Carrier capture by attractive centres has been discussed in a number of papers.The principal results a r e reviewed in /1/ where a criticism of the fundamental papers /2, 3/ is given. The calculation of capture cross sections is carried out by the Pitaevski method in which the expressions for cross sections in high and low temperature ranges, respectively, a r e given in the form
In /1/ the ground state of the F-centre was calculated without taking into account the spatial dependence of the permittivity ~( r ) though it is known that when r-QE-1 /2/. In most ionic crystals the F-centre may be considered a s a defect forming a deep level whose electron cloud is mainly concentrated within the first coordination sphere. Then to correctly solve the problem of the electron energy spectrum of the F-centre it is necessary t o allow for the spatial dependence of ~( r ) .In this note the ground state electron energy of the F-centre is calculated by the Feynman variational method /4/ taking into account the ~( r ) dependence.If we attribute the charge to the centre of force attracting an electron, then at z = 1 and z = 0 it is possible to obtain solutions for the F-centre and polaron, respectively ( z e is the defect charge).
Optical I' impurity-band" transitions have been studied in various models. For example, optical transitions in which shallow impurities take part a r e well described within the frames of a hydrogen-like model /l/. Deep acceptor photodeionization is considered in the delta-function potential model of Lucovsky /2/. Optical transitions involving not too deep and not too shallow impurities are calculated using the Hulten potential /3/.Thus, optical transition problems with shallow donor levels created by group V elements and with deep donor levels created by singly ionized ions of group VT are solved by means of various approaches, though from the physical viewpoint there is no difference between these defects except for the central charge. The same is true for shallow and deep acceptors created by group ITI elements and singly charged group I f ions, respectively. Besides, the above mentioned theories suffer from a serious disadvantage -introduction of a fitting parameter taking into account the increase of the optical transition amplitude due to the fact that the effective local field exceeds the averagemacroscopicfield in the crystal.Tn /4/ we proposed a model in which the impurity centre potential can be presented in the formwhere z* is the effective charge of the impurity centre which is estimated according to the Slater rules. Since this z* corresponds to the free atom ionization energy differing sufficiently from its experimental value, we make a correction to the Slater rules, so that z* found with allowance for this correction 1) Prospekt Chavchavadze 1, 380028 Tbilisi, USSR.
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