Semi-empirical pertci ionization spectra of some cumulenes and cumulene-like systems. Low-energy non-koopmans' ionizations

Abstract

A theoretical approach to the question of satellite structure in the photoelectron spectra of cumulenes and cumulene-like systems is made. PERTCI calculations including all singles and doubles with respect to the Koopmans' and non-Koopmans' configurations were made using semi-empirical MNDO, CNDO/S and LNDO/S wavefunctions. The method was applied to dimethylcarbodiimide, fulvenallene, fulvenone, fulventhione, diazocyclopentadiene, butatriene, tetrafluorobutatriene, pentatetraene, carbon suboxide and carbon subsulfide. Calculated ionization spectra for all these systems are presented. They are compared with the He-I photoelectron spectra. For butatriene, the results are in accord with preceding ab initio findings. Except for dimethylcarbodiimide the calculated ionization spectra exhibit low-energy satellite structure (i.e. more ionizations of reduced intensity than expected on the basis of the Koopmans' configurations alone). From the comparisons of the calculated and measured spectra, hints to possible shake-up structures in the He-I spectra of the molecules considered are given. The results call for refined theoretical and especially experimental investigations in order to substantiate the present theoretical predictions.