Formation of negative ions from cobalt tricarbonyl nitrosyl Co(CO)3NO clusters

Electron attachment and corresponding dissociative electron attachment (DEA) to cobalt tricarbonyl nitrosyl (Co(CO)3NO) clusters have been studied by co-expansion with Ar gas into a high vacuum. A monochromatic electron beam was utilized to generate negative ions and the resulting reaction products were identified using mass spectrometry. The ion fragments corresponding to Co(CO)3NO monomers closely resemble results from earlier gas phase experiments and studies conducted on Co(CO)3NO in He nanodroplets. However, contrary to the gas phase or He nanodroplet ion yields, a resonance structure comprising several peaks at energies above ∼4 eV was observed both in the case of molecular clusters [Co(CO)3NO]n- (with n = 1, 2, 3) and clusters comprising DEA fragments. Additionally, the ion yields of numerous other clusters such as ions without nitrosyl ([Co(CO)4]-, [Co2(CO)5]-), clusters consisting of two fragments such as ([Co2(CO)NO]-, [Co2(CO)(NO)2]-, [Co2(CO)2NO]-, [Co2(CO)2(NO)2]-, [Co3(CO)(NO)3]-, [Co3(CO)8(NO)3]-, [Co3(CO)(NO)2]-, [Co3(CO)3(NO)2]-, and [Co3(CO)5(NO)2]-) were recorded. Moreover, NO bond dissociation was confirmed with the [Co(CO)2N]-ion and with N- or O-retaining cluster ions, such as [Co2(CO)(NO)N]-, [Co2(CO)2(NO)N]-, [Co3(CO)2(NO)N]-, [Co3(CO)3(NO)N]- and [Co3(CO)(NO)2N]-, or [Co2(CO)2O]-, [Co2(CO)3O]-, [Co3(CO)3O]-, [Co3(CO)4O]-and [Co3(CO)2(NO)O]- respectively.

Methane Cluster Fragmentation by Fast Electron Impact

We investigate the fragmentation of the CH4 cluster by fast electron impact at stagnation pressures from 0.5 bar to 16 bar. By measuring the time of flight spectrum (TOF), two types of ions, including (CH4)n−1CH5+ and (CH4)n−2(C2Hm)+, are observed. In the 1D TOF spectrum, it is shown that for the stagnation pressure larger than 4 bar, the former ion is predominant for each n, similar to the previous experimental result. However, as the pressure decreases to 0.5 or 2 bar, the contribution of the C2Hm+ ion is dominant over that of the CH4CH5+ ion. In the 2D coincident TOF spectrum, the above two patterns of ions are also distinguished, and the enhancement of C2Hm+ is observed at 4 bar pressure. The phenomena appearing in 2D and 1D TOF spectra imply that the C2Hm+ ion prefers to survive in a smaller cluster, while the stabilization of the protonated ion needs a more massive cluster environment.

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.

Impurity diffusion in magic-size icosahedral clusters

Atomic diffusion is at the basis of chemical ordering transformations in nanoalloys. Understanding the diffusion mechanisms at the atomic level is therefore a key issue in the study of the thermodynamic behavior of these systems and, in particular, of their evolution from out-of-equilibrium chemical ordering types often obtained in the experiments. Here, the diffusion is studied in the case of a single-atom impurity of Ag or Au moving within otherwise pure magic-size icosahedral clusters of Cu or Co by means of two different computational techniques, i.e., molecular dynamics and metadynamics. Our simulations reveal unexpected diffusion pathways, in which the displacement of the impurity is coupled with the creation of vacancies in the central part of the cluster. We show that the observed mechanism is quite different from the vacancy-mediated diffusion processes identified so far, and we demonstrate that it can be related to the presence of non-homogeneous compressive stress in the inner part of the icosahedral structure.

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.

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.

Krypton-methanol spectroscopic study: Assessment of the complexation dynamics and the role of the van der Waals interaction

The Kr-CH₃OH (Krypton-Methanol) system has several technological applications, such as the determination of diffusivity coefficients, their use in the development of detectors and combustion techniques among others. We report an extensive theoretical study concerning the stability of such complex. A mix between molecular dynamics, electronic structure calculations and solution of the nuclear Schrodinger equation lead to investigation of spectroscopic constants, lifetime of the complex and its Quantum Theory Atom in Molecules (QTAIM) properties. The study of the Potential Energy Curves (PEC) suggested three configurations to be stable as their potential well were able to harbor 9 vibrational levels. Properties from the curves also allowed us to obtain the lifetime of the complex, whose values were >1 ps regardless of the conformation. Furthermore, topological investigations of the charge density profile of the complex, in the scope of QTAIM properties, show that van der Waals type interactions takes place between the noble gas and the methanol molecule. These features are in consonance to the experimental fact that this complex is stable.

Ion-induced molecular growth in clusters of small hydrocarbon chains

We report on studies of collisions between 3 keV Ar⁺ projectile ions and neutral targets of isolated 1,3-butadiene (C₄H₆) molecules and cold, loosely bound clusters of these molecules. We identify molecular growth processes within the molecular clusters that appears to be driven by knockout processes and that could result in the formation of (aromatic) ring structures. These types of reactions are not unique to specific projectile ions and target molecules, but will occur whenever atoms or ions with suitable masses and kinetic energies collide with aggregates of matter, such as carbonaceous grains in the interstellar medium or aerosol nanoparticles in the atmosphere.

Coulomb fission in multiply charged molecular clusters: Experiment and theory

A series of three multiply charged molecular clusters, (C₆H₆)_n^z+ (benzene), (CH₃CN)_n^z+ (acetonitrile), and (C₄H₈O)_n^z+ (tetrahydrofuran), where the charge z is either 3 or 4, have been studied for the purpose of identifying the patterns of behaviour close to the charge instability limit. Experiments show that on a time scale of ∼10^-4 s, ions close to the limit undergo Coulomb fission where the observed pathways exhibit considerable asymmetry in the sizes of the charged fragments and are all associated with kinetic (ejection) energies of between 1.4 and 2.2 eV. Accurate kinetic energies have been determined through a computer simulation of peak profiles recorded in the experiments and the results modelled using a theory formulated to describe how charged particles of dielectric materials interact with one another [E. Bichoutskaia et al., J. Chem. Phys. 133, 024105 (2010)]. The calculated electrostatic interaction energy between separating fragments gives an accurate account for the measured kinetic energies and also supports the conclusion that +4 ions fragment into +3 and +1 products as opposed to the alternative of two +2 fragments. This close match between the theory and experiment reinforces the assumption that a significant fraction of excess charge resides on the surfaces of the fragment ions. It is proposed that the high degree of asymmetry seen in the fragmentation patterns of the multiply charged clusters is due, in part, to limits imposed by the time window during which observations are made.

Doubly charged CO2 clusters formed by ionization of doped helium nanodroplets

Helium nanodroplets are doped with carbon dioxide and ionized by electrons. Doubly charged cluster ions are, for the first time, identified based on their characteristic patterns of isotopologues. Thanks to the high mass resolution, large dynamic range, and a novel method to eliminate contributions from singly charged ions from the mass spectra, we are able to observe doubly charged cluster ions that are smaller than the ones reported in the past. The likely mechanism by which doubly charged ions are formed in doped helium droplets is discussed.

Monocarbon cationic cluster yields from N2/CH4 mixtures embedded in He nanodroplets and their calculated binding energies

The formation of monocarbon cluster ions has been investigated by electron ionization mass spectrometry of cold helium nanodroplets doped with nitrogen/methane mixtures. Ion yields for two groups of clusters, CHmN2(+) or CHmN4(+), were determined for mixtures with different molecular ratios of CH4. The possible geometrical structures of these clusters were analyzed using electronic structure computations. Little correlation between the ion yields and the associated binding energies has been observed indicating that in most cases kinetic control is more important than thermodynamic control for forming the clusters.

Adsorption of hydrogen on neutral and charged fullerene: experiment and theory

Helium droplets are doped with fullerenes (either C60 or C70) and hydrogen (H2 or D2) and investigated by high-resolution mass spectrometry. In addition to pure helium and hydrogen cluster ions, hydrogen-fullerene complexes are observed upon electron ionization. The composition of the main ion series is (H2)(n)HC(m)(+) where m = 60 or 70. Another series of even-numbered ions, (H2)(n)C(m)(+), is slightly weaker in stark contrast to pure hydrogen cluster ions for which the even-numbered series (H2)(n)(+) is barely detectable. The ion series (H2)(n)HC(m)(+) and (H2)(n)C(m)(+) exhibit abrupt drops in ion abundance at n = 32 for C60 and 37 for C70, indicating formation of an energetically favorable commensurate phase, with each face of the fullerene ion being covered by one adsorbate molecule. However, the first solvation layer is not complete until a total of 49 H2 are adsorbed on C60(+); the corresponding value for C70(+) is 51. Surprisingly, these values do not exhibit a hydrogen-deuterium isotope effect even though the isotope effect for H2/D2 adsorbates on graphite exceeds 6%. We also observe doubly charged fullerene-deuterium clusters; they, too, exhibit abrupt drops in ion abundance at n = 32 and 37 for C60 and C70, respectively. The findings imply that the charge is localized on the fullerene, stabilizing the system against charge separation. Density functional calculations for C60-hydrogen complexes with up to five hydrogen atoms provide insight into the experimental findings and the structure of the ions. The binding energy of physisorbed H2 is 57 meV for H2C60(+) and (H2)2C60(+), and slightly above 70 meV for H2HC60(+) and (H2)2HC60(+). The lone hydrogen in the odd-numbered complexes is covalently bound atop a carbon atom but a large barrier of 1.69 eV impedes chemisorption of the H2 molecules. Calculations for neutral and doubly charged complexes are presented as well.

Methane Adsorption on Graphitic Nanostructures: Every Molecule Counts

Bundles of single-walled nanotubes are promising candidates for storage of hydrogen, methane, and other hydrogen-rich molecules, but experiments are hindered by nonuniformity of the tubes. We overcome the problem by investigating methane adsorption on aggregates of fullerenes containing up to six C(60); the systems feature adsorption sites similar to those of nanotube bundles. Four different types of adsorption sites are distinguished, namely, registered sites above the carbon hexagons and pentagons, groove sites between adjacent fullerenes, dimple sites between three adjacent fullerenes, and exterior sites. The nature and adsorption energies of the sites in C(60) aggregates are determined by density functional theory and molecular dynamics (MD) simulations. Excellent agreement between experiment and theory is obtained for the adsorption capacity in these sites.