The Hel photoelectron (PE) spectra of anthracene and ten of its
9-substituted derivatives with methyl, t ert-butyl, fluoro; chloro,
bromo, methoxy, aldehyde, cyano, nitro and phenyl groups as substituents
have been recorded. Systems arising from Jt-ionizations
are assigned. The assignment is supported by molecular orbital calculations.
The effect of the substituent on the lowest ionization
energy is well described by Hammett substituent constants based
on the benzene system