Nonlinear susceptibilities of atoms for frequencies exceeding the ionization potential

A correct calculation of the third harmonic generation coefficients β and hyperpolarizabilities γ for the bound and excited states of hydrogen and alkali metal atoms is performed for the first time at superthreshold frequencies. Some features of the physical interpretation of the susceptibilities connected with the ionization of the atoms in the light field are considered. Rapid decrease of β and γ at high frequencies is noted. The corrections to the photoelectric effect in a strong field which are defined by the imaginary part of γ are analyzed. It is demonstrated that approximate method of calculation, which is based on the neglection of the contribution from continuous spectrum intermediate states in perturbation theory composite matrix elements, is invalid even for evaluation of the order of magnitude of the nonlinear susceptibilities