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Chemistry at extremely low temperatures

Final Activity Report Summary - COOL CHEMISTRY (Chemistry at extremely low temperatures)

The activity of the chair holder was divided between research activities and teaching and training.

The main aim of his research programme was to advance fundamental knowledge related to molecular collisions at low and ultralow temperatures. The research objectives of the chair period could be summarised as the investigation of:
1. collisional processes at very low temperatures for astrochemistry, referring to a temperature greater than 10 K;
2. collisional processes at extremely low temperatures for comparison with theory, implying a temperature between 1 and 10 K;
3. the growth of clusters or nanoparticles at a molecular level.

Regarding the collisional processes at very low temperatures for astrochemistry, chemical reactions between uncharged species at temperatures down to 10 to 30 K were shown by the chair holder to play an important role in the synthesis of interstellar molecules in situ. Moreover, they were of importance in other low temperature environments, such as extraterrestrial atmospheres like the one of Titan. The research that was previously undertaken by the chair holder in this area focussed on the measurement of the rate constants, which provided an efficiency measure for reactions that increased the carbon chain length, specifically for reactions involving two abundant highly reactive species, namely diatomic carbon (C2) and C4H, using the cinétique de réaction en ecoulement supersonique uniforme, or reaction kinetics in uniform supersonic flow (CRESU) technique. Furthermore, in order to improve our understanding of why some reactions became more rapid as the temperature fell, being therefore important for astrochemistry, while others slowed down, the chair holder directed a systematic study of the rate constants of reactions of oxygen atoms with alkenes, i.e. hydrocarbons containing a double bond.

In terms of the collisional processes at extremely low temperatures for comparison with theory, it should be firstly noted that the comparison of experimental measurements of rate constants for reactive and inelastic collisions with theory was of major importance at very low temperatures, since the rate constants were most sensitive to the finer details of the forces between the atoms that made up the reacting system. Therefore, the comparison was most revealing. In order to descend below the current limit of 10 K approximately, it was necessary to introduce a novel design of pulsed valve at the throat of the Laval nozzle that was at the heart of the CRESU apparatus, which would enable higher Mach numbers, and thus lower temperatures, to be achieved in the range of 1 to 10 K with no pre-cooling. The new pulsed CRESU was anticipated to enter the commissioning stage in early 2008.

Regarding the growth of clusters or nanoparticles at a molecular level, the formation of a condensed phase in a homogeneous gaseous medium was an elementary thermodynamic and kinetic problem which was not completely understood, despite its numerous practical applications. It was of wide scientific and technological importance in fields as diverse as nanotechnology, i.e. nucleation of nanoparticles, and combustion, i.e. soot formation. The first steps in these processes were measured, particularly the formation of dimers of large organic molecules called Polycyclic aromatic hydrocarbons (PAHs). These measurements provided a strong challenge to the principal proposed model for soot formation in flames.

Finally, in terms of the teaching and training activities, the chair holder implemented a multilevel training programme under the following headings:
1. training by research;
2. postgraduate and undergraduate teaching programmes;
3. teaching to wider audiences.

The PhD work of four students was supervised during the project. Two of these students graduated with a PhD, while two were continuing their successful studies by the time of the project completion. Lectures were delivered at undergraduate level (L3 physique), Masters' level (M1 and M2) and graduate level. Public lectures were also delivered in the context of the world year of physics in 2005, as well as at the science festival in Rennes in 2006.