Photoionization spectroscopy of radicals of astrophysical interest in the vacuum ultraviolet

The evolution of matter in astrophysical environments, such as the interstellar medium, is governed by complex chemical processes involving numerous stable and radical species in the gas phase. Identifying and studying interstellar molecules constitute fundamental steps toward understanding the chemical processes in space, offering insights into the origin of life in the universe, star formation processes, and interstellar environments’ characterization.
The quest for identifying interstellar species and constructing chemical models relies heavily on laboratory-based experimental studies. This doctoral work is dedicated to generate new, highquality spectroscopic laboratory data for gas-phase compounds of astrophysical interest, with a particular focus on radical species, using photoionization spectroscopic techniques. My work can be broadly divided into two main components :
 the first aspect involves the exploration of a stable and efficient radical source device to produce radicals in situ ;
 the second aspect entails the development and application of high-resolution photoionization spectroscopic techniques that enable the investigation of the (ro)vibrational structure of cations of radicals. This thesis documents experiments conducted using the high-resolution VULCAIM setup at ISMO and the medium-resolution SAPHIRS setup at the DESIRS beamlines of the SOLEIL synchrotron.
The VULCAIM setup, unique in France and one of the only three worldwide, boasts the capability to provide high-resolution tunable VUV laser radiation from 6 to 17 eV. This thesis highlights developments performed on this unique apparatus, including the introduction of a broadly tunable OPO laser to extend the range of the VUV laser, the exploration of radical sources through pyrolysis and electric discharge, and the implementation of a reflectron. The coupling of the pyrolysis radical source to the VULCAIM setup achieved a significant result : the production of vibrationally excited CH₃ radicals, providing highresolution laboratory data via PFI-ZEKE photoelectron spectrosocpy that support experimentally the detection of CH₃⁺ in space for the first time. Additionally, a novel high-resolution and high signal-to-noise ratio PRFI-ZEKE PES method has been proposed and applied within the research domain.
SAPHIRS, the second setup used in this thesis at the synchrotron, couples a fluorine flow-tube reactorbased radical source with a double-imaging photoelectron/photoion coincidence (i²PEPICO) spectrometer and the synchrotron radiation. This configuration allowed for the simultaneous recording of ion yield and mass-selective photoelectron spectra at vibrational resolution for various Si_xC_yH_z radicals, offering essential insights into the vibronic structure of their cations. In this manuscript, I present experimental results encompassing SiH, Si₂, SiC, and SiCH. These studies have provided reliable photoionization energy values and information on the vibronic structures of cations and Rydberg states of the neutral species observed via autoionization. This work represents a first step toward understanding the chemistry of silicon within interstellar clouds.