Photon and electron emission during halogen adsorption on sodium

The adsorption of chlorine, bromine and iodine on UHV evaporated sodium films is accompanied by the emission of photons and electrons. Although the emission probabilities are small (～10^?7 and ～10^?5 per adsorbed molecule respectively), the effect provides a sensitive means of monitoring the adsorption process. For moderate exposures, the photon and electron yields are proportional to the pressure. At high exposures, due to the formation of a halide film on the surface, the photon yield decreases rapidly, eventually becoming proportional to the square root of the pressure. The high sensitivity allows use of the light signal to measure diffusion through the halide layer. The spectral distributions of the observed photons are characterized by wide (～1 eV) emission bands in the range 4000–8000 A. The spectra are influenced by surface composition, as demonstrated most clearly in the Na + I₂ spectrum, where a peak occurs on the high energy side of the main band when the NaI film starts to form. The origin of the spectra, their comparison with gas phase chemiluminesccnce and a model of the reaction kinetics are discussed.