Abstract
To elucidate the mechanism of photochromism of H2 and H3 centres, i.e. H2( H3 −) + X + + h ̵ ω → H3 + X 0 , a study has been made of the recovery process of optical absorption and the photosensitive electron spin resonance (ESR) and thermoluminescence. The ESR measurement indicates that the neutral state concentration of nitrogen is increased by illumination. This fact gives evidence for one of the unknown traps X + to be ascribed to ionized nitrogen, N +. The glow curve in thermoluminescence after photoexcitation denotes the existence of at least four traps besides N +. The recovery appears logarithmic with respect to time and is faster when the concentration of remaining neutral nitrogen is larger. The recovery rate is almost independent of temperature below 420 K, while it increases with increasing temperature above 420 K. As a result, it has been concluded that the dominant process of the recovery arises via the tunnelling of an electron from a neutral nitrogen atom to an H3 centre rather than via the thermal release of a trapped electron.
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