Competition between Above-Threshold Ionization and High-Order Harmonic Generation

Abstract

Both above threshold ionization and high-order harmonic generation are the result of atomic excitation in a manner which cannot be described by lowest-order perturbation theory. By high-order harmonic generation, more correctly, strong-field harmonic generation, we are referring to the generation of harmonic radiation (typically greater than the 7th harmonic) which is not a monotonically decreasing function of harmonic order. This was recently observed by MacPherson, et al1 and later by Ferray, et al.2 On the other hand, above-threshold ionization, the process by which an atom absorbs more photons than the minimum required to ionize, is now a well established phenomena in the interaction of intense radiation (>1013W/cm2) with atoms.3