Modelling of ultra-cold helium nanodroplet dynamics
Résumé
For the past 30 years, helium nano-droplets have been of particular interest to experimentalists.
Indeed, this is a cold dense finite liquid environment which is an excellent matrix for
spectroscopy applications, in atomic and molecular collision physics as well as in ultra cold
chemistry with ability of isolating interesting reactions intermediates like HOCO in OH + CO
→ CO2 + H or to synthesize new materials as metallic nano-wires. These phenomena are
consequences of the quantum collective behaviour of helium atoms at temperature T < 1 K
which classify helium nano-droplets as quantum liquids. In this thesis work we propose to
model photo-dissociation [1] of immersed diatomic as molecular iodine, doping and rotational
excitation of these droplets by using an home-made molecular dynamics developed by Pr.
Marius Lewerenz, the CLUSTER code. To take the quantum nature of helium atoms into
account the methodology exploits the quantum effective potential technique [2] at experimental
work temperature 0.37 K.
With the photo-dissociation, an excitation above a molecular fragmentation limit leads to two
possible scenarios depending one the amortised energy by the helium droplets. The first one is
the fast recombination so-called ’cage effect’ [3] and the second is where photo-fragments
reside in the helium bath. But with nano-meter scale of droplets a third scenario is possible with
fragments leaving the cluster surrounded by a helium solvating shell. Results are shown for
helium nano-droplets containing thousands atoms (2500 to 9000 He) and statistics on size and
velocity of I@Hen photo-fragments have been computed and are similar to experimental
outcomes for analogous system CH3I [4].
According experiments using 1 fs X-ray flashes [5], the helium nano-droplets are geoids which
is a clue indicating a rotation on itself as the Earth. Our code can set precise angular momentum
in order to study the pattern of distortion from fission to multiple splitting. To evaluate the
numbers of sub-cluster a Complete Clustering algorithm based on graph theory has been
programmed exhibiting a linear complexity.
To conclude a study of xenon doping is presented where the recombination dynamics unveils a
metastable state called Atkins’ snowball [6] and different patterns of aggregation as nano-
crystals and nano-wires well known by experimentalists