The He(II) photoelectron spectra of the fluorosubstituted ethylenes and their analysis by the Green's function method

The 30.4 nm He(II) photoelectron spectra of the fluorosubstituted ethylenes have been recorded. The assignment of all main bands is obtained from many-body Green's function calculations. The results from the semi-empirical HAM/3 method lead to a nearly identical assignment. For the mono- and difluoroethylenes, an unambiguous interpretation of the spectra can be established from empirical considerations alone. The set of spectra has been reexamined with respect to the “perfluoro-effect” rules. Also discussed are the ionisation energies as a function of the geminal FCF bonding angle and the similarity of the spectra of the cis and trans isomers of 1,2-difluoroethylene. Additional weak bands were detected in the energy region 21–24 eV in all spectra and were attributed to “shake-up” transitions on the basis of 2ph-Tamm-Dancoff Green's function calculations. The orbital model of ionisation breaks down for the ionisation out of the F(2s) and C(2s) orbitals in general. The calculations reveal a charge transfer character of the excitations accompanying ionisation from the C(2s) orbitals.