ULTRA-Strong Light Fields

In ultrastrong light fields, our common understanding of light - matter interactions begins to fail. The speed of the photoelectron becomes relativistic and the magnetic field of light affects the way light interacts with matter. Our recent research results have characterized this progression from the strong- to ultrastrong-field and shown this can have a significant effect on strong field "rescattering physics" that is responsible for high harmonic generation and multielectron ionization for atoms in strong laser fields.

Instrumentation for the Ultra-Strong Field, 2008

Ultrahigh intensities mean relativistic interactions. Where traditional laser-matter interactions result in photons and ionization products with eV to 1 keV of energy, the energy scale of ultrastrong fields is an MeV. Such energetic particles require new detection methods closer to nuclear instrumentation. High energy particles transmit right through normal micro-channel plates and time of flight is very limited since all velocities are near the speed of light. The figure above (a) represents the type of chamber currently used to detect photoelectrons for ultrahigh field ionization with high contrast, 40fs pulses (b,c). Fast scintillation materials and photomultiplier tubes are used as detectors and magnetic field deflection and 150 picosecond timing electronics are used to help analyze the electron energies.

MeV Photoelectrons from Ultrastrong Fields, 2008

Results from recent photoelectron measurements from ultrastong field interactions with atoms are shown in the above photoelectron spectrum for the ionization of xenon at 10¹⁹W/cm². The photoelectrons have the highest energy observed from the photoionization of a single atom by optical frequency photons. The angular distibutions of the photoionization (shown just above the spectrum as inserts) have also been measured and are currently being analyzed to provide insight into the fundamentals of the ionization and propagation of the photoelectrons. In any sense of the definition this interaction is highly nonperturbatuve with the light-atom interaction resulting in the absorption of nearly one-million photons during ionization.

The Collective Molecular to Atomic Transistion in Ultrastrong Fields, 2008

When molecules or clusters are exposed to strong fields, the atoms and molecules begin to field ionize and then undergo complex enhanced ionization and resonant excitation in the field that further excites and ionizes the system. We have begun to address questions on how ionization proceeds for molecules and clusters in ultra-strong, relativistic fields.

Whether Coulomb and collective molecular mechanisms play a dominant role as the molecule ionizes to higher charge states; or if molecular characteristics are eventually lost with the atom-field interaction becoming dominant. Our experiments (results shown below) show C⁺² and C⁺³ ions from methane are produced through a molecular response, however, as one proceeds to C⁺⁴ ions and removes the last valence electron, the ionization mechanism reflects both molecular and atomic character.

Finally, the ionization of the inner shell is relativistic, the C⁺⁵ ions from methane are produced entirely from an atomic-like response in an ultra intense field, including cross-shell rescattering ionization and a photoelectron spectrum in excellent agreement with an atomic model of the ionization.

Lorentz Force Deflection of Strong Field Rescattering, 2007

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.

• "Impact of the laser magnetic field on recombination and bremsstrahlung radiation from atomic ionization rescattering in ultraintense fields," Ghebregziabiher I, Palaniyappan S, MacDonald J, Walker BC, PHYSICAL REVIEW A 73, 033419 (2006).
• "Emergence from nonrelativistic strong-field rescattering to ultrastrong-field laser-atom physics: A semiclassical analysis," Palaniyappan S, Ghebregziabher I, DiChiara A, MacDonald J, Walker BC, PHYSICAL REVIEW A 74, 033403 (2006).

HIGHLY-CHARGED MULTIELECTRON Ionization in Strong Fields, 2007

In ultrastrong light fields the light interacts with tightly bound electrons in atoms. Normally, lasers do not remove more than one or two electrons from atoms, however in ultrastrong laser fields the laser may remove all of the outer electrons from an atom and begin removing electrons from the inner electron shells.

The amount of multielectron ionization is a current question for the fundamental atom-laser interaction as it may be affected by the laser magnetic field (see above) or effected by the fact that inner shell electrons are more tightly bound than the outermost valence electrons for an atom.

These measurements are some of the first to characterize this multielectron ionization and revealed a surprisingly large amount of ionization - orders of magnitude more than expected. The measurements show that in addition to one electron "knocking-off" another electron via rescattering it is more likely to remove two or even three more electrons, that is field assisted (e,4e) processes. These effects act to counter the reduction of multielectron ionization by the light magnetic field deflection of the photoelectron as its speed becomes relativistic. Furthermore, the measurements show that an electron ionized from the outer shell of an atom in a laser field, is correlated with the ionization of the inner shell electrons, i.e. the ionization of outer electrons may remove not only another nearby electron but also ionizes several more tightly bound electrons nearer to the atomic nucleus.

• "Multielectron ultrastrong laser field ionization of Arⁿ⁺, Kr^m+ and Xe^l+ (n <= 9, m <= 9, l <= 12) at intensities from 10¹⁵ W/cm² to 10¹⁸ W/cm²," Palaniyappan S, DiChiara A, Ghebregziabher I, Huskins EL, Falkowski A, Pajerowski D, Walker BC, JOURNAL OF PHYSICS B-ATOMIC MOLECULAR AND OPTICAL PHYSICS 39, S357-S369 (2006).