The lowest electronic excited state on the Si(100) surface and its coupling to the ground electronic state have been investigated using first-principles theory. The energy difference between the optimal geometry in the two states is small enough for a significant equilibrium population of the excited state to exist under common reaction conditions. The kinetics of crossing between spin states have been determined by explicit calculation of the minimum-energy crossing point and the spin-orbit coupling between them. The predicted excited-state lifetime is very short, except at low temperature.