Graduate Thesis Or Dissertation

 

High resolution ionization-detected simulated Raman spectroscopy Public Deposited

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  • A new experimental apparatus was built at Oregon State University to perform high resolution stimulated Raman spectroscopy in a pulsed molecular jet at state densities of the order of 10¹¹ cm⁻³. The technique uses a stimulated Raman step to first populate a vibrational/rotational level and then a resonantly enhanced multiphoton ionization (REMPI) step to subsequently probe the Raman pumped upper state. The resulting ions are accelerated down a Wiley-McLaren time of flight mass spectrometer (with mass resolution of 182 amu) and are detected with a home built microchannel plate detector, making mass selective Raman spectroscopy possible. Instrumental linewidths of 0.001 cm⁻¹ were demonstrated for benzene transitions, possibly being the highest resolution yet obtained for stimulated Raman spectroscopy. One reason for this narrow linewidth is that all the spectroscopy is performed in a cold molecular beam. This is advantageous because the rotationally resolved spectra are simplified to a great extent due to the low rotational temperatures (on the order of 10 K) and the collisional and Doppler contributions to the linewidths are reduced to less than the instrumental resolution. This form of ionization-detected stimulated Raman spectroscopy (IDSRS) was performed on N2 for the first time. This is important because nitrogen was ionized using a difficult 2 + 2 REMPI step for detection of the Raman signal. Even so, the detection limit was improved by a factor of 10⁴ over optical stimulated Raman spectroscopy. These results demonstrate that IDSRS is not limited to the aromatic molecular systems (which are easily ionized with 1 + 1 REMPI) that have been studied almost exclusively to date. Finally, the unusually high resolution of this experiment has enabled a qualitative study of the AC Stark splittings that come about through the induced dipole moment of the benzene molecule. To model the experimental spectra it was determined that good fits can only be achieved by including saturation and temporaVspatial broadening with the Stark splittings. Due to the unique power dependent lineshapes of the Stark split rotational transitions, the Stark effect can be a useful spectroscopic tool for Raman rotational assignments within a particular vibrational band.
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