Role of shallow electron traps in the fast transient optical phenomena of alkali halide crystals

We present additional evidences that the same shallow electron traps-atomic alkali impurity centrcs [M(+)](e)(0)e(-) are responsible for both classes (A and B) of the transient IR-absorption bands: (A) bands with maximum at 0.27-0.36 eV in NaCl, KCl, KBr, KI and RbCl (due to "shallow electron traps" or "bound polarons") and (B) bands with maximum at 0.15-0.36 eV in NaI, NaBr, NaCl:I, KCl:I, K-Br:I, RbCl:I and RbBr:I (due to "on-centre STE" or "on-centre STE localised at iodine dimier"). Both classes of the IR bands have the same location, similar shape (both exactly coincide for KCl:I and KCl at 10 or 80 K), half-width, vibration structure. It is established that the same Mollwo-Ivey plot curves E(0)=a/d(n) (d is the nn anion-cation distance, n is the exponent, a is the constant) take place for both IR band classes, if we plot instead of the IR band peak energy values the more definite values of the IR band zero-phonon line energy Eo (for NaCl, KCl, KBr, RbCl and KCl:l) and/or the IR band low-energy edge Eo (+/-0.03 eV) values (for NaBr, NaI, NaCl:I, KBr:I, RbCl:I and RbBr:I). Two types of the [M+](c)0e(-) centres are predominant: (i) [Na(+)](e)(0)e(-) in KX and RbX host crystals, for which the relation E(0)approximate to6.15/d(2.74) is valid, and (ii) [Li(+)](e)(0)e(-) in NaX host crystals for which the relation E(0)approximate to29.4/d(4.72) is valid. The Mollwo-Ivey relation E(0)approximate to18.36/d(2.70) is valid as well for the F band in NaCl, KCl, KBr, KI, RbCl, RbI, if we use the F centre optical binding energy E, values. Under irradiation at 77 or 4.2 K the shallow trapped electron centres ([M(+)]c(0)e(-) and F centres) and their associations with the V(k)-tYpe centres, i.e., dipolar pairs ([M(+)](e)(0)e(-)...V(k)} and F(1) ...V(k)} are created. Such a "quasi-molecular electronic transients" have a very wide lifetime spectrum (r greater than or equal to 1 ns) and can play a great role in the intense pulse excitation experiments.