Contrast in photon yield between phase-separated domains of reduced and photo-oxidized alkanethiols comprising a mixed molecular monolayer on Au(111) was observed using an ultrahigh vacuum scanning tunneling microscope. The photon quantum yield of the two surface species was measured to be independent of tunneling gap width, within experimental uncertainty, suggesting a novel photon emission mechanism. The spatial resolution of the photon maps was 20 nm. These results indicate that photons emitted by a scanning tunneling microscope reveal chemical information with nanometer spatial resolution on molecularly heterogeneous surfaces.