We have investigated atomic ionization dynamics in Kr in the transition regime from nonrelativistic to relativistic laser intensities (10(16) to 10(18) W/cm2) by measuring yields of highly charged ions stemming from an inner shell. Interpretation of the data is focused on the applicability of the single active electron description, which requires fully relaxed core states between successive ionization steps. In particular, we are concerned with transient core polarization or alignment effects originating from the strong dependence of the ionization rates on the magnetic quantum number. We found that for intense laser pulses with 40 fs pulse width internal m-mixing processes appear to be sufficiently fast to erase any transient core polarization.
We have studied the ionization process of Kr atoms in strong laser fields by measuring the laser intensity dependence of highly charged ion yields. The measurements have been performed in an intensity regime between 1016 W cm−2 and 1018 W cm−2, which covers the transition from a nonrelativistic to a relativistic regime. While it is reasonable to assume that the initial tunnelling ionization process is well described nonrelativistically, the continuum dynamics of the photoelectrons change significantly. In particular, it has been found that the rescattering mechanism, which is dominant for lower charge states at low intensity, is largely diminished. We discuss three possible reasons: a decreasing cross section for electron impact ionization for higher charge states, an increasing spatial spread of the returning electronic wavepacket and the growing importance of the magnetic field component, which pushes the returning electron away from the core. Including these effects in the ionization rates based on the Ammosov, Delone and Krainov (ADK) tunnelling theory, we solve rate equations to predict ion yields, which agree with our experimental data.
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