Photoelectron spectra of the nucleobases cytosine, thymine and adenine

The complete valence shell photoelectron spectra of cytosine, thymine and adenine have been investigated experimentally and theoretically. Vertical ionization energies and spectral intensities have been evaluated using the manybody Green's function method, thereby enabling theoretical photoelectron spectra to be derived. In cytosine, the influence of tautomers and rotational confomers has been investigated. The calculated spectra display a satisfactory agreement with the experimental data and this has allowed most of the photoelectron bands to be assigned. Photoelectron asymmetry parameters have been determined from angle resolved spectra recorded with synchrotron radiation. The experimental data show that the electronic configuration of the five outer orbitals in cytosine, thymine and adenine is pi, sigma, pi, sigma, pi. Vertical ionization energies have been measured for all the outer-valence orbitals even though some of the associated bands overlap significantly.