Theory of Atomic and Molecular Interactions with High Intensity Light

Our research into the theory of light-matter interactions addresses the role of relativity and the magnetic field from the light in the dynamics. Recent highlights from the group’s research are shown below.

Relativistic continuum dynamics for electrons from the ionization of atoms in an ultraintense (1017 W/cm2 to 1020 W/cm2) laser focus are analyzed using a semi-classical wavelet model. The results quantify the energy and angle resolved photoionization yields due to the developing relativistic dynamics in ultraintense fields. Using the final state momentum, the bremsstrahlung radiation yield is calculated and shows a linear relationship between the radiation cutoff and the laser intensity. At 1020 W/cm2 photons with energies out to 10 MeV should be observed. The results are quantitatively comparable to the observed angle resolved photoelectron spectra of current ultraintense laser-atom experiments. The results show the azimuthal angular distributions becoming more isotropic with increasing intensity.

Movie of the ionization wave function probability from one Ne+7 ion at the center of a f#/1.5, 2 1017 W/cm2 peak focus (LEFT).

Movie of the ionization wave function probability from one Ar+15 ion at the center of a f#/1.5, 1 1019 W/cm2 peak focus (RIGHT). The x-axes extends to the exp(-2) beam radius and the z-axes to one Raleigh length.

Limits of Strong Field Rescattering in the Relativistic Regime

Rescattering between a photoelectron and parent ion is an essential physical process supporting two decades of advances in x-ray radiation generation, multielectron dynamics, and the nascence of attosecond science. Recombination with the parent ion in higher intensity collisions requires taking into account the presence of relativistic effects. Via a strong field quantum description and Monte Carlo semiclassical approach, recollision in the relativistic regime is investigated. We find the relativistic recollision energy cutoff is independent of the ponderomotive potential Up, in contrast to the well-known 3.2Up 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.

We show the ultimate energy cutoff and highest rescattering flux is best realized by intense, short-wavelength lasers and cannot exceed a few thousand Hartree, indicating that hard x-rays via recollision-induced high-order harmonic generation can be extended to photon energies of 60 keV and setting a boundary for recollision based attosecond physics.

This material is based upon work supported by the National Science Foundation under Grants No. 1607321 and No. 1307042 and the Program of Introducing Talents of Discipline to Universities Grant No. B12024.


MeV Photoelectrons from Ultrastrong Fields

Recent photoelectron measurements from ultra-strong field interactions with atoms are shown in the above photoelectron spectrum for the ionization of argon at 1019W/cm2. The photoelectrons have the highest energy observed from the photoionization of a single atom by optical frequency photons. In any sense of the definition, this interaction is highly nonperturbative with the light-atom interaction resulting in the absorption of nearly one million photons during ionization. Comparisons with relativistic calculations show the two major features in the photoelectron spectrum come from the ionization of the inner shell electrons (dashed line calculation) peaked at 0.6 MeV and the low energy, valence electrons at less than 200 keV. The calculation including both the valence and inner shell electrons is shown as a solid line in the figure.



Lorentz Force Deflection of Strong Field Rescattering

The above figure is a snapshot of the ionizing electron from an atom. Normally, in laser fields the electron leaves the atom and ionizes along the electric field direction. As the light wave oscillates, the photoelectron may then recollides with the atom as it quivers. As the laser strength increases (b-c), the outgoing electron experiences a force not only from the electric field, but also the magnetic field of light. This results in a deflection of the electron away from the atom by several nanometers (z-direction in figure). The impact of the deflection is that both radiation and multielectron ionization from strong laser fields and rescattering may be suppressed. This can occur at intensities as low as 1014 W/cm2 for mid-IR light (2 to 5 micron wavelengths) or as high as 1018 W/cm2 for UV radiation.

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