High Sensitive Laser Absorption Spectroscopy Of Transient Molecules: An Application On The C₂ And O₂ Molecules

This dissertation is concerned with both the theoretical aspects and experimental implementation of the optical heterodyne magnetic rotation enhanced concentration modulation spectroscopy (OH-MR-CMS) technique among with the application on the C2 transient molecule and the excited O2 molecule. The molecular species are produced in an electric glow discharge and a dye laser source is used in the visible region, finally a computerized sensitive detection system is used to record the spectra.A brief historical review of the molecular absorption spectroscopy of transient and excited molecules, followed by a brief review of all the components of the OH-MR-CMS technique and features; a discussion of their combinations is provided in the two first chapters. Chapter 3 introduces the basic principles of some different mechanisms of generating transient species with a special emphasis on the electric discharge methods which is used in this work. A 40 cm long absorption cell has been constructed and serves to produce the C2 molecule from a mixture of C2H2 and Ar gases; the O2 molecule is generated from pure O2 in He gas buffer. The absorption spectra of the produced molecules have been recorded using the OH-MR-CMS technique.In the study of the C2 molecule presented in chapter 4, more than 150 lines have been observed in the spectral range 17730-17900 cm-1 and assigned to the (0,1) band of theswan system. Some 30 △Ω≠0 transitions were observed allowing ageneral fit of all the molecular constants. This fit to the effective Hamiltonian for state yield precise spectroscopic constants for this band.The OH-MR-CMS spectra of the O2 molecule was recorded in the region 17650-17800 cm'1, the analysis of a part of this spectrum leads to a preliminary assignment for the first time, of the (7,3) band of the O2 forbidden transition. A fit of the observed lines to the calculated ones displays some discrepancies which we try to explain. A set of molecular parameter for the (v = 7) state resulting from this least square fit ispresented. However this work constitutes a first step of the study of the O2 atmospheric forbidden transition including high vibrational number of the lower state. This work ends by a general conclusion and some recommendation for future laboratory modifications to our spectrometer.