Attosecond Control and Measurement: Lightwave Electronics

data stays consistently below 0.1 rad and thus shows
excellent agreement all the way down to ca. 2 U p (below
this value, noise is dominating the curves due to the low
amplitude of the asymmetry). We have demonstrated
that velocity-map imaging is not only a powerful tool
for measuring the full momentum distributions of ATI
in rare gases but also provides a new way to determine
the CEP of few-cycle laser pulses from the angular
distribution of the emitted electrons.
WAVEFORM CONTROl OF ElECTRON lOCAlIZATION
IN MOlECUlES
A previously explored single-color control scheme for
attosecond electron localization in molecules is difficult
to extend to systems with dissociation times much longer
than the duration of the waveform-controlled fewcycle
laser field (in previous experiments, 5 fs pulses at
760 nm were used) as the field strength of the laser will
be to low to coherently couple two electronic states
close to the break-up of the molecule.
As a demonstration for a suitable way to extend the
control scheme, in collaboration with the groups of Anne
L’Huillier (Lund), Marc Vrakking (Amolf), Franck Lepine
(Lyon) and Mauro Nisoli (Milan), we have performed
a two-color experiment, where an isolated attosecond
laser pulse ionizes D and part of the ionized molecules
2
dissociate. The single attosecond pulse is generated by
means of high-harmonic generation in Krypton using the
polarization gating technique. In the photo-ionization
+ by the attosecond pulse the repulsive 2pσ state is
u
populated when the photon-energy of the pulse is high
enough (above ca. 29 eV). Interaction of this dissociating
wave packet with a time delayed moderately strong IR
field localizes the electron on the upper or lower D + ion.
By varying the delay between the XUV pulse and the
few-cycle IR pulse the asymmetry in the ejection of D +
ions can be controlled with attosecond time resolution.
This experiment may be viewed as a first example of
the observation of attosecond time-resolved electron
dynamics in molecular physics. As seen in Figure 2 the
asymmetry shows a strong dependence on the kinetic
energy of the D + fragment.
A model solving the 1D time-dependent Schrödinger
equation (TDSE) was used to confirm the interpretation
and to give insight into the origin of the observed
energy dependence. In the model a nuclear wave
+ packet is projected on the repulsive 2pσ state and
u
+ propagated on the bound 1sσ and the repulsive
g
+ 2pσ state of the molecular ion in the presence of a
u
few-cycle IR field. The IR field couples the two states
which generates a coherent superposition where the
electron can be localized on one or the other ion. Just as
in the experiment the timing of the IR field determines
the electron localization. The kinetic energy dependence
1 . 3 AT T O S E C O N D A N D H I G H - F I E L D D I V I S I O N
Figure 2: A) D + ion kinetic energy spectra from the
dissociative ionization of D 2 versus the delay between
the 300 as XUV pulse at 30 eV and a phase-stable
6-fs IR pulse; B) Asymmetry of the D + ion emission
along the laser polarization as a function of the D +
kinetic energy and the delay between the XUV and
IR pulses.
of the electron localization, which is observed in the
experiment and the calculations, likely, originates from
the initial spread of the wave packet.
Electron transfer processes are ubiquitous in chemistry.
The present combination of one- and two-color
experiments, where electron localization is first
controlled on attosecond timescales using the controlled
waveform of a few-cycle IR laser pulse and then using
the controlled delay between an isolated attosecond
pulse and a few-cycle IR laser pulse, are first examples of
strong-field control of chemical processes on attosecond
timescales and of the direct observation of attosecond
electron dynamics in molecules, paving the way towards
attempts to observe and control electron dynamics in
more complicated molecules and nanostructures. The
results of these studies are currently written up.
ATTOSECOND NANOPlASMONIC MICROSCOPY
Nanoplasmonics deals with collective electronic
dynamics on the surface of metal nanostructures, which
arises as a result of excitations called surface plasmons.
150 Max-Planck-Institut für Quantenoptik • Progress Report 2007/2008