M-Center Fluorescence in NaCl and KI Crystals

The first excited singlet state ${\Sigma }_u^{+}$ of the $M$ center fluoresces in the near infrared. This fluorescence has been observed at low temperatures and the decay time found to be 41 nsec for NaCl and 60 nsec for KI. In KI, a maximum time (5 nsec) is established for filling the ${\Sigma }_u^{+}$ state from a higher-excited state ${\Pi }_u$. In addition, the fluorescence spectrum of the $M$ center in KI is reported for the first time. At ∼ 10 °K, the fluorescence intensity peaks at 0.818 eV = 1.52 μ and has a half-width of 0.100 eV.