Thermal radiation from small particles

We derive an expression for the emission of electromagnetic radiation from small, highly excited, and isolated particles in a cold environment. The result is a generalization of Planck’s blackbody formula on two counts. One is due to the finite level densities of small particles. It is most pronounced for very small particles and for high photon energies. The other effect is the absence of stimulated emission which influences the low-energy part of the spectrum. We discuss some consequences for the interpretation of experimental emission spectra.