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Abstract As atomic matter interacts with ultrastrong fields, the bound electrons are polarized and have ionization energies changed by Stark-shifting. The unprecedented range of laser intensities from 10¹⁵W cm⁻²to 10²⁴W cm⁻²can take the interaction from the neutral atom to a bare nucleus. We have used an outer, single active electron approximation to calculate the polarization and Stark-shifted binding energy for ultraintense lasers interacting with highly charged ions at intensities from 10¹⁴W cm⁻²to 10²²W cm⁻². The polarization of the bound state can result in a dipole moment and Stark shift that may be 0.1 e a₀and 50 E_h, respectively. At these high intensities, relativistic effects must also be considered. Across the intensity range of these studies, the magnetic field of the laser does not comparably affect the bound state of the atom; the impact of polarization and Stark shift exceed changes to the bound state wave function and binding energy from including relativity. 

The ionization of CO, CO+, and CO2+ are quantified in an ultrafast, strong laser field. Measurements were performed over the intensity range from 1014 to 1017 W cm−2. Across this span, the intensity-dependent ionization yields were quantified over eight orders of magnitude in the dynamic range. Both sequential and (e, 2e) nonsequential ionization processes were observed. We calculate the electron states for CO and its molecular ions interacting with a strong laser field using a traditional field-free approach, the single active electron approximation, and present the results for the all-electron interaction of CO with the laser field. By comparing the calculated ionization with the experimental yields, we determined that the electron wave function polarization and Stark shifts were accurately treated with an all-electron Hartree–Fock calculation. The calculated field–molecule interaction included the core electron polarizability, which is not captured using field-free or frozen-core single-electron approximation. 

Abstract The discovery of laser-driven rescattering and high harmonic radiation out to a maximum photon energy of 3.17 times the ponderomotive energy ( U p ) laid the groundwork for attosecond pulse generation and coherent X-rays. As the laser field drives the interaction to higher energies, relativity and the Lorentz force from the laser magnetic field enter into the dynamics. We present the results of recent studies of laser rescattering, including these effects, to give a quantitative description of rescattering dynamics in the high-energy limit, ie, recollision energies of order 1,000 hartree (27 keV). The processes investigated include inner K- and L-shell excitation and the ultimate limit of high harmonic generation via rescattering bremsstrahlung. The results indicate the path to the frontier area of x-ray strong field processes. 

We address the challenge of finding the optimal laser intensity and wavelength to drive high-energy, strong field rescattering and report the maximum yields of K-shell and ${\mathrm{L}}_{\mathrm{I}}$-shell hole creation. Surprisingly, our results show laser-driven rescattering is able to create inner shell holes in all atoms from lithium to uranium with the interaction spanning from the deep IR to x-ray free electron laser sources. The calculated peak rescattering follows a simple scaling with the atomic number and laser wavelength. The results show it is possible to describe the ideal laser intensity and wavelength for general high-energy laser rescattering processes.