Dynamics in Ultrahigh Fields
The Collective Molecular to Atomic Transistion in Ultrastrong Fields:
- 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:
- 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.
HIGHLY-CHARGED MULTIELECTRON Ionization in Strong Fields:
- 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.
I. Ghebregziabiher, S. Palaniyappan, J. MacDonald, and B. C. Walker, Impact of the laser magnetic field on recombination and bremsstrahlung radiation from atomic ionization rescattering in ultraintense fields, Phys. Rev. A 73, 033419 (2006). [url]
S. Palaniyappan, I. Ghebregziabher, A. DiChiara, J. MacDonald, B. C. Walker, Emergence from nonrelativistic strong-field rescattering to ultrastrong-field laser-atom physics: A semiclassical analysis, Phys. Rev. A 74, 033403 (2006). [url]
S. Palaniyappan, A. DiChiara, I. Ghebregziabher, E. L. Huskins, A. Falkowski, D. Pajerowski, B. C. Walker, Multielectron ultrastrong laser field ionization of Arn+, Krm+ and Xel+ (n <= 9, m <= 9, l <= 12) at intensities from 1015 W/cm2 to 1018 W/cm2, J. Phys. B: At. Mol. Opt. Phys. 39, S357 (2006). [url]