High-resolution infrared spectroscopy of large molecules and water clusters using quantum cascade lasers

High-resolution infrared spectroscopy is a powerful tool for obtaining detailed information about molecules and molecular clusters. This dissertation presents several high-resolution spectroscopic studies of large molecules and water clusters which have been obtained using a quantum cascade laser (QCL) based infrared spectrometer coupled to a supersonic expansion source. The spectrometer operates near 8.5 ?m and has been developed for the purpose of obtaining a high-resolution gas phase spectrum of C60 for use in astronomical observations of this molecule. Details of the development of the spectrometer and initial results measuring methylene bromide are included as appendices in the dissertation. The spectrometer was used to attempt to observe the high-resolution spectrum of C60, but the spectrum has not yet been observed. Calculations of the expected signal-to-noise ratio reveal that the spectrum should have been observable, and the lack of an observed spectrum is attributed to inefficient vibrational cooling of C60 in the supersonic expansion. This finding is in contrast to previous studies which showed good cooling of polycyclic aromatic hydrocarbons (PAHs), including high-resolution spectroscopy of pyrene (C16H10) near 1184 cm-1 using the QCL spectrometer. Details of the high-resolution infrared spectrum of pyrene and the good cooling which was observed are included as an appendix in the dissertation. The QCL spectrometer has also been utilized to study two small deuterated water clusters: Ar-D2O and (D2O)2. Several bands of the bending mode of Ar-D2O were observed in Ar/D2O supersonic expansions. The observed bands were fit to a pseudo-diatomic model which treats D2O as a nearly free rotor within the complex, and accurate molecular constants were obtained. The deviations in the fits to this model ranged from 0.0002 to 0.0005 cm-1, and two previously unobserved bands of the complex were identified. The bending mode of the hydrogen bond donor of deuterated water dimer ((D2O)2) has also been observed with rotational resolution, which represents the first high-resolution study of the bending modes of water dimer. Two perpendicular sub-bands were observed and interpreted in the context of the tunneling motions which occur in the water dimer complex. Excitation of the donor bending mode was found to have little effect on the tunneling motion of the acceptor molecule, but caused significant perturbations on the tunneling motion which exchanges the roles of the hydrogen bond donor and acceptor. An accurate value for the gas phase vibrational frequency of the donor bend was also obtained, which was found to be quite similar to previous theoretical predictions obtained from an ab initio potential energy surface. Details of the construction of an external cavity QCL (EC-QCL) system are also presented. The EC-QCL provides a significant improvement in the frequency tunability of the spectrometer, increasing the tuning range from 1180--1200 cm-1 to 1135--1220 cm-1. The procedures for putting the system together and aligning it are outlined, and the details of implementing mode-hop free tuning of the laser are also presented. Mode-hop free tuning of the EC-QCL has been achieved over ~0.6 cm-1, which is sufficient to allow the EC-QCL to be used for high-resolution spectroscopy once the frequency stability of the laser can be improved.