Phenanthrene: establishing lower and upper bounds to the binding energy of a very weakly bound anion

Doubly charged dimers and trimers of heavy noble gases

Many doubly charged heteronuclear dimers are metastable or even thermodynamically stable with respect to charge separation. Homonuclear dicationic dimers, however, are more difficult to form. He22+ was the first noble gas dimer predicted to be metastable and, decades later, observed. Ne22+ is the only other dicationic noble gas dimer that has been detected so far. Here, we present a novel approach to form fragile dicationic species, by post-ionization of singly charged ions that are embedded in helium nanodroplets (HNDs). Bare ions are then extracted by colliding the HNDs with helium gas. We detect homonuclear doubly charged dimers and trimers of krypton and xenon, but not argon. Our multi-reference ab initio calculations confirm the stability of Kr22+, Kr32+, Xe22+, Xe32+, and Ar22+, but put the stability of Ar32+ towards dissociation to Ar+ + Ar2+ into question.

Resonant electron capture by polycyclic aromatic hydrocarbon molecules: Effects of aza-substitution

Resonant electron capture by aza and diaza derivatives of phenanthrene (7,8-benzoquinoline and 1,10-phenanthroline) and anthracene (acridine and phenazine) at incident free electron energies (Ee) in the range of 0-15 eV was studied. All compounds except 7,8-benzoquinoline form long-lived molecular ions (M-) at thermal electron energies (Ee ∼ 0 eV). Acridine and phenazine also form such ions at epithermal electron energies up to Ee = 1.5-2.5 eV. The lifetimes (τa) of M- with respect to electron autodetachment are proportional to the extent of aza-substitution and increase on going from molecules with bent geometry of the fused rings (azaphenanthrenes) to linear isomers (azaanthracenes). These regularities are due to an increase in the adiabatic electron affinities (EAa) of the molecules. The EAa values of the molecules under study were comprehensively assessed based on a comparative analysis of the measured τa values using the Rice-Ramsperger-Kassel-Marcus theory, the electronic structure analysis using the molecular orbital approach, as well as the density functional calculations of the total energy differences between the molecules and anions. The only fragmentation channel of M- ions from the compounds studied is abstraction of hydrogen atoms. When studying [M-H]- ions, electron autodetachment processes were observed, the τa values were measured, and the appearance energies were determined. A comparative analysis of the gas-phase acidity of the molecules and the EAa values of the [M-H]· radicals revealed their proportionality to the EAa values of the parent molecules.

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. Gentle shrinking of the helium matrix of phenanthrene-doped HNDs by collisions with helium gas makes the bare Ph⁻ visible by high-resolution mass spectrometry. From these and previous measurements we conclude, that the EA of phenanthrene is positive and smaller than 24.55 meV.

Phenanthrene anions are stabilized in the ultracold environment of helium nanodroplets. Gentle shrinking of the helium matrix by collisions with helium gas makes the bare phenanthrene anion visible by high-resolution mass spectrometry.