6533b85ffe1ef96bd12c191d

RESEARCH PRODUCT

Role of Shallow Electron Traps in the Fast Transient Optical Phenomena of Alkali Halide Crystals

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We present additional evidences that the same shallow electron traps-atomic alkali impurity centres [M + ] c 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, KBr:I, RbCl:I and RbBr:I (due to on-centre STE or on-centre STE localised at iodine dimer). 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 o =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 E 0 (for NaCl, KCl, KBr, RbCl and KCl:I) and/or the IR band low-energy edge E 0 (±0.03 eV) values (for NaBr, NaI, NaCl:I, KBr:I, RbCl:I and RbBr:I). Two types of the [M + ] c 0 e - centres are predominant: (i) [Na + ] c 0 e - in KX and RbX host crystals, for which the relation E 0 ≃6.15/d 2.74 is valid, and (ii) [Li + ] c 0 e - in NaX host crystals for which the relation E 0 ≃29.4/d 4.72 is valid. The Mollwo-Ivey relation E 0 ≃18.36/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 o 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 + ] c 0 e - …V k } and {F'…V k } are created. Such a quasi-molecular electronic transients have a very wide lifetime spectrum (τ ≥ 1 ns) and can play a great role in the intense pulse excitation experiments.