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

摘要

We present additional evidences that the same shallow electron traps-atomic alkali impurity centres [M~+]_c~0e~- 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, KC1, 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, KC1: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 KC1:I and KC1 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 E_0 (for NaCl, KC1, KBr, RbCl and KC1: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~0e~- centres are predominant: (ⅰ) [Na~+]_c~0e~- in KX and RbX host crystals, for which the relation E_0≈6.15/d~(2.74) is valid, and (ⅱ) [Li~+]_c~0e~- 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, KC1, KBr, KI, RbCl, RbI, if we use the F' centre optical binding energy E_0 values. Under irradiation at 77 or 4.2 K the shallow trapped electron centres ([M~+]_c~0e~- and F' centres) and their associations with the V_k-type centres, i.e., dipolar pairs {[M~+]_c~0e~-...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.