Many physico-chemical processes involve the movement of atoms within a molecule or molecular complex. To understand these processes, we need to identify the forces behind atomic motion. Unfortunately, a force is not an observable in the quantum mechanical sense, which complicates its determination. An elegant way of doing this is to isolate molecules (or complexes) in the gas phase, then observe their deformations using a variety of techniques : laser spectroscopy, for example, complemented by theoretical chemistry approaches to specify the potential energy surfaces over which the system evolves, and then, access the force field at play.
It is in this context that I became interested in the mechanisms behind the large amplitude deformations of floppy molecular complexes. These complexes are fragile and owe their existence to non-covalent bonds, known as weak bonds, of the van der Waals or hydrogen type. One of the original features of this work lies in the use of very cold (≃ 0.4 K) and superfluid helium clusters to create custom-made complexes with controlled stoichiometry. Infrared spectroscopy and quantum chemical calculations were used to obtain structural information about these complexes, and go back up to their high-amplitude movements. These delicate studies required the design and construction of a specific experimental set-up.
Reaction dynamics were also addressed through the study of a chemiluminescent exothermic reaction blocked by a barrier preventing access to the transition state. Once the barrier has been overcome, the spectrum obtained provides information on certain reaction pathways. The reaction medium is a helium cluster, which does not interfere with the dynamics, and enables control of the stoichiometry of the reactants. Information on the reaction mechanism of the (Cax, (N2O)y) complex was thus obtained.
Taking advantage of a scientific reorganization at LIDyl, where the above work was carried out, the project reported in this manuscript reflects a reorientation towards nanotechnologies. It concerns the development of spectroscopic tools for in situ, real-time elemental analysis of nanoparticles.
Keywords: Gas-phase physical-chemistry, floppy complexes (van der Waals, hydrogen bonding), reaction dynamics, helium cluster, nanoparticules, optical characterisation of nanoparticules