Research
Overview
Orbital alignment in
strong-field ionized atoms
We have observed that optical field ionization of krypton
produces a hole orbital that is aligned with the polarization
axis of the ionizing laser. The degree of orbital alignment
is suppressed by internal atomic interactions; i.e., the
recoupling of the orbit with the spin occurs on a timescale
of 6.2 fs [1,2]. We further find in this gas phase system, with
ion/electron density ~1015/cm3, that memory of the
orbital
alignment persists indefinitely until an electron-ion
dealignment collision [3]. The collection of aligned ions, randomly distributed
in space, but all pointing in the same direction can be induced to precess
about an externally applied magnetic field, giving an in situ measure
of magnetic field in a laser-produced plasma [3]. At present, we are investigating
the surprising suppression of dealignment by application of modest external
magnetic fields.
1L.
Young et al., Phys. Rev. Lett. 97, 083601 (2006). X-ray microprobe of
orbital alignment in strong-field ionized atoms.
2R. Santra et al., Phys. Rev. A. 74, 043403 (2006). Spin-orbit
efffect on strong-field ionization of krypton.
3C. Höhr et al., Phys. Rev. A 75, 011403A (2007). Alignment
dynamics in a laser produced plasma.
Electromagnetically induced transparency
for x-rays
Electromagnetically induced transparency has been an topic of much interest
in the visible region of the spectrum. Research has focused primarily
on three-level lambda systems, where the upper level decays radiatively
while the two lower levels are stable. A coupling laser is used to modify
the absorptive and dispersive properties of the system to generate transparency
and slow light, respectively.
In the x-ray regime, the situation is considerably more complex, as shown
in the diagram. Theory demonstrates that the x-ray absorption spectrum
of a neon atom can be rendered transparent at a selected wavelength by
application of a strong optical field (1013 W/cm2
at 800 nm) [1,2]. The actual absorption spectrum is considerably more
complex than the simple three-level model, shown in red, would predict.
This work points the way toward producing ultrafast, shaped x-ray pulses
by laser irradiation of a simple gaseous target. An experimental demonstration
is planned at Berkeley’s Advanced Light Source at the femtosecond
slicing beamline, 6.0.1.
1C. Buth and
R. Santra, Phys. Rev. A 75 , 033412 (2007). Theory of x-ray absorption
by laser-dressed atoms.
2C. Buth, R. Santra, L. Young, Phys. Rev. Lett. 98, 253001
(2007). Electromagnetically induced transparency for x-rays.
X-ray structure of laser-aligned molecules
At intensities of ~1012 W/cm2 molecules align due
to the interaction between the polarized laser field and the anisotropic
polarizability of the molecule. Even three-dimensional alignment can be
achieved for molecules with three distinct moments of inertia using elliptically
polarized light. We aim to align small molecules with laser fields and
determine their structure in the presence of the fields using x-ray methods.
The molecular framework can be distorted from the equilibrium structure
by multiphoton mixing between ground and excited states and a major goal
is to develop methods to predict such structural distortions. In the larger
picture, laser-alignment of single biomolecules is proposed as a method
to simplify structure determination of non-crystalline samples at x-ray
free electron lasers - thus development and characterization of laser-alignment
methods is useful.
We use the x-ray microprobe to measure the structure of an ensemble
of laser-aligned molecules. Due to the high repetition rate (6 MHz) and
relatively long duration (100 ps) of the x-ray pulses at the Advanced
Photon Source it is inconvenient to use either standard adiabatic (Trot<<Tlaser)
or impulsive (Trot>>Tlaser) alignment methods.
Instead we employ laser pulses in the intermediate regime, where strong
alignment, similar to the adiabatic case is predicted (see below). An
initial experiment using x-ray detection demonstrates alignment consistent
with a rotational temperature of 15 K.
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