Kinetics and branching ratio studies of collisional processes at very low temperatures – CRESUSOL

A pulsed version of the CRESU technique will be implemented at SOLEIL in order to perform experimental measurements using a photoionisation/mass spectrometry method.

The 6 first months of this project allowed us to establish the specifications for the design of the experimental chamber, its frame and to look into details of the detection system. It turned out that the quadrupole mass spectrometer had to be replaced by a time of flight mass spectrometer.

The next months will be dedicated to finalize the design of the detection system in order to fully design the chamber to be implemented at SOLEIL. The building of the all system will be therefore undertaken.

Nothing at this date.

Understanding the mechanism of elementary reactions leading to the formation of molecules and clusters in various conditions, especially at very low temperatures (i.e. low collision energies), is of fundamental interest and yields crucial information for modelling gaseous environments encountered in the fields of atmospheric chemistry (of the Earth and other planets) and astrophysics.
The main scientific aim of this project is to monitor the reactants and products of reactions at substantially lower temperatures than has been attempted before (down to 15 K), to measure the rate coefficients of a selection of reactions, including dimerization, as well as their branching ratios at least for some of their potential channels. Knowledge of this information is of crucial importance for properly modelling gaseous environments.
A new, pulsed, version of the well established CRESU (Cinétique de Réaction en Ecoulement Supersonique Uniforme or Reaction Kinetics in Uniform Supersonic Flow) reactor will be associated with a mass spectrometer to probe reactants and products of reaction after threshold photoionisation by the VUV beamline of the synchrotron SOLEIL. This work will lead to the interaction of chemists and physicists expert in experimental and theoretical reaction kinetics and dynamics as well as in mass spectrometry and photoionisation techniques. The grant will allow our teams to (1) develop in Rennes a new transportable experimental chamber equipped with the new, and recently patented, pulsed CRESU system combined with a mass spectrometer, (2) implement this apparatus on the DESIRS beamline of the synchrotron SOLEIL to photoionize reactants and products of reaction and obtain branching ratios and kinetics information by mass spectrometry, (3) compare the experimental results to state-of-the-art calculations undertaken by the third partner of this project from Argonne National Laboratory (USA). The new pulsed CRESU apparatus coupled with a quadrupole mass-spectrometer will be built in Rennes and moved to the cw synchrotron source of SOLEIL in order to implement the photoionisation/mass spectrometry technique to detect and monitor reagents and products of a selection of reactions relevant to the chemistry of the interstellar medium, planetology and atmospheric chemistry, as well as to our understanding of chemical reactivity as a whole.
1- CN + H2C=CH2 ? H2C=CHCN + H (1a)
H2C=CH + HCN (1b)
2- C2H + H2CCHHCCH2 ? + H (2a)
? + H (2b)
3- NH2 + NO ? N2H + OH (3a)
? N2 + H2O (3b)
? N2O + H2 (3c)
4- CH + CH4/CH3D ? C2H4/C2H3D + H (4a)
? C2H3 + H2/HD (4b)
5- C3H3 + C3H3 ? C6H6 (and different isomers) (5a)
? C6H5 + H (5b)
6 - C2H5 + C2H5 ? C4H10 (6a)
? C2H4 + C2H6 (6b)
7 - H2O + H2O ? (H2O)2 (7)
The presently proposed combination of careful state-of-the-art experimental and theoretical studies will yield a synergistically enhanced understanding of the specific reactions. It will also help advance our understanding of the success and limitations of the theoretical methods. From a fundamental point of view, under these low temperature/energy conditions any theoretical description becomes highly dependent on the quality of the potential energy surface (PES) that is used. For modelling cold environments such as interstellar clouds or cold planetary atmospheres, the expected results will be obtained at the relevant temperatures. The branching ratio indeed can be temperature dependent and so measurements at low temperature are critical for photochemical models of planetary atmospheres or interstellar clouds.

The breakthroughs that will be realised will be of benefit both to future studies targeted at systems relevant to cold environments as well as to our understanding of chemical reactivity and homogenous nucleation.