Electronic states of substituted haloacetylene and cyanoacetylene radical cations

Abstract

Photoelectron spectra excited by He(lα), and in some cases also by He(IIα), radiation of the following twelve acetylenic derivatives are reported: CH 3CCX, CF 3CCX, NCCCX with X = Cl, Br, I and NCCCX with X = F, CH 3, CF 3. The ionization energies leading to the doublet states of their radical cations accessible by such photoionization processes have been obtained and interpreted. Vibrational frequencies of some of the totally symmetric modes in the cationic states of these molecules have been inferred from the fine structure discernible on the photoelectron bands. Trends in the ionization energies are pointed out and the models used previously to assign the photoelectron spectra of such species are summarized. Theoretical calculations of the ionization energies of the molecules RCN with R = H, F, CH 3, CF 3, CN and RCCCN with R = H, F, CH 3, CN both in Koopmans' approximation and with a many-body Green's function method permit a complete interpretation of their photoelectron spectra. For the majority of the RCN and RCCCN molecules Koopmans' approximation fails to predict both the correct ordering of ionic states as well as trends in the ionization energies, whereas, the Green's function calculations are in good agreement with experiment in both respects.