Conception, construction and validation of scientific instruments to study the spectrum of cold ionic species
(2021)
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- This doctoral work, carried out at UCLouvain and University of Rennes 1, explored two experimental methods to study molecular ions. These methods are background free photodissociation spectroscopy at UCLouvain, and cavity enhanced absorption spectroscopy at the University of Rennes 1. In both cases, we have developed the scientific instrument from scratch. Regarding the first part, at UCLouvain a pulsed free jet was developed, and it was coupled with two ionization methods. The first is an electric discharge. The second method consisted of using an electron gun. We have also implemented a time-of-flight (TOF) mass spectrometer capable of increasing resolution and consisting of a unit capable of accelerating, gating and re-referencing the ion beam to ground potential. The recorded mass spectra demonstrated the efficiency of the production of protonated water aggregates, other various mixed cationic aggregates, and anions. The first tests were carried out on N2O+. The photodissociation was performed using a frequency-doubled dye laser in the UV range around 323 nm. Laser fragmentation of the parent ion led to the formation of an ionic (NO+) and neutral (N) species. Analysis of the measured spectrum makes it possible to estimate the minimal rotational temperature that we can attain at 40 K. This result is encouraging and suggests the possibility of studying other molecular ions in the near future. The second part of this work, carried out at the University of Rennes 1, consisted of developing an instrument to study molecular species relevant for astrophysics by high-resolution absorption spectroscopy in the visible and near-infrared regions. In particular, the instrument developed will aim to study the large anionic carbon chains (C$^-_3$, C$^-_4$, … C$^-_n$). Due to space charge effects, the density of target species produced in the laboratory is extremely low even in supersonic expansion. An efficient ion source has been developed. The chosen geometry was that of a planar Laval nozzle, adapted to reach a low temperature (40 K). The planar profile is important to have a greater interaction distance between the ions produced and the laser. These ions are produced using a direct current discharge coupled to the nozzle. On the other hand, by using a supercontinuum laser as a wideband incoherent source coupled to a high fineness cavity, we were able to characterize these species by absorption spectroscopy. The absorption spectrum is obtained using a Fourier transform spectrometer. The optimization and development of the instrument consisted of the optimization of the coupling of the laser source and the optical cavity, and then coupling the output into the Fourier transform spectrometer. We successfully measured the static absorption spectra of methane and acetylene to determine the operating parameters of the instrument, and we look forward to record the spectra of different radicals and finally anionic carbon chains.
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Bejjani, R. (2021). Conception, construction and validation of scientific instruments to study the spectrum of cold ionic species. https://hdl.handle.net/2078.5/108689