This PhD thesis deals with the application of recent high resolution spectroscopic methods that are either selective or ultrasensitive. Spectroscopic parameters of stable and transient molecular species, difficult to reach otherwise, are determined.
First, selective Fourier transform spectroscopy is used to characterize TiCl₄/He and N₂O/He plasmas. The spectrum of the whole emission of a plasma and spectra, which select a given class of molecules (for instance short-lived or ions) are simultaneously recorded. In particular, the rovibronic transition X ⁴Φ - C ⁴∆ of the ⁴⁸Ti³⁵Cl radical is observed around 3 µm. Its analysis has shown a very weak perturbation, which affects the C ⁴∆_1/2 state. Moreover, selective spectra of a N₂O/He plasma, expected to only retain the ion lines, have revealed NO rovibronic transitions between the A, C, D and E Rydberg states. Under specific experimental conditions, a Doppler shift of the NO transitions, similar to the one of a positive ion, is observed. An interpretation of this unexpected phenomenon, highlighted for the first time for a neutral molecule, is proposed.
Second, spectra with absorption lengths of several tens kilometers are recorded by coupling intracavity laser absorption spectroscopy (ICLAS) and time-resolved Fourier transform spectroscopy (TRFTS). Two different experiments have make it possible to probe, with high sensitivity, relatively wide infrared spectral ranges. Around 2.5 µm, lots of weak intensity rovibrational transitions of the atmospherically relevant molecule N₂O are identified and analysed. Unstable species created in a N₂/He plasma are also detected for the first time around 1.05 µm.

Keywords : Fourier transform spectroscopy, Intracavity laser absorption, Selective spectroscopy, Time-resolved spectroscopy, Velocity modulation, High sensitivity infrared detection, Doppler shift, TiCl, NO, N₂O, N₂.