  Using time-dependent density-functional theory, we investigate the electronic
stopping power of self-irradiated silicon through non-adiabatic dynamics
simulations. For specific velocities above 0.6 atom units, electronic stopping
shows a generally assumed metallic behavior that is velocity-scaling. While in
the lower velocity regime, the slope of electronic stopping power versus
velocity changes and the overall magnitude are significantly greater than
expectations, leading to a complete vanish of hard threshold. An analysis of
the electron localization function allows us to arrive at the following
conclusion: the long duration of encounter process between the host atoms and
the projectiles with low velocities makes possible the formation of chemical
bonds with relative high bond order. The continuous formation and breaking of
chemical bonds provides an additional effective energy loss channel.
