Electron attachment to complexes of polyaromatic hydrocarbons with O2 and CO2

Clusters of polycyclic aromatic hydrocarbons (PAHs) doped with guest molecules represent a suitable model system for interstellar dust grains. Here, we investigate the electron-induced chemistry in aggregates of naphthalene, anthracene, and phenanthrene, both when they are pure and when they are doped with O2 and CO2. Clusters are experimentally produced by adiabatic expansion and doped by the molecules in a pickup process, are collided with electrons of controlled energy, and the anionic products are mass analyzed. In pure PAH clusters, a strong influence of the monomer electron affinity is observed. We see a surprising effect in doped clusters: while cluster anions with multiple CO2 molecules [i.e., (PAH)m(CO2)n- with n > 1] are detected, only anions with one O2 molecule [i.e., (PAH)mO2-] are formed. In addition, the dissociation of oxygen molecules by dissociative electron attachment at 6.5 eV is quenched on PAH clusters and stable superoxide O2- is formed. This may influence the efficiency of the depletion of molecular oxygen in astrochemical environments with a high abundance of free electrons.