New Temporary Infrared Absorption Bands in the Alkali Halides

Abstract

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.

<title>Shallow electron traps in alkali halide crystals: Mollwo-Ivey relations of the optical absorption bands</title>

“…It was concluded that the transient JR-absorption bands of both classes are with a very similar form (a wide asymmetric band) and exactly coincide (for KC1:I at 10 K14'15 or 80 K14) or overlap (with a small shift -for RbCl:I, KBr:I and NaCl:I at 10 K) with the corresponding transient JR-band (max. at 0.28-0.36 eV102 at 1 1-15 K) in KC1, RbCl, KBr and NaCl due to electrons trapped in shallow traps10'2 or "bound polarons" [8][9][10]12 It is obvious that the vibration structure of the transient JR-absorption band in KC1:I at 10 K14 is very similar to the vibration structure of the trapji electron centre JR-absorption band in KCJ at 4.2-2 1 K10'12.…”