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 (EF), 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 EF + 3.7 eV. The tunneling decay parameter, β, was fitted by β90 K = 0.097 Å(-1) and βRT = 0.13 Å(-1). These values were one order smaller than that at around EF by conventional contact probe methods.

Two-photon photoemission (2PPE) spectroscopy has been employed to probe the electronic states of n-alkanethiolate self-assembled monolayers (SAMs) on an Au(111) substrate fabricated in a wet chemical process. Electronic states newly formed in the SAM formation were observed below and above the Fermi level (E F) for various alkyl chain lengths of C12-, C18-, and C22-SAMs. At 3.5–3.7 eV above E F, an unoccupied state originating from a bond between gold and sulfur atoms appears, a state that shows little dispersion with parallel to the surface. The peak position of the unoccupied state depends on the substrate temperature, and it was stabilized with increasing temperature; E F +3.7 eV at 85 K and E F +3.5 eV at 330 K for C18-SAM. The stabilization of the state is attributed to the increase of intermolecular interaction at sulfur atoms with their neighboring S atoms, which is caused by a change in the tilt angle of the alkyl chains in the SAM: on increasing the temperature, the interaction between S atoms in the SAM is promoted by the more upright alkyl chains.

The electron dynamics in alkanethiolate selfassembled monolayers (Cn-SAMs; n = 6−18, where n is the number of alkyl carbons) formed on Au(111) surfaces has been investigated by time-and angle-resolved two-photon photoemission spectroscopy. The time evolution of photoexcited electrons flowing down into image potential states (IPSs) formed on standing-up structure of SAMs is resolved twodimensionally; the electron lifetime in the IPS increases with chain length, from sub-ps to 100 ps. The chain length dependence of the IPS lifetime is particularly marked at shorter chain lengths of n = 6−10, whereas it becomes milder at chain lengths above n = 10, whose alkyl layer thickness is ≈10 Å. This thickness dependence can be explained by two competitive channels for the decay of IPS electrons: one is electronic coupling of IPS with unoccupied bulk Au states and an interfacial state localized at the Au−S linkage, and the other is IPS electron decay to the Au substrate through a tunneling barrier of insulating alkyl chains. The former is most influential at shorter chain lengths, while the latter is solely dominant at longer chain lengths. In addition, the photon energy dependence of the IPS intensity revealed that electron injection into the IPS is mediated effectively by an electron excitation into interfacial resonance formed in the alkyl layer.