On nature of the transient IR-absorption STE-like bands at 0.15–0.36 eV in alkali halides

New evidence is given that two classes (A and B) of the transient IR-absorption bands: (A) with max. at 0.15–0.36 eV (in NaCl : I, NaBr, NaI, KCl : I, KBr : I, RbCl : I, RbBr : I), due to “on-centre self-trapped exciton” and (B) with max. at 0.27–0.36 eV (in NaCl, KCl, KBr, RbCl), due to “shallow trapped electrons” or “bound polarons”, are caused by the same defect—shallow trapped electron (e−) at the substitutional (cation: c-site) alkali impurity cation (M+): [M+]c0e−. The A- and B-class IR bands have the same location, similar shape, half-width (exactly coincide for KCl : I and KCl at 80 or 10 K with the same vibration structure). It is established that the same Mollwo–Ivey plot curves E0=a/dn (d—nn anion–cation distance, n—exponent, a—constant) are fulfilled for both IR band classes, if we plot instead the IR band peak energy values the more definite values of the IR band zero-phonon line energy E0 (for NaCl, KCl, KBr, RbCl and KCl : I) and/or the IR band low-energy edge energy E0 (±0.03 eV) values (for NaBr, NaI, NaCl : I, KBr : I, RbCl : I and RbBr : I). These data are significant additional evidence that the A-class IR bands are caused by the same type defects as the B-class IR bands—by the shallow electron traps, i.e., centres [M+]c0e−. Two types of the [M+]c0e− centres are predominant: (i) [Li+]c0e− in NaX host crystals with E0≈29.4/d4.72 and (ii) [Na+]c0e− in KX and RbX host crystals with the relation E0≈6.15/d2.74.