DATE : le Mardi 06 février 2018 à 10h00
LIEU : la salle des thèses (bâtiment 1 du Campus de Beaulieu)
Résumé : (en anglais)
The main subject of the present thesis is the measurement of the dissociative recombination (DR) rate coefficients for several molecular ions of astrophysical and atmospheric interest. DR is a process in which a positive molecular ion recombines with an electron and subsequently dissociates into neutral fragments. The measurements were performed using the Flowing Afterglow Langmuir Probe Mass Spectrometer (FALP-MS) at the University of Rennes 1. In this apparatus, a flowing helium plasma is created by microwave excitation and subsequently converted into an argon plasma. Reagent gases are then introduced into the flow and the decay of the product positive ions and the electrons due to recombination are tracked downstream of the injection location by a movable mass spectrometer and Langmuir probe, respectively. Electron attachment can also be studied (when it occurs) by following the decay of the electron density along the flow. A number of different data collection methods are employed and the data collected are processed to derive the desired rate coefficients.
The DR reaction at room temperature has been investigated for molecular ions of acetone, trimethylamine and dimethylamine. In the case of acetone, the formation of adduct and cluster ions complicated the measurements but the techniques used have allowed the DR rate coefficients for these individual ionic species to be determined. DR rate coefficients values for these different species were found to fall within the range from 1 to 2×10-6 cm3s-1. In addition, the recombination rates coefficients for trimethylamine (TMA) and dimethylamine (DMA) ions were also examined at room temperature using two different techniques of several ions present in the plasma. While the rates found for TMA derived ions were in line with those for other polyatomic species, those found for DMA ions were considered to be unreasonably high (> 10-5 cm3s-1). This could be explained by formation of negative ions in the DMA plasma. It is found that neutral DMA attaches electrons with a rate of (5.0 ± 1.5) x 10-10 cm3s-1 while there was no electron attachment to neutral trimethylamine. The so-called multiple ions/z fixed method, which allows processes only dependent on electron density (i.e. DR) to be studied and used to extract the rate coefficients failed in the case of DMA. This is considered to be due to the influence of negative ion-positive ion mutual neutralization reactions which, unlike ion-neutral molecule reactions, are not independent of the electron density in the plasma. It was found that there was extensive clustering with DMA. This molecule has been found to be increase the formation rate of cloud nuclei in the terrestrial atmosphere, (even in very low concentration) and its ability to easily cluster must surely play a role in this enhancement phenomenon.