Charge transfer from core-excited argon adsorbed on clean and hydrogenated Si(100): Ultrashort timescales and energetic structure

Using the well-established core hole-clock method under resonant Auger Raman conditions we have measured the charge transfer (CT) times for the 4s electron on 2p_3/2 core excited Ar atoms adsorbed on Si(100). The influences of the doping (p- or n-type), surface condition (clean or covered with monohydride) and varied excitation energy have been examined. The data for the Si-H surfaces are most extensive and distinct and undisturbed by background or losses. The CT times, which are identical for n- and p-type materials, are found to be about 2 fs at resonance. They show a distinct energy dependence when broadly tuning the excitation energy through the Ar core resonance. The CT times on clean Si(100), for which the data are not as extensive, are shorter by a factor of ~2 compared to the Si-H surfaces and again about the same for n- and p-type Si(100). The unexpectedly short CT times found, as well as the energetic structure seen, are discussed in terms of possible influences of the projected surface electronic structure of Si(100) in the energy range of the Ar 4s electron, and of other explanations. Theoretical modeling would be highly desirable.