Nonlinear Raman spectroscopy in supersonic jets Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/6108vd68t

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  • The techniques of coherent anti-Stokes Raman spectroscopy (CARS) and inverse Raman spectroscopy (IRS) are used to examine carbon dioxide aggregation in a supersonic jet expansion. To promote the formation of small clusters, a BMW injector valve is modified to permit its use as a pulsed nozzle at very cold temperatures. This results in a substantial enhancement in the concentration of small aggregates and permits an analysis of relative spectral intensities as a function of initial temperature, pressure and composition. CARS probing of CO₂ in jets reveals several new peaks in the region of the Fermi doublet, v₁ and 2v₂, which appear only in the cold, highly condensing conditions of the expansion. This study focuses upon several peaks which are red shifted from the 1285.5 cm⁻¹, vi monomer peak. The dependence of the relative peak intensities upon initial temperature and CO₂ concentration along with the relative shift from the monomer has enabled assignment of peaks at 1281.3 cm⁻¹, 1278.7 cm⁻¹ and 1275.3 cm⁻¹ to the dimer, trimer and higher polymers respectively. The dimer structure has been the subject of much controversy and the present results are enlightening in this regard. The CARS dimer peak is shifted -4.1 cm⁻¹ from the monomer resonance and comparison with a published infrared spectrum, showing a dimer band with a -0.7 cm⁻¹ shift, shows that the dimer adheres to the rule of mutual exclusion and must therefore, have a structure with a center of symmetry. Earlier experimental and theoretical studies gave conflicting evidence for one of two structures, a T-shaped configuration with C2v symmetry and an offset parallel structure with C₂h symmetry. Since only the C₂h form has a center of symmetry, it is concluded that the most stable arrangement for the dimer of CO₂ is the nonpolar, offset parallel structure. The Fermi doublet of CO₂ is also used to characterize the performance of a new high resolution IRS spectrometer assembled as part of this thesis work. A pulse amplified, actively stabilized ring dye laser provides the Stokes beam for IRS studies while the probe beam is a chopped single-mode Ar ion laser. Almost completely resolved static and jet spectra of the v₁ Q branch of CO₂ are reported and an instrumental resolution of 0.0033 cm⁻¹ is determined.
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