Competition between relaxation and decay of a core-level electron ionized state in a charge transfer system

Abstract

In a weak charge transfer (CT) system, such as an adsorbate–substrate system a core-level electron ionized state in an adsorbate observed by photoelectron spectroscopy (PES) as the smallest binding energy state, may be a metastable state. On time scale of core–hole decay it may relax further to the most stable state by CT core–hole screening via transfer of an electron from the substrate to an unoccupied adsorbate discrete level pulled down below the Fermi level of the substrate by an attractive core–hole potential. The latter state is a CT shakedown state, which is not observable by PES but its presence is visible in a decay spectrum, such as Auger-electron spectroscopy (AES) spectrum. The CT shakedown state resembles a resonantly excited core–hole state lying below the core–hole state of the smallest binding energy. If this is so, the AES spectrum measured at far above the core-level electron ionization threshold has then an additional spectral feature, which resembles the autoionization spectrum by the core–hole decay of the resonantly excited core–hole state. Such an example is the N 1s AES spectrum of N 2 molecule physisorbed on graphite reported by Björneholm et al. [Q. Björneholm, A. Nilsson, A. Sandell, B. Hernnäs, N. Mårtensson, Phys. Rev. Lett. 68 (1992) 1892]. The competition between relaxation and core–hole decay of a core-level electron ionized state in a weak CT system is formulated by a many-body theory and discussed. The present theory provides a theoretical foundation of the interpretation by Björneholm et al. They determined the CT time from the intensity ratio of the AES spectrum by the decay of the CT shakedown state to that by the decay of the core–hole state observed by PES as the smallest binding energy state. However, the CT time cannot be determined, unless the branching ratio of the former decay rate is equal to that of the latter one.