Atomic, Molecular and Optical
Physics Group

Research Themes

Intrinsic x-ray processes can be altered and controlled by judicious application of intense optical fields in the range of 10¹⁵ W/cm² to 10¹² W/cm². Optical control of x-ray processes are demonstrated in simple atoms and molecules to develop understanding from first principles. Ultrashort x-ray pulses from a variety of sources with unique properties (Argonne's Advanced Photon Source, Berkeley's femtosecond sliced soft x-ray beamline, Ohio State University's attosecond/femtosecond EUV source, eventually Stanford's Linac Coherent Light Source, world's first x-ray free electron laser) are used to probe electron and molecular dynamics with atomic-scale temporal, spatial, and spectral resolution.

X-ray dichroism induced by optical field ionization
Modern ultrafast optical lasers, when focused to intensities of 10¹⁵W/cm², provide field strengths (10 volts/Angstrom) comparable to the binding energies of electrons in outer orbits of atoms. When an atom is subjected to these field strengths, the Coulomb barrier is suppressed and an outer electron tunnels into the continuum – to create a hole orbital aligned with the field polarization. The polarization dependence of the hole orbital can be used to control x-ray absorption (as shown to the right).

Electromagnetically induced transparency (EIT) for x-rays
Subjecting an inert gas atom to an optical intensity of 10¹³ W/cm² is insufficient for ionization. However, this intensity is sufficient to cause Rabi flopping rates in resonant dipole transtions to be comparable to inner shell decay rates. This effect can be used to induce transparency on an isolated inner shell resonance by creating a coupled three-level lambda system. Because the laser-induced effect is reversible, EIT can shape x-rays pulses using ultrafast optical pulses. Shown is the creation of two short x-ray pulses from a single long pulse using two short laser control pulses.

Laser-controlled molecular alignment

   ^e_laser

Subjecting a molecule to a non-resonant, linearly-polarized laser field of intensity of 10¹² W/cm² causes the molecule to align along its unique axis. Such methods are expected to be useful for studying b iomolecule structure with intense x-ray free-electron lasers. At Argonne, we are developing x-ray methods to understand quantitatively the structure of molecules aligned by non-resonant laser fields.