STM excitation of individual biphenyl molecules on Si(1 0 0) surface: DIET or DIEF?

We have studied the excitation of individual biphenyl molecules adsorbed on Si(1 0 0)-(2 × 1) surface by using a scanning tunnelling microscope in ultra-high vacuum at room-temperature. Exciting these molecules in their unstable state with a voltage pulse can stabilise them. We have investigated in detail this transformation in order to determine the reaction mechanisms induced by the pulse: is it due to an electronic excitation or is it an effect of the electric field between the tip and the sample during the pulse? It is quite difficult to distinguish between the dynamics induced by electronic transition (DIET) and the dynamics induced by electric field (DIEF): when increasing the number and the energy of electrons for electronic excitation (DIET), the electric field under the tip can be strong enough to induce reaction by lowering the energy barrier (DIEF). By using a program to compute the electric field in three dimensions, we conclude that the transformation of the unstable configuration into the stable one is due to an electronic excitation.