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The role of clusters in planetary atmospheres and the interstellar medium is potentially important. Investigating such a role requires basic experimental information, however lacking. The goal of this thesis was to develop an efficient experimental set-up to produce clusters in the laboratory in concentrations large enough to allow their high resolution spectra to be recorded, thus providing the necessary data allowing the physico-chemical properties of the clusters to be studied.<p>The study of this subject however suffers from the lack of basic experimental data. The goal is therefore to produce clusters in the laboratory in concentration large enough to record their high resolution spectrum. This is the initial aim of the present thesis.<p>During this work, we have built and extensively tested a new experimental set-up called FANTASIO (``Fourier trANsform, Tunable diode and quadrupole mAss spectrometers interfaced to a Supersonic expansIOn'). With the help of this new device, various experiments on jet-cooled species have been performed.<p>The cartography of the supersonic expansion was established, using the mass spectrometer as a moving pressure probe. This enabled us to characterize the geometrical properties of the supersonic jet produced by circular and slit nozzles and to determine the position of the virtual nozzle. The effect of the axisymmetric expansion geometry on the R(0) lineshape in the nu_3 band of N_2O, recorded by FTIR, was also investigated.<p>The rotational temperature of the jet-cooled molecules was determined to be a few K by measuring the intensity of lines in spectra recorded by FTIR spectroscopy.<p>Vibrational energy transfer occuring in the expansion between N_2O molecules and different collision partners was investigated on the nu_2+nu_3-nu_2 band of N_2O, again using FTIR spectroscopy. The trend of these transfers was found to be related to the energy difference between the v_2=1 level of N_2O and the closest vibrational state in the collision partner, with the largest population.<p>The sensitivity of the set-up was enhanced by a factor of 5 by increasing the absorption path length, using a multipass system. A procedure to remove the residual gas contribution from the IR spectra was developped, based on the mass spectrometer. Thanks to this sensitivity increase, broadband absorption features of clusters were observed for a C_2H_2-Ar mixture in circular and slit expansions.<p>The optical sensitivity of FANTASIO was again increased by the implementation of the CW-CRDS system. The enhancement over FTIR was calculated to be over a factor 750. Thanks to this drastic improvement, spectral signatures of various clusters were recorded, such as C_2H_2-Ar, C_2H_2 multimers, C_2H_2-N_2O and C_2H_2-CO_2, at high resolution.<p>The role of clustering in generating unusual line shapes of acetylene in an axisymmetric expansion was investigated. We demonstrated that C_2H_2 aggregates produced in the expansion are responsible for central dips observed in the monomer absorption. These acetylene clusters thus appear to be formed in the centre of the expansion, while, unexpectedly, acetylene-Ar complexes are formed at the edge of the conical expansion.<p>Various research prospects were explored during this thesis thanks to the FANTASIO device, opening new research directions. FANTASIO is today operational and defines a useful tool to achieve the study of small clusters by infrared spectroscopy./<p><p>Le rôle des agrégats dans les atmosphères planétaires et dans le milieu interstellaire est potentiellement important. Cependant, les études sur ce sujet souffrent du manque de données expérimentales. Le but de cette thèse était de développer un dispositif expérimental efficace pour produire au laboratoire des agrégats en quantité suffisante pour permettre l'enregistrement de leur spectre infrarouge à haute résolution et donc l'étude de leurs propriétés physico-chimiques.<p>Durant ce travail, nous avons construit et testé un nouveau dispositif expérimental appelé FANTASIO, basé sur un jet supersonique couplé à un spectromètre de masse, un spectromètre à transformée de Fourier et un système CRDS. Grâce à cet appareillage, différentes expériences sur des molécules à basse température ont été menées.<p>L'expansion supersonique a été cartographiée en utilisant le spectromètre de masse comme une sonde de pression mobile. Cette cartographie nous a permis d'établir les propriétés géométriques des jets supersoniques produits par les orifices circulaire et de type fente, et de déterminer la position de l'orifice virtuel. L'effet de la géométrie de l'expansion sphérique sur le profil de la raie R(0) de la bande nu_3 de N_2O, enregistré par FTIR, a aussi été étudié.<p>Une température rotationnelle de quelques K a été déterminée pour les molécules refroidies en jet supersonique par mesure de la distribution d'intensité de raies dans les spectres enregistrés par FTIR.<p>Le transfert d'énergie vibrationnelle entre des molécules de N_2O et différents partenaires collisionnels a été étudié en analysant l'intensité de la bande nu_2+nu_3-nu_2 de N_2O, enregistré également par spectroscopie FTIR. Il a été trouvé que la tendance de ces transferts est liée à la différence d'énergie entre le niveau v_2=1 de N_2O et l'état vibrationnel le plus proche et le plus peuplé du partenaire collisionnel.<p>La sensibilité du dispositif a été augmentée d'un facteur 5 dû à l'allongement du chemin d'absorption, grâce à l'utilisation d'un système multipassage. Une procédure basée sur l'utilisation du spectromètre de masse et visant à enlever la contribution du gaz chaud résiduel dans les spectres infrarouges a été mise au point. Grâce à cette augmentation de sensibilité, des structures d'absorption non résolues d'agrégats ont été observées dans des expansions en trou et en fente d'un mélange de C_2H_2-Ar.<p>La sensibilité optique de FANTASIO a encore été augmentée par l'ajout au dispositif d'un système CW-CRDS. L'amélioration par rapport au spectromètre à transformée de Fourier a été calculée comme étant supérieure à un facteur 750. Grâce à cette importante amélioration, les signatures spectrales de divers agrégats, tels que C_2H_2-Ar, des multimères de C_2H_2, C_2H_2-N_2O et C_2H_2-CO_2, ont été enregistrées à haute résolution.<p>Le rôle de l'agrégation dans la génération de profils de raie inhabituels dans une expansion en trou de l'acétylène a été étudié. Nous avons démontré que les agrégats de C_2H_2 produits dans le jet supersonique sont responsables des creux observés dans le profil d'absorption du monomère. Ces agrégats apparaissent donc comme étant formés au centre de l'expansion, tandis que, de manière inattendue, les agrégats de C_2H_2-Ar sont formés aux bords de l'expansion conique.<p>Plusieurs idées de recherche ont été explorées durant cette thèse grâce au dispositif FANTASIO, ouvrant de nouvelles directions de recherche. FANTASIO est aujourd'hui opérationnel et se présente comme un outil utile dans l'étude des petits agrégats par spectroscopie infrarouge. / Doctorat en Sciences / info:eu-repo/semantics/nonPublished

This thesis presents HELIOS, an in-house laboratory for time-resolved pump-probe spectroscopy with extreme-ultraviolet (XUV) probe radiation. A wide span of pump wavelengths can be generated using commercial laser equipment while XUV probe radiation is generated via a high harmonic generation process in a noble gas delivering probe photons with energies between 20 eV and 72 eV. The XUV beam path features a time-preserving monochromator and was constructed and built in-house. HELIOS features an overall time resolution of about 50 fs when using 800 nm pump and 41 eV probe photons. An energy resolution of 110 meV at 41 eV photon energy can be achieved. HELIOS features two beamlines. One µ-focus beamline with an XUV focal size of about 20 µm can be used with experiments that require such a small XUV focal size as well as with different end stations. The other beamline features a semi-permanently mounted end station for angle-resolved photoelectron spectroscopy under ultra-high vacuum conditions. Experiments demonstrating the usability of HELIOS and the two beamlines are presented. A pump-probe measurement on graphene demonstrates the capability of determining a large part of the k-space in only one measurement due to the use of an ARTOF angle-resolved time-of-flight electron spectrometer. A non-angle-resolved pump-probe measurement on the conducting polymer PCPDTBT demonstrates the high signal-to-noise ratio achievable at this beamline in non-angle-resolved photoelectron-spectroscopy pump-probe measurements. The usability of the µ-focus beamline is demonstrated with time-resolved measurements on magnetic samples employing an in-house-designed spectrometer. These experiments allow the retrieval of element-specific information on the magnetization within a sample employing the transversal magneto-optical Kerr effect (T-MOKE). Additionally, a Fourier transform spectrometer for the XUV is presented, the concept was tested at a synchrotron and it was used to determine the longitudinal coherence of the XUV radiation at HELIOS.

This thesis describes laboratory measurements of the complex refractive index of volcanic ash particles. These measurements are needed to model the radiative impact of volcanic ash, vital for accurate satellite remote sensing. Three experimental methods have been developed, and the results for the complex refractive index and optical properties of a wide range of volcanic ash samples are presented. Measurements were made of the spectral transmission of radiation through suspended volcanic ash particles inside an aerosol cell, using a Fourier transform spectrometer at infrared wavelengths and two diffraction grating spectrometers covering ultraviolet, visible, and near-infrared wavelengths. In addition to the optical measurements, a suite of sampling and sizing instruments were connected downstream of the aerosol cell to measure the particle size distribution. The method was calibrated using two quartz samples. Mass extinction coefficients for nine volcanic ash samples, at 0.3-14 μm, are presented and show considerable variation. These variations are linked to the composition of the samples, measured using X-ray fluorescence (XRF) analysis. The complex refractive index, at 0.3-14 μm, of the two quartz samples and two samples of volcanic ash from the 2010 Eyjafjallajökull eruption were retrieved from the extinction measurements. The forward model used Mie theory and a classical damped harmonic oscillator (CDHO) model to represent the complex refractive index of the samples in terms of a finite set of band parameters, as well as the real refractive index of the sample in the small wavelength limit. Previous studies have shown that there is a redundancy in the retrievals between the band strength parameters and the real refractive index in the small wavelength limit, which can lead to spurious values for the retrieved complex refractive index. This problem was overcome by using an independent measurement of the real refractive index at a visible wavelength, to constrain the model parameter of the real refractive index in the short wavelength limit. Independent measurements of the complex refractive index at visible wavelengths are also important because the extinction produced at these wavelengths is highly sensitive to the particle size distribution, and any uncertainty in the measured size distribution will contribute to significant systematic error in the refractive index retrieved from extinction. The retrieved spectral complex refractive index of Eyjafjallajökull ash was applied using the ORAC retrieval scheme to measurements of the 2010 Eyjafjallajökull eruptionmade by theMODIS instrument aboard NASA's Terra satellite. Significant difference were found in the retrieved plume parameters of optical path, effective radius, and plume altitude, compared to assuming a literature measurement for the refractive index of pumice. For three discrete visible wavelengths (450, 546.7, and 650 nm) an optical microscope was used to make measurements of the complex refractive index of the volcanic ash samples. The long-established Becke line method was used to measure the real refractive index of the samples. For the imaginary refractive index, a new and novelmethod was developed involving measurements of the attenuation of light in individual particles. A strong linear correlation was found between the SiO₂ content of the samples and both their real and imaginary refractive indices at the visible wavelengths investigated. Furthermore, from the XRF compositional analysis of the samples values were calculated for the ratio of non-bridging oxygen atoms per tetrahedral cation (NBO/T), and it was found that NBO/T was an even stronger predictor of real refractive index at visible wavelengths. The optical microscope measurements could only be applied to particles with a radius larger than 10 μm. A new refractometer method was investigated for retrieving the real refractive index of submicron particles from colloidal reflectance measurements close to the critical angle in an internal reflection configuration. A coherent scattering model (CSM) was used to model the coherent reflection from a half-space of monodisperse or polydisperse particles, and a simple extension of the model is presented to properly account for the modified size distribution at the interface in an internal reflection set-up. A rigorous sensitivity analysis was performed to determine how experimental uncertainties propagate into uncertainty associated with the retrieved real refractive index, and the uncertainty due to non-spherical effects was estimated using T-matrix methods. Experimental reflectance data at a wavelength of 635 nm were obtained for spherical monodisperse polystyrene calibration particles, a polydisperse sand sample, and a polydisperse volcanic ash sample. The retrieved values for the real refractive index agreed, within propagated uncertainties, with values measured using other techniques. The method is shown to be a viable technique for measuring the real refractive index of small quantities of submicron particles, and can also retrieve the concentration and size of particles.