Core Relaxation in Atomic Ultrastrong Laser Field Ionization

We have investigated atomic ionization dynamics in Kr in the transition regime from nonrelativistic to relativistic laser intensities (10¹⁶ to 10¹⁸   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.