AUTOIONIZATION VERSUS PHOTOIONIZATION AND AUGER DECAY OF PHYSISORBED MOLECULAR ADSORBATES - CONDENSED BENZENE ON CU(110)

Autoionization spectra following core-to-bound excitation of benzene physisorbed on Cu(110) are presented. The inner valence region of the monocationic ion states, where the single particle picture is known to break down, dominates the spectrum. While autoionization and photoionization probe the same ion states transition intensities are different due to different selection rules. Especially the comparison between inner and outer valence region allows the identification of rather pronounced discrepancies. Core-to-continuum excitation leaves the molecule in a core ionized state which decays radiationless into outer valence double ionized states (Auger spectrum). Our interpretation is based on ab initio Green's function calculations for the inner and outer valence region followed by explicit calculations of the radiationless autoionization rates. In order to compare the transition rates for autoionization and Auger decay we have furthermore calculated the core-valence-valence Auger spectrum of benzene after C Is excitation by the particle-particle Green's function method with subsequent calculation of the radiationless Auger rates. The simulated spectra after resonant and continuum excitation basically explain the experimental findings. The results show the usefulness of combining experimental and theoretical methods to study the radiationless decay even for large molecules.