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

Quite a few molecules do not form stable anions that survive the time needed for their detection; their electron affinities (EA) are either very small or negative. How does one measure the EA if the anion cannot be observed? Or, at least, can one establish lower and upper bounds to their EA? We propose two approaches that provide lower and upper bounds. We choose the phenanthrene (Ph) molecule whose EA is controversial. Through competition between helium evaporation and electron detachment in HePh clusters, formed in helium nanodroplets, we estimate the lower bound of the vertical detachment energy (VDE) of Ph as about -3 meV. In the second step, Ph is complexed with calcium whose electron affinity is just 24.55 meV. When CaPh ions are collided with a thermal gas of argon, one observes Ca product ions but no Ph, suggesting that the EA of Ph is below that of Ca.