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Design, construction, and characterization of an ultra-sensitive, high-precision fast ion-beam spectrometer for the study of molecular ions

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Title: Design, construction, and characterization of an ultra-sensitive, high-precision fast ion-beam spectrometer for the study of molecular ions
Author(s): Mills, Andrew
Advisor(s): McCall, Benjamin
Contributor(s): Scheeline, Alexander; Eden, James G.; Gruebele, Martin H.
Department / Program: Chemistry
Discipline: Chemistry
Degree Granting Institution: University of Illinois at Urbana-Champaign
Degree: Ph.D.
Genre: Doctoral
Subject(s): Physical Chemistry Analytical Chemistry Frequency Comb Gas Phase Spectroscopy High Resolution Spectroscopy N2+ Molecular Ions
Abstract: This dissertation details the design, construction and characterization of a sensitive, fast ion beam spectrometer to record the absorption spectra of molecular ions. It is the first work to use the noise immune cavity-enhanced optical heterodyne molecular spectroscopy (NICE-OHMS) technique in conjunction with a fast ion beam. Additionally, it is the first work outlining direct electronic spectroscopy in an ion beam. Direct spectroscopy of a fast molecular ion beam offers many advantages over competing techniques, including the generality of the approach to any molecular ion, the complete elimination of spectral confusion due to neutral molecules, and the mass-identification of individual spectral lines. Fast ion beam spectroscopy allows for sub-Doppler absorption spectra to be recorded. While some techniques, such as velocity modulation spectroscopy, provide semi-rigorous ion/neutral discrimination, the spectrometer employed in this dissertation provides a rigorous, spatial discrimination against background neutrals. The major challenge is the intrinsic weakness of absorption and dispersion signals resulting from the relatively low ion density in the beam. The primary goal of designing this instrument is to enable acquisition of cold, rotationally-resolved, gas-phase spectra of molecular ions in the 2 - 5 μm region. For this dissertation in particular, the goal was to build an instrument able to acquire the uncooled, rotationally-resolved, gas-phase spectrum of N2+ in the near IR at 920 nm, using an ion beam. By characterizing this instrument in terms of linewidth and sensitivity, we are confident that we can move on to the next steps, of building a difference frequency generation laser, performing mid-IR spectroscopy, and implementing supersonic cooling. The spectra obtained in the mid-IR will be valuable for applied astronomical and fundamental spectroscopic research. Direct infrared spectroscopy of an ion beam was pioneered by the Saykally group in 1989-1990, but has not been attempted since that time. Our ion beam instrument uses a Ti:Sapphire laser to produce near-IR (NIR) radiation at 920 nm. NICE-OHMS is set up using two electrooptic modulators and a medium finesse cavity (F=450) with a free spectral range of 113 MHz. The optical cavity increases the path length of absorption, and the heterodyne spectroscopy reduces the noise to near the shot noise limit. The laser is calibrated to sub-MHz accuracy using an optical frequency comb. A fast ion beam instrument was constructed, using the most recent advances in ion beam technology, including cylindrical benders and a beam-modulated time-of-flight mass spectrometer. The cylindrical benders help to maximize the ion density in the interaction region, and the mass spectrometer offers improved resolution and online mass analysis. The ion beam is made collinear with the laser beam inside of the NICE-OHMS cavity. The NIR NICE-OHMS setup has a noise equivalent absorption of 2x10^−11 cm^−1 Hz^−1/2, which is 50 times more sensitive than the previous instrument. Several transitions of the 1 – 0 band of the A ^2 Pi_u – X ^2 Sigma^+_g Meinel system of N_2^+ with linewidths of ~120 MHz have been measured. An optical frequency comb is used to calibrate the absolute value of the transition frequencies to within 8 MHz of the values obtained for a sub-Doppler calibrated positive column discharge. The rotational temperature of the ions in the plasma was measured to be ~750 K.
Issue Date: 2012-02-06
Genre: thesis
URI: http://hdl.handle.net/2142/29803
Rights Information: Copyright 2011 Andrew Allen Mills.
Date Available in IDEALS: 2012-02-06
Date Deposited: 2011-12
 

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