Quantum Theory of High Harmonic Generation via Above Threshold Ionization and Stimulated Recombination

Fully quantum treatment explicitly presents the high harmonic generation as a three-stage process: above threshold ionization (ATI) is followed by the continuum electron propagation in a laser field and subsequent stimulated recombination back into the initial state. The contributions of all ATI channels add up coherently. All three stages of the process are described by simple, mostly analytical expressions. 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. Under the influence of an intensive electromagnetic field an atom can emit electrons and photons. The number of photons absorbed from the field in the first process generally exceeds the minimum necessary for ionization, i.e. the photoelectrons are characterized by their distribution over above threshold ionization (ATI) channels. The photon production corresponds to the harmonics generation (HG) for the incident (monochromatic) laser radiation. Both ATI and HG are capable of populating the channels with remarkably high energy, as has recently been registered in experiments (see, e.g., L.’Huillier et al 1991, Macklin et al 1993, Schafer et al 1993) and tackled by the theory. An idea that the two processes referred above are interrelated has been articulated quite long ago. Since in the HG process an active electron ends up in the initial bound state, it is appealing to represent it as ionization followed by recombination. This mechanism presumes a strong interaction between the emitted electron and the core that is omitted in the standard Keldysh (1964) model of multiphoton ionization. The importance of this interaction for a variety of processes was first pointed out by Kuchiev ‡ E-mail: [email protected] § Permanent address: Institute of Physics, The University of St Petersburg, 198904 St.Petersburg, Russia; E-mail: [email protected]