The methods of vacuum-ultraviolet and thermal activation spectroscopy were used to measure the excitation spectra of impurity radiation in the fundamental absorption band of K 2 SO 4 -Cu crystals at 15 K and 300 K. An energy transfer was detected from the base to Cu + impurities. The band gap of crystals K 2 SO 4 was estimated. In K 2 SO 4 -Cu crystals, recombination radiation ...