New temporary infrared absorption bands are observed in several alkali halides. I n NaCl a short-lived centre with a transition at 4 pm is formed by excitation of the electron excess centres at low temperatures. Decay times have been measured simultaneously in the M, F, and 4 pm band. They indicate a bimolecular decay with the same time constant for the three bands. The 4 pm band is attributed to a transition of an electron in a shallow trap. Simultaneous absorption measurements were made in the F, M, and 4 pm band after ruby laser excitation in the F band, and in the F, F , and 4 pm bands after laser excitation in the F band. From these measurements it waa concluded that the shallow trap is the F centre. This implies the existence of a metastable form of the F centre. A triplet state is postulated. Absorption bands analogous to the 4 pm band in NaCl have been observed in KC1 at 6 pm, in NaF a t 2.4 pm, in KBr, and KI.
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