The effect of solvation on electron capture revealed using anion two-dimensional photoelectron spectroscopy

Although electron-driven chemistry is ubiquitous, how molecular electron capture is altered by solvent remains poorly understood. Now, using anion two-dimensional photoelectron spectroscopy, it is shown that the presence of water molecules can enhance electron capture and that considering the mechanism from the perspective of the anion offers further understanding. The reaction of low-energy electrons with neutral molecules to form anions plays an important role in chemistry, being involved in, for example, various biological and astrochemical processes. However, key aspects of electron-molecule interactions, such as the effect of incremental solvation on the initially excited electronic resonances, remain poorly understood. Here two-dimensional photoelectron spectroscopy of anionic anthracene and nitrogen-substituted derivatives-solvated by up to five water molecules-reveals that for an incoming electron, resonances red-shift with increasing hydration; but for the anion, the excitation energies of the resonances remain essentially the same. These complementary points of view show that the observed onset of enhanced anion formation for a specific cluster size is mediated by a bound excited state of the anion. Our findings suggest that polycyclic aromatic hydrocarbons may be more efficient at electron capture than previously predicted with important consequences for the ionization fraction in dense molecular clouds.

Automated summary

The presence of water molecules can enhance electron capture, with resonances red-shifting with increasing hydration for incoming electrons, while excitation energies of resonances remain essentially the same for the anion. This suggests that polycyclic aromatic hydrocarbons may be more efficient at electron capture than previously predicted, with important consequences for the ionization fraction in dense molecular clouds.