Valence and core-level X-ray photoemission spectroscopy of light-sensitive molecules: lumazine and alloxazine

Valence and core-level photoemission spectra of two gaseous nitrogen containing heterocycles, lumazine and alloxazine, have been studied experimentally and theoretically. Comprehensive information about the electronic structure of these aromatic molecules has been acquired. The outer-valence Green's function (OVGF) approach was used to calculate vertical ionization energies and relative intensities (pole strengths) of the valence band spectra. The C, N, and O 1s core level spectra of both compounds were measured and assigned on the basis of Δ-Kohn–Sham calculations. The relative stability of different tautomers of lumazine and alloxazine was investigated through DFT calculations. Only the single, most stable tautomer for lumazine and alloxazine was populated at the present experimental conditions and this finding is confirmed by the core-level photoemission signal, which is able to discriminate between different isomers of a molecule. Other tautomers of lumazine and alloxazine have not been clearly identified spectroscopically, however their relative free energies and Boltzmann populations were calculated and compared with previous studies.