About rotating and aligning molecules inside superfluid helium using femtosecond lasers.
This thesis is about laser-induced alignment and rotation of molecules
solvated inside cold (0.37 K) nanodroplets of superfluid helium. A
laser pulse with a duration of about a picosecond or less induces
coherent rotation of the molecules. After a short delay, the rotation
causes the molecules to align along the laser polarization roughly at
the same time. In this way, it is e.g. possible to conduct experiments
in the molecular frame of reference.
For a long time, overwhelming spectroscopic evidence showed that
molecules rotate freely inside superfluid helium. This is in fact a
defining property of superfluids, since in a normal fluid like water, the
rotation would break down due to collisions with the solvent. Over the
past 5 years, however, alignment experiments have shown radically
different and contradictory results, namely that the free rotation in
helium breaks down immediately.
The work presented in this thesis represents significant progress
towards reconciling molecular alignment with spectroscopy. New
theoretical and experimental findings improve the understanding of
how rotating molecules interact with superfluid helium. Among other
things, the results open new directions for studies of solute-solvent
dynamics.