The temperature dependence of the intensity of the G vibronic band is reported. Absorption data are given for the range 2 to 200 K, and luminescence data for the range 2 to 90 K. The data show that the molecular structure of the G centre is invariant in these ranges. From 2 to 30 K the luminescence intensity undergoes a specimen-dependent growth. We suggest the growth is caused by the thermal ionisation of competing shallow exciton traps. Above 30 K the luminescence intensity undergoes an intrinsic decrease, and is quenched to 0.2% of the low-temperature value by 77 K.

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