March 17, 2022
16:15
Louvain-La-Neuve
Auditorium SUD08
Systematic study of mutual neutralization reactions between atomic species using the merged beam method and an asymptotic model by Arnaud DOCHAIN
Pour l’obtention du grade de Docteur en sciences
The mutual neutralization reaction A⁺ + B⁻ → A⁰ + B⁰ plays an important role whenever an anion populated plasma is involved (interstellar molecular clouds, ionospheres, stellar atmospheres, etc.), and modifies its radiative emissions which are used for estimating stars metallicity. Additionally, the measurement of mutual neutralization can serve as a benchmark for the numerical models estimating the inelastic collisions used in these models of reaction dynamics.
In this manuscript, we present results for several low energy mutual neutralization reactions measured with the UCLouvain single pass merged beam apparatus: He⁺/Li⁺ + H⁻/D⁻, C⁺/N⁺/O⁺ + D⁻/C⁻/O⁻/Si⁻/P⁻/S⁻, Na⁺ + O⁻/S⁻, and Ne⁺ + C⁻/O⁻/S⁻. We detail how a pair of time and position sensitive detectors allows us to obtain accurate branching ratios and partial differential cross sections.
In order to rationalize the branching ratios we have measured, we developed a multi-channel Landau-Zener model combined with an updated version of the anion centered asymptotic (analytical) model (ACAM). The overall agreement between theory and experiment allows us to use this model to describe the mutual neutralization process. The ACAM validity domain is also studied, revealing additional processes responsible for the discrepancies between theory and experiment, such as the production of Rydberg series of the neutral atoms.
The differential cross section was also computed using two models: the interrupted Rutherford scattering model and the impact parameter method. The former is analytical and describes well the collision energy dependency of the differential cross section while the latter gives quantitative results. It is also shown how differential cross section analysis provides a better insight into the mutual neutralization process, e.g. which molecular states are temporarily populated.
As a conclusion, this manuscript suggests several ideas for improving both the theoretical models and the experimental setup.
Jury members :
- Prof. Xavier Urbain (UCLouvain), supervisor
- Prof. Arnaud Delcorte (UCLouvain), chairperson
- Prof. Clément Lauzin (UCLouvain), secretary
- Prof. Bernard Piraux (UCLouvain)
- Prof. Nathalie Vaeck (ULB)
- Prof. Paul Barklem (Uppsala University, Sweden)
- Prof. Arnaud Le Padellec (Univ. Toulouse III, France)
Pay attention :
The public defense of Arnaud Dochain scheduled for Thursday 17 March at 4:15 p.m will also take place in the form of a video conference