Studies of surface-induced luminescence relevant to spacecraft glow, using a seeded arc-jet

An experimental investigation has been undertaken of surface-induced luminescence under conditions of energetic rarefied flow. A two chamber differentially pumped vacuum system and a dc arc-jet source were used to produce beams seeded with oxygen, nitrogen or nitrous oxide. The beams have been characterised using a combined cylindrical mirror analyser and quadrupole mass spectrometer instrument. Typically, beams of 0.2eV (1.55kms-1) atomic oxygen seeded in argon, and 0.9eV (3.29kms-1) atomic oxygen seeded in helium were obtained, with absolute fluxes of 1.1x1015cm-2s-1 on entry into the test chamber. In the luminescence experiments, the beam was passed through a rotating two disc velocity selector in order to isolate the exiting modulated gas beam from the intense source light. Faint luminescence was detected under conditions in which an atomic oxygen arc-jet beam and an additional effusive beam of nitric oxide were simultaneously incident on an aluminium target coated with Chemglaze Z306 matt black paint. Experiments have shown that this luminescence is not a reaction between O1 and the surface, NO and the surface, nor He/Ar and the surface. The luminescence intensity is proportional to [O1][NO], it has no significant dependence on O1 translational energy in the range 0.2-0.9eV, and it increases at longer wavelengths into the deep red (> 712nm). Analysis of the observations indicates that the process is surface-mediated and that the emissions result from excited particles leaving the surface with a radiative lifetime in the range 15-150μs. The average reaction probability to form an excited particle has been calculated to be ≈4x10-5 per oxygen atom and ≈1x10^-4 per nitric oxide molecule. These results are consistent with the formation, desorption and subsequent radiative relaxation of excited nitrogen dioxide. An extensive review of the spacecraft glow phenomenon has been carried out. It has been found that much of the anomalous behaviour, including that between different spacecraft and altitudes, can be reconciled by a single hypothesis based on emissions from excited NO_2^*, via a mechanism similar to the laboratory luminescence. The additional critical process which must be included, under orbital conditions, is the mechanism for provision of sufficient NO flux on the surface. The combination of ambient NO, atom exchange O_1+N_2→NO+N1, and surface recombination N1+O1→NO can provide such a flux. With certain realistic assumptions the majority of spacecraft glow observations can be explained using the new model.