Charge separation at the molecular monolayer surface: Observation and control of the dynamics

Charge separation dynamics relevant to an electron transfer have been revealed by time- and angle-resolved two-photon photoemission spectroscopy for an n-alkanethiolate self-assembled monolayer (SAM) on a Au(111) surface fabricated by a chemical-wet process. The electron was photoexcited into an image potential state located at 3.7 eV above the Fermi level (E _F), and it survived well for more than 100 ps on dodecanethiolate (C12)-SAM. The degree of electron separation is precisely controlled by selecting the length of the alkyl chain (C10-C18). We have also evaluated molecular conductivity at the specific electron energy of E _F + 3.7 eV. The tunneling decay parameter, β, was fitted by β _{90 K} = 0.097 Å ^-1 and β _RT = 0.13 Å ^-1. These values were one order smaller than that at around E _F by conventional contact probe methods.