The tip of a scanning tunneling microscope (STM) may be used as an extremely localized source of low-energy electrons to locally excite photon emission from a variety of metal films. The detection of locally excited luminescence at the junction of a STM tip provides access to electron dynamic properties at the surface, which makes it possible to study luminescence phenomena of nanometer-sized structures. We analyzed maps of the photon intensity emitted from electrically isolated silver nanoparticles self-organized as a 2D network on a gold (111) substrate. We observed unexpectedly strong variations of photon-emission efficiency from isolated nanoparticles depending on how tightly they are embedded within the network site. The quenching site observed in the STM photon emission map is interpreted as an enhanced interaction of electrons with surface phonon modes.