High-Resolution Pulsed Slit Jet Spectrometer with QCL and OPO probes for Infrared Spectroscopy on van der Waals Clusters
Creators: name: Rezaei, Mojtaba
Publisher: name: Graduate Studies
Year: 2014
Subjects: subject: Physics--Molecular
subject: Optics
subject: high-resolution IR spectroscopy
subject: supersonic jet
subject: van der Waals cluster
subject: Quantum Cascade Laser
subject: Optical Parametric Oscillator
subject: Signal processing

description: Infrared absorption spectra of weakly-bound van der Waals complexes provide the information necessary to develop intermolecular potential energy surfaces. These surfaces can then be used to better understand condensed phases. The work in this thesis concentrates on retrofitting a high resolution supersonic pulsed slit jet spectrometer with a continuous wave external cavity Quantum Cascade Laser (QCL) and an Optical Parametric Oscillator (OPO). Various techniques were implemented to make the QCL and OPO well suited for high-resolution spectroscopy of weakly-bound van der Waals molecular clusters. A new data acquisition scheme was implemented to increase the effective scanning speed,which is limited by the PZT element in QCL and OPO. In addition, two new digital signal processing techniques were developed to eliminate the effect of QCL jitters and OPO power fluctuations. These techniques and operation of the apparatus will be discussed in detail. Performance of the spectrometer is illustrated by recording combination bands of the van der Waals complexes (N2O)2 and CO2−C2D2 in the N2O nu1 and CO2 nu3 fundamentals, respectively. These observations demonstrate the successful application of a continuous wave external cavity quantum cascade laser and an optical parametric oscillator in our supersonic jet expansion apparatus and confirm that QCL and OPO are reliable sources for high-resolution and high sensitivity spectroscopy of van der Waals molecular complexes. Several weakly-bound van der Waals molecular complexes formed in a pulsed supersonic slit-jet expansion are studied using the spectrometer. The infrared spectra of CS2 clusters are measured in the CS2 nu3 fundamental (C-S asymmetric stretch) region. Analyzing these spectra indicates a tendency for CS2 to form highly symmetric clusters. The CS2 dimer turns out to have a non-planar cross-shaped structure with D2d symmetry, very different from most dimers in the isovalent "CO2 family" which are planar with mostly parallel monomers. The CS2 trimer exhibits a twisted barrel-shaped structure, a bit like the non-cyclic CO2 trimer but with higher symmetry (D3), in contrast to the barrel-shaped trimers of CO2, N2O, and OCS, which do not exhibit threefold rotational symmetry axes. Intensity alternation observed in the CS2 tetramer spectrum is consistent with D2d symmetry, and we believe the tetramer structure is that of a "staggered barrel", similar to the oblate isomer of the N2O tetramer. Finally, excellent signal-to-noise ratio in the experimental setup enabled us to observe combination bands of CO2−C2H2, CO2−C2D2, N2O−CO2, N2O−OCS, and N2O−N2 van der Waals complexes. Information concerning the low-frequency intermolecular modes were extracted from these combination bands. The extracted information is especially useful for probing the intermolecular potential surfaces and testing the ab initio calculations.