We showed that high harmonic generation cannot only be used to image microscopic distances,
but it is also capable to retrieve time-resolved information about a
complex system such as an expanding cluster. The proposed method is based on
a pump-probe laser scheme
in which the first pulse induces the cluster expansion and the time-delayed second pulse
is used to generate higher-oder harmonics in the exploding cluster. The results of nur
numerical simulations show that the dependence of the harmonic signal on the pump-probe delay
provides information about the particle density and the average static electric field in the
cluster.
We studied high-harmonic generation as a tool to image molecular
structure on an ultrafast time scale. Theoretical predictions for harmonic signals, generated in
highly symmetric fullerenes (C20 to C180),
based on an extension of the so-called Lewenstein
model exhibit modulations in the plateaus of the spectra at
mid-infrared wavelengths (upper panel).
Analysis of the results showed that the minima are due to
multislit interference effects arising from the contributions of the
different atomic centers to the dipole moment. The modulations
disappear, if the atomic contributions are added incoherently (middle panel).
The position of the minima are found to be in good agreement with
those in the recombination matrix element (lower panel),
which can be used to retrieve the
radius of the fullerenes from the spectra.