Multielectron ultrastrong laser field ionization of Ar^{n +}, Kr^m+ and Xe^l+ (n ≤ 9, m ≤ 9, l ≤ 12) at intensities from 10¹⁵ W cm^-2 to 10¹⁸ W cm^-2

Ionization yields are reported for Ar, Kr and Xe in ultrastrong fields from 10¹⁵ W cm^-2 to 10¹⁸ W cm^-2. Non-sequential ionization (NSI) is shown to be a robust and general feature in ultrahigh field ionization. NSI yields measured are consistent with the trends predicted by a rescattering model, but as one proceeds to higher Z atoms more NSI is observed than predicted theoretically. Additional recollision mechanisms that may need to be considered in future theories of ultrastrong field-atom interactions include 'chain' NSI, NSI from excited states of the atom (e.g. Rydberg states or inner-shell holes) and the possibility of ultrastrong field enhanced recollision/impact processes.