We employ an extreme ultraviolet (XUV) pulse to impulsively excite dipole polarization in atoms or molecules, which corresponds to coherently prepared superposition of excited states. A delayed near infrared (NIR) pulse then perturbs the fast evolving polarization, and the resultant absorbance change is monitored in dilute helium, dense helium, and sulfur hexafluoride (SF6) molecules. We observe and quantify the time-dependence of various transient phenomena in helium atoms, including laser-induced phase (LIP), time-varying (AC) Stark shift, quantum path interference, and laser-induced continuum structure. In the case of dense helium targets, we discuss nonlinear macroscopic propagation effects pertaining to LIP and resonant pulse propagation, which account for the appearance of new spectral features in transient lineshapes. We then use tunable NIR photons to demonstrate the wavelength dependence of the transient laser induced effects. In the case of molecular polarization experiment in SF6, we show suppression of XUV photoabsorption corresponding to inter-valence transitions in the presence of a strong NIR field. In each case, the temporal evolution of transient absorption spectra allows us to observe and understand the transient laser induced modifications of the electronic structure of atoms and molecules.

Faraday modulation spectroscopy (FAMOS) is a laser-based spectroscopic dispersion technique for detection of paramagnetic molecules in gas phase. This thesis presents both a new theoretical description of FAMOS and experimental results from the ultra-violet (UV) as well as the mid-infrared (MIR) regions. The theoretical description, which is given in terms of the integrated linestrength and Fourier coefficients of modulated dispersion and absorption lineshape functions, facilitates the description and the use of the technique considerably. It serves as an extension to the existing FAMOS model that thereby incorporates also the effects of lineshape asymmetries primarily originating from polarization imperfections. It is shown how the Fourier coefficients of modulated Lorentzian lineshape functions, applicable to the case with fully collisionally broadened transitions, can be expressed in terms of analytical functions. For the cases where also Doppler broadening needs to be included, resulting in lineshapes of Voigt type, the lineshape functions can be swiftly evaluated (orders of magnitude faster than previous procedures) by a newly developed method for rapid calculation of modulated Voigt lineshapes (the WWA-method). All this makes real-time curve fitting to FAMOS spectra feasible. Two experimental configurations for sensitive detection of nitric oxide (NO) by the FAMOS technique are considered and their optimum conditions are determined. The two configurations target transitions originating from the overlapping Q22(21=2) and QR12(21=2) transitions in the ultra-violet (UV) region (227nm) and the Q3=2(3=2)-transition in the fundamental rotational-vibrational band in the mid-infrared (MIR) region (5.33 µm). It is shown that the implementations of FAMOS in the UV- and MIR-region can provide detection limits in the low ppb range, which opens up the possibility for applications where high detection sensitivities of NO is required.

NMR relaxation and Raman lineshape analysis are well known methods for the study of molecular reorientational dynamics in liquids. The combination of these two methods provides another approach to tackle the characterization of molecular dynamics in liquids. Investigations presented here include (1) NMR relaxation study of polycyclic compounds in solution, (2) the study of nitromethane reorientational dynamics using the NMR and Raman methods, and (3) Raman lineshape analysis of reorientation hexafluorobenzene/benzene mixtures.

No description available.

Ce travail est consacré aux études théoriques et expérimentales du profil spectral des raies d'absorption du dioxyde de carbone, une espèce clé dans l'atmosphère de la Terre. Le but de ce travail est de tester les différents modèles du profil spectral des raies d'absorption de CO2. Dans un premier temps, des mesures des paramètres spectroscopiques du CO2 pur dans l'infrarouge proche en utilisant un système de diode laser à cavité externe ont été effectuées. Différents modèles de profil spectral ont été utilisés pour ajuster les spectres mesurés. Les résultats montrent que le profil de Voigt mène à de larges différences avec les spectres mesurés et qu’il est nécessaire de prendre en compte à la fois des changements de vitesse et des dépendances en vitesse des paramètres collisionnels pour décrire correctement le profil spectral. Le modèle HTP a alors été utilisé pour modéliser le profil spectral de CO2. Ce modèle a donc pu être testé et validé par cette étude. Des simulations de dynamique moléculaire pour CO2 pur et CO2 perturbé par N2 ont ensuite été effectuées afin d’étudier plus précisément les effets non-Voigt observés. Le but a été ici de déterminer l’influence de plusieurs paramètres sur ces effets. Nous avons alors pu comparer les simulations à des mesures. Pour le CO2 pur, nous avons pu montrer que le choix du potentiel intermoléculaire pour modéliser les interactions existantes n’avait pas d’influence sur l’évolution de ces effets avec la pression. Aucune dépendance rovibrationelle n’a pu être mise en évidence. Les résultats pour CO2 dans N2 ont montré une dépendance de l’évolution de ces effets en fonction du rapport de mélange utilisé. / This work is devoted to the theoretical and experimental studies of the spectral shape of isolated absorption lines of carbon dioxide, a key species of the Earth's atmosphere. The objective of this PhD thesis is to test the different line-shape models that take into account various velocity effects affecting the spectral shape of CO2 absorption lines. The experimental part consists of measurements of spectroscopic parameters of pure CO2 using a tunable External Cavity Diode Laser setup. In the theoretical part, different spectral profiles were used to fit the measured spectra. The results show that the Voigt profile leads to important residuals and it is thus necessary to take into account both Dicke narrowing and the speed dependence of collisional parameters to adequately describe the experimental spectral profile. The HTP profile, developed recently, was also used to model the spectral profile of CO2 lines. This model has been validated by this study. Molecular dynamics simulations for pure CO2 and CO2 perturbed by N2 were also conducted to study more precisely the “non-Voigt” effects observed. The goal here was to determine the influence of several parameters on these effects. We compared the theoretical simulations with our measurements. For pure CO2, we could show that the intermolecular potential chosen to model the existing interactions had no influence on the evolution of these effects as a function of pressure. Furthermore, no rovibrational dependence could be found. The results for CO2 mixed in N2 showed a dependence of the evolution of these effects depending on the CO2/N2 mixing ratio.

Nuclear magnetic resonance (NMR) provides a convenient probe for the study of molecular reorientation in liquids because nuclear spin-lattice relaxation times are dependent upon the details of molecular motion. The combined application of Raman and Infrared (IR) lineshape analysis can furnish more complete information to characterize the anisotropic rotation of molecules. Presented here are the studies of NMR relaxation times, together with Raman/IR Mneshape analysis of the solvent and temperature dependence of rotational diffusion in 1,3,5-tribromobenzene and 1,3,5-trifluorobenzene. In these experiments, it was found that the rotational diffusion constants calculated from Perrin's stick model were two to three times smaller than the measured values of D, and D,,. Similarly, rotational diffusion constants predicted by the Hu-Zwanzig slip model were too large by a factor of 2. Application of the newer Hynes-Kapral-Weinberg model furnished rotational diffusion constants that were in reasonable agreement with the experimental results. The vibrational peak frequencies and relaxation times of the isotropic Raman spectra of the υ1 modes of CD2Br2 and CHBr3 were studied in solution. The frequency shifts in non-interactive solvents were explained well on the basis of solution variations in the dispersion energy. In Lewis bases, the displacements were in some, but not all, cases greater than predicted. On the other hand, it was found that the vibrational relaxation times of the C-H/C-D modes decreased dramatically in all Lewis base solvents. Therefore, it was concluded that relaxation times of the υ1 modes, rather than frequency shifts, furnish a more reliable measure of hydrogen bonding interactions of halomethanes in solution.

No description available.