were derived from meta-dynamics simulations and subsequent DFT computations. The obtained interaction energies were attributed to London dispersion attraction. Our findings demonstrate that diamantane units readily form assemblies even at low pressures and near-zero Kelvin temperatures, confirming the importance of the intermolecular dispersion effect for condensation of matter.

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. This study is essential in developing our understanding of both low-energy electron phenomena in clusters that can bridge the complexity gap between gas and realistic systems and the effect of hydration on electron-induced processes.

Control of chemical reactions and synthesis by low-energy electrons

Controlling the outcome of reactions is a central issue of chemical research. Physical tools can achieve this if they are able to precisely dissociate specific bonds of a molecule. However, to control synthesis, such tools must induce the formation of new bonds between two reactants to yield a more complex product. In the ideal case of an atom efficient synthesis, this product would contain all or at least most of the initial material. An electron beam is a physical tool that is capable of preparing molecules in reactive states or, at low electron energies, of initiating highly selective bond dissociation. The resulting fragments in turn can react with other molecules to yield stable products. This tutorial review focuses in particular on such low-energy electron-initiated molecular syntheses and their applications in the modification of surfaces. It thus emphasizes strategies towards the controlled and predictable formation of more complex products from small reactants initiated by interaction with low-energy electrons either through selective bond dissociation or formation of specific reactive molecular species. However, selective bond dissociation is not always desirable. This is briefly illustrated by the case of electron beam induced deposition where additional strategies may be required to control product formation.

Kinetically controlled formation of formamide trimer from first principles

The formation of formamide trimers was simulated using Car-Parrinello molecular dynamics. A variety of different initial setups were compared to study the effects of spatial arrangement, concentration, and temperature on the trimer product distribution. A total of nine different trimer species were obtained by simulation. Although a triangular initial arrangement of the three monomers is found to favour a less energetically stable chain-like product at high concentration, the more compact global minimum structure is expected to be the most abundant species overall in experiment. This is because there is evidence of a low activation barrier for conversion of the chain-like trimer to the lowest-energy structure. For one, this process is observed upon increasing the length of the trajectories. Furthermore, a slight rise in temperature drastically reduces the number of chain-like trimers. With regard to the intermolecular forces driving the aggregation dynamics, dispersion corrections to the underlying density functional theory description have a strong effect on the product distribution, again favouring the global minimum species. Certain local minimum structures are significantly destabilised relative to the global minimum by dispersion correction while the relative energies of the majority of species are practically unchanged.

Ionization of doped helium nanodroplets: residual helium attached to diatomic cations and their clusters

Electron impact ionization of helium nanodroplets containing a dopant, M, can lead to the detection of both M(+) and helium-solvated cations of the type M(+)·He(n) in the gas phase. The observation of helium-doped ions, He(n)M(+), has the potential to provide information on the aftermath of the charge transfer process that leads to ion production from the helium droplet. Here we report on helium attachment to the ions from four common diatomic dopants, M = N(2), O(2), CO, and NO. For experiments carried out with droplets with an average size of 7500 helium atoms, the monomer cations show little tendency to attach and retain helium atoms on their journey out of the droplet. By way of contrast, the corresponding cluster cations, M(n)(+), where n ≥ 2, all show a clear affinity for helium and form He(m)M(n)(+) cluster ions. The stark difference between the monomer and cluster ions is attributed to more effective cooling of the latter in the aftermath of the ionization event.

Review: gas-phase ion chemistry of the noble gases: recent advances and future perspectives

This review article surveys recent experimental and theoretical advances in the gas-phase ion chemistry of the noble gases. Covered issues include the interaction of the noble gases with metal and non-metal cations, the conceivable existence of covalent noble-gas anions, the occurrence of ion-molecule reactions involving singly-charged xenon cations, and the occurrence of bond-forming reactions involving doubly-charged cations. Research themes are also highlighted, that are expected to attract further interest in the future.