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Ne to Ne 8 ionization yields in 10 14 W=cm 2 to 10 18 W=cm 2 laser fields are reported over a 10 9 dynamic range. A 3D relativistic rescattering model incorporating e; 2e and e; 3e electron impact ionization, single-and double-excitation is compared to the data. For double ionization the agreement is excellent; however, for higher charge states the model accounts for only 15% of multielectron nonsequential ionization. Rescattering is not affected by the laser magnetic field until 10 17 W=cm 2 .Atomic ionization and radiation processes in strong fields have remained at the forefront of time resolved dynamics and laser science for the past 15 years. Recent interest has focused on quantum [1] and classical mechanisms [2] behind multielectron, multiphoton ionization and the new area of attosecond science [3,4]. These studies, which often address multielectron nonsequential ionization (NSI) [5] and high harmonic generation (HHG) [6], involve a field driven rescattering mechanism. As the field increases, higher charge states, relativistic effects, and the laser magnetic field (B laser ) will affect NSI, HHG, and rescattering physics. This Letter begins to address these topics with a precision, ultrahigh field ionization experiment and relativistic, semiclassical ionization and rescattering model. Rescattering [7] occurs when a photoelectron, which is oscillating in the continuum with the laser electric field (E laser ), is driven back toward the parent ion. For laser intensities from 10 13 W=cm 2 to 10 15 W=cm 2 , inelastic rescattering between the photoelectron and the parent ion may result in collisional ionization (NSI) or radiation (HHG). Two-electron NSI has been observed [8] to exceed by 10 5 the expected doubly ionized species from a sequential ionization (SI) mechanism, in which the laser field ionizes the atom one electron at a time. Recent theoretical [9-12] and experimental efforts have made significant progress towards understanding two-electron NSI mechanisms including resonantly enhanced multiphoton ionization [13,14] and rescattering impact excitation and ionization [15].Beyond two-electron NSI of the neutral atom, multielectron NSI has been reported [16] but is not well understood at this time. Fully differential rates for the multiphoton ionization of ground state Ne [17], correlated electron emission measurements for multiphoton double ionization [18], and ion momentum measurements [19] indicate rescattering is likely to become a dominant NSI mechanism in ultrahigh fields with high charge states. Chowdhury [20] and Dammasch [21] measured multielectron ionization from 10 15 W=cm 2 to >10 17 W=cm 2 and found NSI decreased at higher intensities. Diminished NSI was at first believed to result from a reduction in rescattering due to the Lorentz force on the photoelectron. In the ''Lorentz force paradigm,'' B laser and the significant photoelectron velocity force the rescattering into the laser propagation direction (k v B) and away from the parent ion. As we will show, B laser does not play as large of a ...

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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.

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