Theoretical Atomic Physics

Kathryn Levin

See Prof. Levin's entry under Theoretical Condensed Matter Physics.

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 Robin Santra

Robin Santra

Ph.D., University of Heidelberg, 2001.
Physicist, Chemical Science & Engineering Division, Argonne National Laboratory. Associate Professor (part time), Department of Physics.
Theoretical physics, atomic, molecular and optical physics, chemical physics.

My current research interests revolve around the following topics: ionization dynamics and inner-shell physics of atoms, molecules, and clusters; strong-field and electron-correlation effects in the EUV and x-ray regimes; applications of short-wavelength free-electron lasers; ultrafast laser-induced phenomena; electronic many-body theory; and non-Hermiticity in quantum mechanics.

Using x rays, matter may be probed in an element-specific way with high spatial resolution and penetration depth. These specific characteristics of x rays have led to, for instance, significant progress in protein structure determination. A central question I have been working on in recent years is, Can we control x-ray processes using strong laser fields? In collaboration with colleagues at Argonne, I have been investigating three different laser-intensity regimes. At intensities of the order of 1011 to 1012 W/cm2, molecules in the gas phase can be spatially aligned along the laser polarization axis. The suppression of random molecular orientations can be exploited for both x-ray absorption and x-ray scattering. This may make it possible some day to perform x-ray diffraction imaging of molecules that cannot be crystallized. If the intensity is increased by an order of magnitude, the electronic states reached via x-ray absorption are modified. For instance, it becomes possible in this way to substantially suppress x-ray absorption at an otherwise strong x-ray absorption resonance. This form of electromagnetically induced transparency for x rays may be exploited to imprint the shape of one or more laser pulses onto an x-ray pulse. Finally, at intensities of the order of 1014 to 1015 W/cm2, the optical field is strong enough to ionize even noble-gas atoms via a nonperturbative multiphoton absorption process. As an example, resonant x-ray absorption studies of laser-ionized krypton atoms made it possible for the first time to directly observe the spatial alignment of the orbital hole produced by the strong laser field. This work has interesting implications for attosecond science.

Another theoretical effort of mine focuses on scientific applications of future x-ray free-electron lasers (FELs). The first of the x-ray FELs to come online is expected to be the Linac Coherent Light Source (LCLS), which is currently under construction at the Stanford Linear Accelerator Center. The pulse peak intensity at LCLS will be about nine orders of magnitude (!) higher than state-of-the-art synchrotron radiation sources such as Argonne's Advanced Photon Source, the currently brightest source of hard x rays in the United States. Other important characteristics of LCLS are the sub-picosecond duration of its pulses and the full transverse coherence. The high-intensity aspect of x-ray FELs will allow us to extend concepts of nonlinear optics and strong-field physics to the x-ray domain. With the advent of the x-ray free-electron lasers, it will become possible for the first time to study x-ray/matter interactions beyond the one-photon absorption or scattering regime. For instance, our calculations predict that the focused beam of LCLS will be intense enough to completely ionize, in a single x-ray pulse, all 10 electrons of atomic neon. In addition to the fundamental interest in understanding x-ray/matter interaction at high intensity, this topic is critical for planned single-shot single-biomolecule experiments, which seek to overcome the current limitations of protein crystallography.

Selected Publications:

  • An x-ray probe of laser-aligned molecules, E. R. Peterson, C. Buth, D. A. Arms, R. W. Dunford, E. P. Kanter, B. Kraessig, E. C. Landahl, S. T. Pratt, R. Santra, S. H. Southworth, and L. Young, Appl. Phys. Lett. 92, 094106 (2008).
  • X-ray nonlinear optical processes using a self-amplified spontaneous emission free-electron laser, N. Rohringer and R. Santra, Phys. Rev. A 76, 033416 (2007).
  • Electromagnetically induced transparency for x rays, C. Buth, R. Santra, and L. Young, Phys. Rev. Lett. 98, 253001 (2007).
  • Quantum state-resolved probing of strong-field-ionized xenon atoms using femtosecond high-order harmonic transient absorption spectroscopy, Z.-H. Loh, M. Khalil, R. E. Correa, R. Santra, C. Buth, and S. R. Leone, Phys. Rev. Lett. 98, 143601 (2007).
  • X-ray microprobe of orbital alignment in strong-field ionized atoms, L. Young, D. A. Arms, E. M. Dufresne, R. W. Dunford, D. L. Ederer, C. Hoehr, E. P. Kanter, B. Kraessig, E. C. Landahl, E. R. Peterson, J. Rudati, R. Santra, and S. H. Southworth, Phys. Rev. Lett. 97, 083601 (2006).

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