Plasmonic gaps offer the possibility to develop a great variety of applications in nanooptics that range from optimization of optical signal in field-enhanced spectroscopy and microscopy, to testing quantum transport processes within the cavity gap at optical frequencies. With use of a plasmonic gap, doubly resonant with the vibronic transition of a sample molecule, it is possible for example, to obtain single-molecule chemical identification using tip-enhanced Raman scattering (TERS) with subnanometric resolution. Furthermore, when the separation distance in plasmonic gaps becomes subnanometric, new quantum phenomena emerge as a result of the quantum tunneling of electrons between the metallic surfaces forming the gap. This regime has been identified experimentally very recently. Additionally, when an emitter is located in a plasmonic gap, the dynamical processes involving the electronic states of the emitter and those of the metal become richer and more complicated. Generally, the strong coupling regime between the quantum emitter and the gap plasmon gives rise to plexcitonic splitting of the optical response, however, for distances of the emitter to the surface below 1 nm, direct electron transfer from the emitter's localized state into the continuum of states of the plasmonic particle can further modify the optical response, even making the fingerprint of the emitter to disappear from the spectrum.
We study these quantum effects in the optical response of subnanometric plasmonic gaps with use of time-dependent density functional theory (TDDFT) and compare fully quantum and classical approaches for each case. Resonant electron transfer (RET) can be relevant in many situations of nanooptics dealing with quantum information, field-enhanced spectroscopy, catalysis, and photochemistry in general.