Photoelectron Spectroscopy of Heterocycles . Fluorene Analogues *

The He(I) photoelectron spectra of fluorene and its analogues, biphenyl, dibenzofuran, carbazole, dibenzothiophene, and phenanthrene were measured and analyzed. The assignment of the ^-electron ionizations using orbital interaction and symmetry arguments for the constituent parts of the molecules indicates that the energy levels of the fluorene analogues can be reproduced within a few percent of error. A close linear correlation between the "z-ionization energies and the eigenvalues of the corresponding HMO-orbitals