High-harmonic generation in hydrogenic ions

We have calculated the frequency-dependent nonlinear susceptibilities of hydrogenic ions up to the 70th order of nonlinearity, in the framework of lowest-order perturbation theory. In this approximataion, the critical driving intensity, at which the radiated harmonic intensity of the (n+2)th order exceeds that of the nth order, is found to decrease rapidly with n. The computed values of critical intensity are comparable to those at which plateau behavior is observed in recent experiments.