the low-temperature photoluminescence of hgi sub 2 is reported three main luminescence bands are observed, with peaks at approx 2.30, 2.20, and 2.00 ev at 77 k. at 4.2 k, the highest energy peak shows considerable structure. the temperature dependence of these lines indicates both free and bound exciton recombination, and very small exciton binding energies (approx. 3 to 5 mev) have been estimated. a comparison of the results of sublimation and doping experiments suggests that the lowest energy band may be related to impurities, whereas the middle-energy band is related to i content the two strongest bound exciton lines comprising the high-energy band show systematic correlations with the middle-energy, i-related band further correlations between these spectral features and the performance of nuclear radiation detectors fabricated from these samples are also noted. the temperature coefficient of the band gap is estimated from the spectral shift of luminescence lines to be approximately -1.13 x 10 exp -4 ev/k between 32 k and 45 k. (era citation 08:008204)