In principle, chemical information is obtainable from metal surfaces by means of photon emission from the scanning tunneling microscope (STM). However, the photon emission varies significantly with topography and choice of tip. We address the important issue of geometry by studying the emission characteristics of Ag and Au spheres. First, photon maps of Ag clusters, consisting in some cases of just a few atoms, demonstrate that they can be uniquely identified from other nonmetallic particles. Then, the bias at which there is an onset of photon emission for 1 nm Ag and Au clusters is measured and found to be ≈ 3.3 and ≈ 2.1 V, respectively. This allows for the demonstration of the ability of the STM to distinguish different metal particles by their photon emission. The value of the onset bias for each metal can be made almost invariant to sample topography by an appropriate choice of tip; only then is the photon emission related purely to the optical properties of the surface. We envisage a form of chemical microscopy, whereby the bias of emission onset is mapped. © 1998 American Institute of Physics.