Cette thèse présente une étude expérimentale et théorique de petites chaines peptidiques modèles en phase gazeuse. Le premier objectif de ce travail consistait à déterminer les conformations préférentiellement adoptées par ces molécules isolées, en vue d’obtenir des informations sur les interactions intra- et inter-moléculaires intervenant dans ces systèmes flexibles. La stratégie expérimentale utilisée associait la vaporisation laser à une détente supersonique et reposait sur la spectroscopie laser de double résonance IR-UV. L’attribution finale des structures a ensuite été réalisée par comparaison des spectres expérimentaux à des spectres issus de calculs de chimie quantique au niveau DFT-D. Dans un deuxième temps, il s’agissait d’étudier la dynamique de relaxation électronique de ces systèmes par spectroscopie pompe-sonde et mesures de fluorescence, et en particulier la dépendance de celle-ci avec la structure secondaire des peptides modèles. La question de la population conformationnelle de molécules flexibles en phase gazeuse est un sujet délicat et bien souvent éludée car les distributions observées expérimentalement résultent d’un passage hors équilibre lors de la détente supersonique, définissant ainsi une température conformationnelle effective. Un modèle statistique a été développé, décrivant le refroidissement et les isomérisations subis durant la détente par une molécule. Les résultats de ces modélisations reproduisent les tendances d’évolution des rapports d’abondances entre conformations observés expérimentalement et permettent de fournir des ordres de grandeurs relatifs aux processus mis en jeu (nombre de collisions efficaces, trajectoire dans la détente après désorption, températures finales) ainsi qu’une meilleure compréhension des processus de refroidissement et de relaxation conformationnelle. Les études conformationnelles ont été appliquées à deux systèmes modèles choisis pour étudier des interactions structurantes intervenant dans les protéines : les interactions protéines-solvant et les interactions hydrophobes. L’étude des complexes (Ac­Phe­NH₂ : H₂O) et (Ac­Phe­NHMe : H₂O) ont permis d’identifier les sites de solvatation préférentiellement occupés par une molécule d’eau et ainsi de proposer des mécanismes de formation des complexes dans la détente supersonique. Le rôle structurant très fort des interactions hydrophobes entre chaînes latérales aromatiques a pu être mis en évidence en étudiant deux peptides modèles contenant un enchainement de plusieurs acides aminés phénylalanine : Ac­Phe­Phe­NH₂ et Ac­Phe­Phe­Phe­NH₂. L’étude des dynamiques de relaxation du premier état excité ππ*, réalisée sur divers peptides modèles, a permis de démontrer la présence d’effets conformationnels importants. Des calculs de chimie quantique (TDDFT et CC2) réalisés sur les systèmes Ac-Phe NH₂ et Ac-Phe NHMe ont montré que cet effet pouvait être expliqué par un transfert d’excitation depuis le cycle aromatique présent sur la chaîne latérale vers les liaisons peptidiques de la chaine principale. Enfin, l’ajout d’une molécule d’eau sur le peptide Ac-Phe NH₂ semble ouvrir de nouvelles voies ultrarapides de relaxation non-radiative. / The very good spectral resolution of laser spectroscopy achieved in the gas phase is a powerful tool to study the folding properties and the hydrogen bonding network of flexible molecules such as small peptide chains. The experimental strategy used in this work to determine the structural properties of these systems is based on IR-UV double resonance spectroscopy and combines laser vaporisation with a supersonic expansion. The final assignment then requires a comparison between experimental spectra and DFT-D calculations. The conformational selectivity brought by gas phase laser spectroscopy also makes it possible to study the dependence of the dynamics of relaxation of electronic excited states of model peptides with their secondary structure by using pump-probe methods or fluorescence detection. The issue of the conformational population of flexible molecules cooled in a supersonic expansion is a difficult issue, often disregarded due to the nonequilibrium processes that control the distributions experimentally observed. A statistical model was developed in order to describe this collisional cooling and the isomerizations experienced by one molecule during the expansion. These calculations were consistent with the experimental trends in the population ratios between conformations, they have provided orders of magnitude for the different processes involved (number of collision, trajectory in the expansion after desorption, final temperatures) and a better understanding of the cooling processes and the conformational relaxation. The conformational studies have been applied to two model systems selected to investigate structural interactions involved in proteins: protein-solvent interactions and hydrophobic interactions. The microhydrated protected phenylalanines (Ac­Phe­NH₂ : H₂O) and (Ac­Phe­NHMe : H₂O) were used to locate the solvation sites preferentially occupied by a water molecule, which then helped to propose a mechanism for the formation of hydrates in the supersonic expansion. The strong structuring properties of hydrophobic interactions between aromatic side chains has been revealed by studying two model peptides containing a sequence of phenylalanine amino acids: Ac­Phe­Phe­NH₂ and Ac­Phe­Phe­Phe­NH₂. A comparative study of the relaxation dynamics of the first ππ* excited state performed on various model peptides has demonstrated the existence of a strong conformational effect. TDDFT and CC2 calculations carried out on the protected phenylalanines have shown that this effect could be explained by an excitation transfer from the aromatic ring of a side chain toward a peptide bond of the backbone. Finally, adding a water molecule to the protected phenylalanine is also found to open new ultrafast channels of nonradiative deactivation.

This thesis presents work examining the characteristics and applicability of quantum cascade lasers. An introduction is given explaining both the desire for a widely tunable, narrow bandwidth device working in the midinfrared, as well as detailing the ways in which quantum cascade lasers (QCLs) fulfill these requirements. The development and manufacture of QCLs are then discussed. The experimental section of this thesis is then split into three parts. Chapter 2 concerns the characterisation and application of several pulsed QCLs. The intrapulse mode of operation is employed and the effect of the resulting rapid frequency chirp upon molecular spectra is investigated in the form of rapid passage signals. The evolution of said rapid passage signals is then investigated as a function of chromophore pressure and identity, with different QCLs, chirp rates, and optical path lengths. The prospect of producing population transfer with chirped lasers is discussed. Chapters 3, 4, and 5 are then concerned with the application and characterisation of continuous wave QCLs. In these chapters a widely tunable commercially produced EC-QCL is utilised as well as two DFB QCLs, one of which is used in tandem with a home-made mount and temperature controller. In Chapter 3 a number of sensitive detection techniques are compared with the employment of wavelength modulation spectroscopy, long path cells and optical cavities, and the narrow bandwidth of QCLs utilised to determine a previously unknown spectral constant of DBr. Chapters 4 and 5 then utilise the high power of an external cavity quantum cascade laser in sub-Doppler Lamb-dip and polarisation spectroscopy measurements and then a pump-probe experiment. The laser linewidth is investigated on a millisecond timescale returning a current noise limited value of c.a. 2 MHz and the fundamental linewidth of the device investigated by altering the injection current. Chapter 5 is concerned with the pump-probe experiment, directly measuring the hot band absorption in a ladder like transition (R(6.5)$_\frac{1}{2}$ $v=1\leftarrow0$ and P(7.5)$_\frac{1}{2}$ $v=1\leftarrow0$). The Bennett peak in the hot band is observed with a DFB-QCL swept at $\sim 0.15$ MHz ns$^{-1}$ and is seen not just as a pump bandwidth limited lineshape, but as a highly velocity selected rapid passage signal. The effect of pressure, pump and probe scan rate and power upon this rapid passage signal is also studied. It is further noted that rapid thermalisation occurs within $v=1$ such that at pressures above c.a. 30 mTorr a broad NO doublet absorption is observed beneath the Bennett peak from which a total population transfer of c.a. $16 \%$ can be estimated. Finally an experiment is discussed in which this population transfer could be increased for use in secondary applications. Chapter 6 then presents initial measurements with two prototype pulsed 3.3 \si{\micro\metre} QCLs considering the prospects of such devices. A Fabry-P\'rot device is first studied using a Fourier transform spectrometer and temperature tuning used to produce a spectrum of the Q-branch of CH$_4$ around 3025 cm$^$. Experiments are then performed using a DFB QCL investigating the chirp rate of the system as an indicator of the rate of heat accumulation within the system. Heat management is of particular consideration when the sea-change is made from pulsed to continuous devices. For this device absorption spectra of two CH$_4$ transitions at 2971 cm$^$ are used to determine the chirp rate, which is found to be c.a. 1.8 GHz ns$^$, at least an order of magnitude higher than that of the longer wavelength pulsed devices considered in Chapter 2.