8.

Processes leading to the creation of an exciton without tunneling to or from the HOMO or LUMO, of interest for the case when the bias is in the conduction gap. In (a) a singlet is created and in (b) a triplet. In (a) the electrons tunneling between the substrate and the molecule and between the molecule and the tip have the same spins, while in (b) they have different spins. In a first step an electron tunnels from a low-lying MO to the tip. In (a), in a Coulomb induced transition, the HOMO electron is then excited into the LUMO and an electron in a low-lying level fills the hole created in the first step. Finally the just created hole is filled by a substrate electron. In (b), in a Coulomb induced transition, the HOMO electron falls down into the hole created in the previous process and another electron is excited into the LUMO. The hole just created is then filled by a substrate electron.