The electronic structures of alkanethiolate self-assembled monolayers (SAMs) associated with their molecular-scale geometry have been evaluated by combining two types of scanning tunneling spectroscopy (STS): current-voltage (I/V) and distance-voltage (z/V) spectroscopy. In the STS spectra, in addition to electronic states originating from alkyl-chain and SAM/Au interface, an image potential state (IPS) formed in a vacuum gap is clearly observed. The result is well consistent with our two-photon photoemission spectroscopy (2PPES) measurements. Furthermore, by using time- and angle-resolved 2PPES, we found that the electron excited into IPS is highly separated from the Au substrate and has an extremely long lifetime of the order of 100 ps owing to the excellent insulating property of SAM. The lifetime can be prolonged by increasing the length of alkyl chain and by lowering the substrate temperature, which indicates that the degree of electron separation can be precisely controlled by designing the molecular layer.