| Abstract |
Coherent light pulses of few to hundreds of femtoseconds (fs) duration have prolifically served the
field of ultrafast phenomena. While fs pulses address mainly dynamics of nuclear motion in
molecules or lattice in the gas, liquid or condensed matter phase, the advent of attosecond (asec)
pulses has in recent years provided direct experimental access to ultrafast electron dynamics.
However, there are processes involving nuclear motion in molecules and in particular coupled
electronic and nuclear motion that occur in the few fs or even sub-fs time scale. Electronic
excitations in molecules are commonly in the VUV/XUV spectral region. Until recently most of the
XUV sources were lacking either sufficient pulse energy (HOHG sources) or ultrashort pulse
duration (free electron lasers (FEL), thus preventing access to XUV-pump-XUV-probe
measurements in the 1fs or asec temporal scale.
In this work, by loose focusing a multi-cycle, high power, fs IR laser (35fs,<160mJ/pulse, 805nm)
into a Xenon gas jet, the non-linear interaction results in frequency up-conversion and, under proper
experimental conditions for Phase Matching, in the formation of attosecond pulse trains. The
application of IPG to this scheme allows the generation of energetic, broadband, coherent extremeultraviolet
(XUV) continuum radiation. Although the temporal properties of the XUV pulse are
affected by the lack of CEP stabilization, the XUV intensity is high enough to induce XUV multiphoton
absorption in atomic or molecular system. So far, experimental efforts on this time scale
have been restricted to XUV-IR pump-probe schemes, or in-situ electron-ion collision methods. As
the IR field induced potential may distort the molecular potential, the use of IR-free technique to
investigate the intrinsic molecular dynamics is of central importance.
The present work focuses on the development of techniques for the investigation of such dynamics
utilizing solely XUV radiation and demonstrates proof of principle time resolved and XUV pump -
XUV probe experiments, tracking ultrafast molecular dynamics. Exploiting the short duration of the
above mentioned high energy pulses, all the optically allowed excited states of H2 are coherently
populated. Monitoring the pump-probe delay dependent yield of protons, nuclear and electronic 1fs
scale dynamics are subsequently investigated and compared to the results of ab initio calculations.
The revealed dynamics reflects the intrinsic molecular behavior as the XUV probe pulse, despite its
still high intensity, hardly distorts the molecular potential.
It was further feasible to follow the opening of the dissociative ionization channel through the
2Σ+g(2pσu) state, due to the stretching of the molecule. This is visible as a build-up of non-zero
kinetic energies proton signal during the first fs of the delay time.
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Studies of 1fs scale dynamics in atoms and molecules
