. at om - p h ] 2 6 Ju n 19 99 Quantum Theory of a High Harmonic Generation as a Three - Step Process

Fully quantum treatment explicitly presents the high harmonic generation as a three-step process: (i) above threshold ionization (ATI) is followed by (ii) electron propagation in a laser-dressed continuum. Subsequently (iii) stimulated (or laser assisted) recombination brings the electron back into the initial state with emission of a high-energy photon. Contributions of all ATI channels add up coherently. All three stages of the process are described by simple, mostly analytical expressions that allow a detailed physical interpretation. A very good quantitative agreement with the previous calculations on the harmonic generation by H − ion is demonstrated, thus supplementing the conceptual significance of the theory with its practical efficiency. The virtue of the present scheme is further supported by a good accord between the calculations in length and velocity gauges for the high-energy photon.