Multi Peak ESCA Bands in Substituted Benzenes and Heterocyclic Compounds

The ESCA spectra of several substituted benzenes and heterocyclic compounds were obtained. Multi peak structures are observed in the various energy regions. The effects are generally interpreted as due to shake up phenomena involving intramolecular charge-transfer transitions between a "donor" and an "acceptor" group in the molecule. A quantitative model for such kind of phenomena is given. It rationalizes the fact that: 1) These shake up processes are, to a large extent, specific to the atoms of the acceptor groups; 2) the shake up energies found for the atoms of the donor part of the molecule (ED) are generally higher than that (E\) observed for the atoms pertaining to the acceptor group; when it is possible to correlate the shake up transitions to an U.V. one, the energy of the ultraviolet absorption transition seems generally intermediate between ED and E x . The analytical, spectroscopic and chemical importance of multi peak structures of ESCA bands is emphasized. In particular, the fact that it is not generally recognized that the shake up phenomena may cause strong broadening of the bands is criticized in connection with both the evaluation of ionization energies and the deconvolution procedures.