  Here we report the direct observation of single electron charging of a single
atomic Dangling Bond (DB) on the H-Si(100) 2x1 surface. The tip of a scanning
tunneling microscope is placed adjacent to the DB to serve as a single electron
sensitive charge-detector. Three distinct charge states of the dangling bond,
positive, neutral, and negative, are discerned. Charge state probabilities are
extracted from the data, and analysis of current traces reveals the
characteristic single electron charging dynamics. Filling rates are found to
decay exponentially with increasing tip-DB separation, but are not a function
of sample bias, while emptying rates show a very weak dependence on tip
position, but a strong dependence on sample bias, consistent with the notion of
an atomic quantum dot tunnel coupled to the tip on one side and the bulk
silicon on the other.
