Investigations of the luminous events associated with thebreakdown along the surface of large-band-gap insulators such as alumina(polycrystalline) and silica (monocrystalline) were made by employing aphototube and a very sensitive photomultiplier tube (PMT). The specialnature of the light emitted during breakdown is resolved (in terms ofthe wavelength) by interfacing a monochromator with the PMT. The onsetof the luminous signal was found to occur much before the onset ofvoltage collapse during breakdown. In addition, the samples were foundto emit light when no breakdown occurs. The early onset and lightemission with no breakdown are found to be in the ultraviolet range. Theresults are discussed in terms of the solid-state defects (e.g., traps)possessing different energy levels
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