Quantum theory of photon emission during strong-laser-field-induced ionization. II. Continuum dynamics

Emission of a photon during the process of above-threshold ionization is investigated. In previous work [D. B. Milošević, ] one of the present authors studied this process in the leading order of the strong-field approximation for the matrix element that describes direct emission of a photon during the transition from the bound state to the continuum. We now show that the correction that corresponds to photon emission while the electron is propagating in the continuum, very significantly modifies the process for all but some intermediate photon energies. In contrast to the lowest-order term of the previously considered expansion, the continuum correction is infrared divergent and resonant when the photon energy agrees with the energy of a driving photon. If the energy of the emitted photon is much larger than the energy of a driving photon, then the continuum correction is very close to the leading-order direct contribution and the two terms interfere constructively. These statements hold true for a variety of laser-field configurations; we investigated various bichromatic linearly and circularly polarized fields including bicircular fields.