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We investigate the charging state of an isolated single dangling bond formed on an unpassivated Si(100) surface with c(4 χ 2) reconstruction, by comparing scanning tunneling microscopy and spectroscopy analysis with density functional theory calculations. The dangling bond is created by placing a single hydrogen atom on the bare surface with the tip of a scanning tunneling microscope. The Η atom passivates one of the dimer dangling bonds responsible for the surface one-dimensional electronic structure. This leaves a second dangling at the reacted surface dimer which breaks the surface periodicity. We consider two possible Η adsorption configurations for both the neutral and the doped situation (n- and p-type). In the case of n-doping we find that the single dangling bond state is doubly occupied and the most stable configuration is that with Η bonded to the bottom Si atom of the surface dimer In the case of p-doping the dangling bond is instead empty and the configuration with the Η attached to the top atom of the dimer is the most stable. Importandy the two configurations have different scattering properties and phase shift fingerprints. This might open up interesting perspectives for fabricating a switching device by tuning the doping level or by locally charging the single dangling bond state. [ABSTRACT FROM AUTHOR]