Coinciding coordinates in the numerator, r e = r h , reflect the fact the exciton is created at a spatial scale small compared with its radius. The integral in the numerator can only be performed for specific models of impurity excitons. However, if the exciton is weakly bound to impurity, hence, the radius of the bound exciton a i satisfies the condition a i ≥ a e x and its wave function of the internal motion Φ Φ e x ( r e − − r h ) is only slightly distorted, then the integral in the numerator can be evaluated as ( a i / a e x ) 3 / 2 . This immediately results in an estimate for f i