Effect of Molecular Structure on the Ionization Probabilities of Aromatic Molecules

The ion yield vs electron energy has been measured for the parent ions of a number of aromatic compounds immediately above the threshold. From the slopes of the linear portion of these curves, the relative ionization probability per molecule is determined. It is found that the ionization probabilities increase with the size of the π‐electron system and correlate quantitatively with the electron‐donating ability of the substituents. Disubstituted benzenes show an additivity of the effect of the substituents on the ionization probability with the meta isomer giving slightly higher values than the ortho and para isomers. Polyphenyl methanes and ethanes show considerably greater ionization probability than fused aromatic compounds with the same number of π electrons.