Electron stimulated desorption from condensed benzene

The electron induced dissociation of condensed benzene (C6H6) in thin films deposited on a Pt substrate is investigated by electron stimulated desorption (ESD) of anions and cations. The desorbed yields are recorded as a function of incident electron energy in the range of 10 to 950 eV for a fixed film thickness of 2 monolayers (ML) and for a fixed energy of 950 eV, as well as a function of film thickness from 0.5 to 8 monolayers (ML) for anions, and from 0.5 to 12ML for cations. Both energy and thickness dependencies are discussed in terms of the three main mechanisms yielding positively and/or negatively charged fragments: dissociative electron attachment (DEA), dipolar dissociation (DD) and dissociative ionization (DI) processes. At the probed energies, DD is the major mechanism, while DEA is predominantly induced by secondary electrons from the Pt substrate. Desorption of the parent positive ion is strongly suppressed. Similar qualitative behaviours are observed for the energy dependence of both anion and cation ESD yields, while some discrepancies exist in the thickness dependence, including a very significant systematic magnitude difference found between such ions formation. An estimation of the effective DD cross-section including the desorption probability is obtained. Feasible mechanisms behind the observed energy and thickness dependences for anion and cation yields are proposed. These results highlight the need for further investigations to better understand the underlying processes of electron induced dissociation in condensed matter.

Ambient-light-induced intermolecular Coulombic decay in unbound pyridine monomers

Intermolecular Coulombic decay (ICD) is a process whereby photoexcited molecules relax by ionizing their neighbouring molecules. ICD is efficient when intermolecular interactions are active and consequently it is observed only in weakly bound systems, such as clusters and hydrogen-bonded systems. Here we report an efficient ICD between unbound molecules excited at ambient-light intensities. On the photoexcitation of gas-phase pyridine monomers, well below the ionization threshold and at low laser intensities, we detected the parent and heavier-than-parent cations. The isotropic emission of slow electrons revealed ICD as the underlying process. π-π* excitation in unbounded pyridine monomers triggered an associative interaction between them, which leads to an efficient three-centre ICD. The cation resulting from the molecular association of the three pyridine centres relaxed through fragmentation. This below-threshold ionization under ambient light has implications for the understanding of radiation damage and astrochemistry.

Structure, Stability, and Spectroscopic Properties of Small Acetonitrile Cation Clusters

Ionization and fragmentation pathways induced by ionizing agents are key to understanding the formation of complex molecules in astrophysical environments. Acetonitrile (CH₃CN), the simplest organic nitrile, is an important molecule present in the interstellar medium. In this work, DFT and MP2 calculations were performed in order to obtain the low energy structures of the most relevant cations formed from electron-stimulated ion desorption of CH₃CN ices. Selected reaction pathways and spectroscopic properties were also calculated. Our results indicate that the most stable acetonitrile cation structure is CH₂CNH⁺ and that hydrogenation can occur successively without isomerization steps until its complete saturation. Moreover, the stability of distinct cluster families formed from the interaction of acetonitrile with small fragments, such as CH_n⁺, C₂H_n⁺, and CH_nCNH⁺, is discussed in terms of their respective binding energies. Some of these molecular clusters are stabilized by hydrogen bonds, leading to species whose infrared features are characterized by a strong redshift of the N-H stretching mode. Finally, the rotational spectra of CH₃CN and protonated acetonitrile, CH₃CNH⁺, were simulated using distinct computational protocols based on DFT, MP2, and CCSD(T) considering centrifugal distortion, vibrational-rotational coupling, and vibrational anharmonicity corrections. By adopting an empirical scaling procedure for calculating spectroscopic parameters, we were able to estimate the rotational frequencies of CH₃CNH⁺ with an expected average error below 1 MHz for J values up to 10.

Unexpected reversal of stability in strained systems containing one-electron bonds

Ring strain energy is a very well documented feature of neutral cycloalkanes, and influences their structural, thermochemical and reactivity properties. In this work, we apply density functional theory and high-level coupled cluster calculations to describe the geometry and relative stability of C₆H₁₂⁺˙ radical cations, whose cyclic isomers are prototypes of singly-charged cycloalkanes. Molecular ions with the mentioned stoichiometry were produced via electron impact experiments using a gaseous cyclohexane sample (20-2000 eV). From our calculations, in addition to structures that resemble linear and branched alkenes as well as distinct conformers of cyclohexane, we have found low-lying species containing three-, four- and five-membered rings with the presence of an elongated C-C bond. Remarkably, the stability trend of these ring-bearing radical cations is anomalous, and the three-membered species are up to 11.3 kcal mol^-1 more stable than the six-membered chair structure. Generalized Valence Bond calculations and the Spin Coupled theory with N electrons and M orbitals were used in conjunction with the Generalized Product Function Energy Partitioning (GPF-EP) method and Interference Energy Analysis (IEA) to describe the chemical bonding in such moieties. Our results confirm that these elongated C-C motifs are one-electron sigma bonds. Our calculations also reveal the effects that drive thermochemical preference of strained systems over their strained-free isomers, and the origin of the unusual stability trend observed for cycloalkane radical cations.