Highly localized plasmons can be excited at atomically smooth metal surfaces by using a sphere-plane type plasmonic cavity. This plasmonic structure can be optimized for use in well-organized self-assembled monolayers formed on various single crystalline metal substrates including highly damping platinum-group catalytic metals, and is therefore useful in the fields of molecular science, catalytic science, and surface science. Surface enhanced Raman scattering (SERS) observation at well-defined molecule-metal interfaces revealed crystallographic orientation dependence not only in adsorption geometry of the molecules but also in the signal enhancement, suggesting a contribution of interfacial charge transfer resonances between metal states and molecular affinity levels. Moreover, this method enables us to increase efficiency of various photochemical processes such as photo-energy conversion when photo-sensitive molecular layers are formed on a substrate. The use of well-defined metal-organic system in plasmonic cavities opens up a new possibility of spectroscopy and photochemistry.