Gas-phase electronic spectroscopy of cold molecular cations of astrophysical interest

This thesis, which is the core of a project started in 2010, is aimed at developing an experimental technique suitable for the measurement of the electronic spectra of cold, large, gas-phase molecular cations. Two approaches were initially targeted: photoinduced charge transfer and photodissociation of ionic complexes with He atoms. Since the latter is both the simplest of the two and a universal method, most efforts would be put toward its implementation. The project included the design and assembly of a new apparatus, a tandem mass spectrometer centered on a 22-pole trap. When this thesis was initiated, the apparatus was partially assembled and as a first major task, the assembly had to be completed and characterized. For this reason, chapters 4 and 6 treat some aspects of the obtained electronic spectra that are not novel, but which are part of the characterization of the apparatus and the method. Chapter 2 presents the several types of spectroscopic features that prove that large molecules do form interstellar and circumstellar media. Observations and laboratory experiments are used to put constraints on the responsible molecules. The experimental apparatus and the procedure to obtain the electronic spectra are detailed in Chapter 3, and some aspects of the characterization of the apparatus are presented in Chapter 4. The electronic spectra of several gas-phase protonated H+PAHs at a rotational temperature of 10K constituted the first results of this apparatus and are presented in Chapter 5. These molecules were not studied with one of the two targeted methods, however, they were the firsts spectra obtained in the gas phase for such large cations for astronomical comparison. Also studying these five H+PAHs allowed us to get a better handling of the setup and to solve experimental issues, as shown in Chapter 6. Chapter 7 presents the first results obtained using one of the targetted methods, the electronic spectra of coronene+ and corannulene+, extrapolated from the photodissociation (PD) spectra of their complexes with He. Chapter 8 addresses the case of protonated C60, a project which has merely been started. Finally, Chapter 9 concludes and gives an outlook about the general challenge of identifying large molecules in the ISM and CSEs.