We consider tunnel ionization of atomic argon in a femtosecond-laser pulse of intensity ${10}^{21}–{10}^{22}{\mathrm{W}/\mathrm{cm}}^2$, aiming to investigate the relativistic dynamics and radiation of photoelectrons released from their parent ions inside the laser focus. Radiation of such electrons accelerated along the laser-field propagation direction appears to have moderate power but can be considerably enhanced by a collision with a relatively weak counterpropagating laser pulse. Using the theory of laser-induced tunneling in atoms and ions and that of nonlinear Thomson scattering, we demonstrate that angular distributions and the spectra of emitted photons can serve as a probe of the peak intensity in the focus. The angular distribution of emitted radiation in the plane of polarization and propagation of the ionizing laser beam appears narrow and peaked around an intensity-dependent angle. The sensitivity of angular distributions and spectra of emitted photons to the peak intensity of the ionizing laser pulse can be used as an additional tool for in situ atomic diagnostics of high-power laser beams.

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• Multiphoton or tunneling ionization & excitation
• Strong electromagnetic field effects
• Ultrafast phenomena