Role of quantum dynamics in coherent and incoherent radiation during tunneling ionization

Radiation during strong-field tunneling ionization is investigated. The spontaneous as well as the coherent components of the radiation are calculated describing the ionization dynamics via the strong-field approximation, and the role of the quantum dynamics at tunneling is analyzed. The competition between different mechanisms in different spectral regions is examined. Transitionlike radiation (Brunel radiation) is dominant at low frequencies, Thomson scattering at the laser frequency, and radiative recombination via the three-step process at high-order harmonics. To distinguish the role of the quantum dynamics, simple man Drude models are developed for coherent as well as for spontaneous radiation; the models are based on the electron trajectory out of the tunneling barrier. The quantum dynamics is shown to modify the coherent Brunel radiation for near-zero frequencies in asymmetric laser pulses. The significant role of free-free transitions is demonstrated for spontaneous radiation in the low-frequency region.