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