Despite the complex dynamics of few-body systems interacting with intense laser fields,
the final state energy and momentum distributions of the particles often show some general
features due to Coulomb repulsion or symmetries of accessible final states. We have analyzed so-called selection rules
for the emission of two electrons from helium following the absorption of a few photons
in a laser field. The increase of the number of absorbed photons leads to alternating
suppression and non-suppression of the back-to-back emission of the two electrons.
The Figure shows a snapshot of the two-electron dynamics following single-photon (inner part of distribution)
and two-photon double ionization (outer part).
The cover page of Journal of Physics B featured the
results for the three- and four-photon processes.
The phenomenon of energy shifts of atomic levels in a static electric field is well-known and referred to as
Stark shift. We have shown that attosecond pulse technology should enable us to observe these shifts
instantaneously in an oscillating intense laser field. In our proposed technique the energy gaps between
ground and (Stark shifted) excited states of an atom
in an infrared field are probed with an exciting single attosecond pulse. The population
in the excited states, generated by the attosecond pulse, is then ionized by the remaining infrared pulse.
Hence, as a function of time delay between the pulses the ionization probability reflects the instantaneous energy gaps
(see Figure). Detailed knowledge of the energy states in the field may have impact on controlling more
complex processes, such as correlated electron emission from the atom.
The population and decay of excited states during laser-driven double ionization is an example for
ultrafast correlated electron dynamics in an atom or molecule. In this so-called RESI process
an initially field ionized electron is driven back to the parent ion by the intense field
and shares its energy with a second electron to excite the ion.
The time span to observe the excitation is less than a femtosecond, since the weakly
bound electron in the excited state is likely to be ionized by the field. Attosecond laser pulse technology should provide
the appropriate tool to observe the instantaneous state of the ion.
Our numerical results show that one can indeed probe the electron in
the excited states via ionization by an attosecond VUV pulse. However,
the attosecond pulse does also initiate additional electron dynamics in the atom, which
makes the trace of the RESI process difficult to separate from other signals.
The Figure shows a snapshot of the complex dynamics.
Computation of the quantum two-electron problem in an intense laser pulse
constitutes a major challenge, since it involves six dimensions in space
and one in time. Model systems in reduced dimensions are therefore often used.
A simple principle, based on Newton's law that any directionality
in the external force will be transfered to the center-of-mass motion of
a many-electron system, helps to reduce the computational costs of the
laser driven two-electron problem (He and H2). The cover page of PRL
featured a time slice of our numerical simulations for the spatial probability for
two helium electrons in a laser pulse.
Points on the "X" lead to single ionization, while those off the "X" result
in double ionization. The model has been used to reveal new pathways
to laser-driven double ionization and their relation to field-free correlated
electron dynamics.
References:
C. Ruiz et al. Phys. Rev. Lett. 96, 053001 (2006)A. Staudte et al., Phys. Rev. Lett. 99, 263002 (2007)
S. Baier et al., Phys. Rev. A 78, 013409 (2008)
Collaborations:
L. Plaja and L. Roso (Universidad Salamanca, Spain)P. Corkum (NRC Ottawa, Canada)
R. Dörner (Universität Frankfurt, Germany)