VUV photoionisation of astrophysical molecules : fundamental and quantitative aspects

The evolution of diluted matter in various astrophysical media (interstellar space medium, planetary atmospheres, comets) results from complex photochemistry involving many stable and reactive species in the gas phase. This photochemistry is induced by galactic fluxes or by solar radiation in the vacuum ultra-violet spectral domain (VUV, wavelength < 200 nm). An accurate model of the photochemistry of these media is required to understand this evolution. This model must account for all photo-induced processes through pertinent quantitative parameters measured in laboratory experiments. One of these processes is photoionisation (M + hν -> M⁺ + e^-), which is the focus of this thesis.

One of the aims of this PhD work was to determine quantitative and often unprecedented parameters to describe photoionisation processes of compounds of astrophysical interest, in particular of reactive species. These were determined experimentally using the medium high-resolution SAPHIRS setup of the DESIRS beamline of the SOLEIL synchrotron and using the new high-resolution laser setup (VULCAIM) of ISMO based on a Zero Electron Kinetic Energy (ZEKE) photoelectron spectrometer. This thesis also presents details on the development of this new setup, unique to France, and on the determination of its main characteristics.

An important quantifying parameter is the photoionisation cross section, relating the number of produced ions to an initial quantity of neutral species. This parameter is not always available in the literature due to experimental challenges, especially for unstable species. This work presents two photoionisation cross section measurements (NH₂ and OH free radicals) and some preliminary work on other free radical cross sections. From a practical point of view, these cross sections also allow the indirect quantification of other relaxation processes using VUV pump –VUV probe experiments (e.g. photodissociation), which are also featured in the manuscript.

On top of the application value of these experiments, these studies offer fundamental insights, mainly via photoelectron spectroscopy, into the fine details of photoionisation processes. At the SOLEIL facility, the vibronic structure of the dicarbon radical, C₂, has been investigated in the vicinity of its first adiabatic ionization potential, and the energy of the one-photon forbidden transition X⁺ ← X has been derived for the first time using only one experimental spectrum. At the ISMO laboratory, the new VUV laser has shown to have the capabilities of undertaking spectroscopic studies at a resolution (down to 0.06 cm^-1) rarely seen in the VUV range. The use of this laser for photoelectron spectroscopy combined with an imaging technique promises to be able to perform high-resolution photoionisation studies with improved signal-to-noise ratio. It will lead to unprecedented spectroscopic information on the rovibronic structure of a number of cations, in particular for radical species.