Université Paul Sabatier - Bat. 3R1b4 - 118 route de Narbonne 31062 Toulouse Cedex 09, France

Accueil > Recherche > Équipe Agrégats

Introduction to the cluster team

The aim of our group is to understand the nucleation phenomenon at a microscopic level.

1. How does a cluster grow ?

2. To what extend do parameters such as temperature and thermodynamic phase modify the nucleation cross section ?

3. How does evolve this cross section as a function of the size of the cluster ?...

In order to answer these questions, an original experiment has been built in which we will measure the unimolecular sticking cross section, i.e. the probability for a cluster of size n to grow by one unit through the sticking of one particle. The experiment will first be applied to the nucleation of sodium clusters. In concrete terms, we will measure the probability of sticking of one sodium atom onto one size selected sodium cluster as a function of its size, its temperature and the collision energy. All these parameters are well controlled.

These experiments are supported by molecular dynamics calculations and Monte Carlo modelling. They are expected to contribute to a better understanding of the nucleation of nanoparticles in the first stage, when the number of particles of the cluster (typically a hundred) cannot be described correctly in the frame of the classical nucleation theories which are based on classical thermodynamics. The principle of the experiment is simple : the clusters, produced in a molecular beam, are first thermalized, mass selected, and slowed down to a very low kinetic energy (a few eV). Clusters of the chosen mass n are next propagated through a cell containing a sodium vapor. Finally, the products are discriminated in a mass spectrometer. We can in this way measure the number of clusters of mass n+1 produced by sticking in the cell. The main difficulty is to produce mass selected clusters with a very low kinetic energy (in the laboratory frame) of the order of 1 eV. This condition is necessary in order to avoid instantaneous collision induced evaporation, which is the exact opposite of what we want to observe ! An original mass spectrometry device has been developed for this purpose. In addition to the sticking cross sections, we will measure laser induced photodissociation rates. Applied to clusters obtained by sticking, these measurements can characterize their thermodynamic phase. The measurement of both nucleation cross section and evaporation rate will allow us to determine an important parameter of all nucleation theories : the critical size, for which the nucleation rate is equal to the evaporation rate.

This work will be extended in the future to systems of atmospheric interest, especially water clusters. Many unknown features remain concerning the first stage of water nucleation in the clouds. Our approach is expected to bring information about important questions : under known temperature and pressure conditions, what is the rate of formation of water droplets ? Will theses droplets be liquid or solid ? There is a considerable interest nowadays concerning the formation of Stratospheric Polar Clouds (PSC), due to their implication in the global climate and the ozone depletion. Our experiments will be applied to pure water clusters but also water clusters containing sulphuric and nitric acid, which are suppose to enhance the nucleation in PSC’s.