Numerical techniques are used to solve and compare the rate equations with the semiolassioal equations. Prom the semiclassical theory, it has been derived that the atomic ionizations vary oscill-attorilly with the pulse propagation distance, which is a new coherent optical phenomenon and far from the predication of the rate equations. This work explains quantitatively the theoritical errors which oome from the rate equations for the laser isotope separation, and forms the basis for the experimental analysis.