  The tip of a scanning tunnelling microscope can inject hot electrons into a
surface with atomic precision. Their subsequent dynamics and eventual decay can
result in atomic manipulation of an adsorbed molecule, or in light emission
from the surface. Here, we combine the results of these two near identical
experimental techniques for the system of toluene molecules chemisorbed on the
Si(111)-7x7 surface at room temperature. The radial dependence of molecular
desorption away from the tip injection site conforms to a two-step
ballistic-diffusive transport of the injected hot electrons across the surface,
with a threshold bias voltage of +2.0 V. We find the same threshold voltage of
+2.0 V for light emission from the bare Si(111)-7x7 surface. Comparing these
results with previous published spectra we propose that both the manipulation
and the light emission follow the same hot electron dynamics, only differing in
the outcome of the final relaxation step which may result in either molecular
manipulation, or photon emission.
