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We investigate forward scattering of ionization from neon, argon, and xenon in ultrahigh intensities of 2 × 10(19) W/cm(2). Comparisons between the gases reveal the energy of the outgoing photoelectron determines its momentum, which can be scattered as far forward as 45° from the laser wave vector k(laser) for energies greater than 1 MeV. The shell structure in the atom manifests itself as modulations in the photoelectron yield and the width of the angular distributions. We arrive at an agreement with theory by using an independent electron model for the atom, a dipole approximation for the bound state interaction, and a relativistic, three-dimensional, classical radiation field including the laser magnetic field. The studies provide the atomic physics within plasmas, radiation, and particle acceleration in ultrastrong fields.

Recollision for a laser driven atomic system is investigated in the relativistic regime via a strong field quantum description and Monte Carlo semiclassical approach. We find the relativistic recollision energy cutoff is independent of the ponderomotive potential U_p, in contrast to the well-known 3.2U_p scaling. The relativistic recollision energy cutoff is determined by the ionization potential of the atomic system and achievable with non-negligible recollision flux before entering a “rescattering free” interaction. The ultimate energy cutoff is limited by the available intensities of short wavelength lasers and cannot exceed a few thousand Hartree, setting a boundary for recollision based attosecond physics

The strong and ultrastrong field-molecule interaction is a complex, many-body process involving multiple ionization processes. We present ion yields and molecular fragment energies for the ionization of chloromethane (CH 3 Cl) in a laser field with intensities spanning from 10 14 to 10 17 W cm −2 . As the laser intensity increases, ionization of CH 3 Cl is observed to pass from molecular tunneling, to enhanced ionization (EI), to an atomic-like response. The energy spectra of the ions show no dependence on the intensity and has its source in dissociative molecular ionization. A classical model of an aligned C-Cl ion is used to model the interaction. Following an initial molecular ionization process, our results show EI is a driving influence in the formation of low charge states until ionization become atomic-like and involves tightly bound ion states whose ionization is unaffected by nearest neighbor ions of similar ion charge.

The ionization of Ne and Kr at 400 nm has been measured from 10 13 to 10 17 W/cm 2 for charge states up to Kr 8+ . Nonsequential ionization is seen in the higher charge states Ne 3+ , Ne 4+ , Ne 5+ , Kr 5+ , Kr 6+ , Kr 7+ , and Kr 8+ when the atomic response to the increasing external field strength moves from multiphoton to tunneling. A similar development in nonsequential ionization is seen in a semiclassical model and an empirical model of rescattering. The models show that nonsequential ionization not apparent in lower charge states can become prominent for higher charge states. It is possible to generalize these results and predict optimal wavelengths for multielectron excitation via nonsequential ionization. The findings indicate that ultraviolet to vacuum ultraviolet wavelengths can give the largest recollision for higher charge states.