  Atomic-scale chemical modification of surface-adsorbed ethyl groups on
Si(001) was induced and studied by means of scanning tunneling microscopy.
Tunneling at sample bias > +1.5V leads to tip-induced C-H cleavage of a
$\beta$-hydrogen of the covalently bound ethyl configuration. The reaction is
characterized by the formation of an additional Si-H and a Si-C bond. The
reaction probability shows a linear dependence on the tunneling current at 300
K; the reaction is largely suppressed at 50 K. The observed tip-induced surface
reaction at room temperature is thus attributed to a one-electron excitation in
combination with thermal activation.
