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The goal of this project is to use chirped-pulse Fourier transform microwave (CP-FTMW) spectroscopy from 8.7 – 18.5 GHz as a probe of gas phase chemical reactions occurring at low pressures (1 – 100 mTorr) and at room temperature.  The original proposal discussed reactions between small unsaturated hydrocarbons (such as propene) and hydroxyl radicals, created from hydrogen peroxide by pulses of UV light.

These measurements utilize the core of a CP-FTMW spectrometer that was constructed at New College of Florida via start-up funding.  Funding from ACS-PRF has provided a UV light source, a 2-meter long “sample cell” of WRD-750 waveguide, and associated optical windows and vacuum components associated with transmitting light and molecules in the gas phase into and out of the sample cell.  To date, this award has also supported two undergraduates for 10 weeks of summer research.  Benjamin (Ben) Kriegel '11 worked in the lab during the summer of 2009, and Sophie Lang '13 worked in the lab during the summer of 2010.  Ben is currently applying to chemistry graduate schools, and Sophie intends to graduate with a B.A. in Chemistry in the spring of 2013.

Work began on this project during the summer of 2009.  Both the microwave and the optical components had long lead times and so a majority of the purchased equipment did not arrive until mid-August, with the last pieces arriving in October.  Despite these delays, we made progress during the summer by using room-temperature CP-FTMW spectroscopy to perform initial characterization of the unsaturated hydrocarbon reactants as well as the closed-shell forms of some of the potential products.  During that summer, Ben Kriegel collected and performed preliminary analysis on the room-temperature rotational spectra of seven molecules – propene, 1-butene, isoprene, 1-propanol, 2-propanol, 1-butanol, and 2-butanol.  As the New College spectrometer had only been commissioned a few months prior, these measurements also served as a general test of instrument sensitivity.  Figure 1 shows a few of the spectra that Ben collected.  As a side benefit of taking these measurements, the 1-propanol spectrum was of interest to a group of spectroscopists who are interested in looking for evidence of propanol in the interstellar medium.  The New College spectrometer operates in a relatively unique temperature and frequency range, and so our observations nicely supplemented existing room temperature data on this molecule from millimeter-wave instruments.  These results have been published in Physical Chemistry Chemical Physics.

During the summer of 2010, Sophie Lang worked in the lab both to implement schemes for microwave-microwave double resonance (to aid in assignment of the complex spectra we expect to find) and to incorporate the UV flash lamp into the experimental setup.  Her double resonance work, which effectively “tags” spectral features that share a quantum state with a pumped feature, was successful (Figure 2).  Reaction initiation with the flash lamp was less successful.  Her initial plan was to monitor depletion of the propene signal from a propene / hydrogen peroxide gas mixture as a function of UV exposure time.  Early attempts at these measurements were unsuccessful due to both the small propene microwave signal, resulting from its low dipole moment of ~ 0.3 D, and the relatively small number of photons produced by the UV flash lamp. 

Figure 1.  Panels A and B (expanded view of A) show the room-temperature microwave spectrum of propene, and panels C and D (expanded view of C) show the room-temperature microwave spectrum of 1-propanol.  Each spectrum represents roughly 16 hours of data collection.

Figure 2.  A microwave-microwave double resonance measurement on acetaldehyde.  The positive-going spectrum is collected in the presence of the double-resonance pulse, and the negative-going spectrum is collected in the absence of the double-resonance pulse.  The double-resonance pulse is tuned to the acetaldehyde n_t = 1, 5_14- – 5₁₅₊ transition at 14548.83 MHz, and has drastically modulated the intensity of the n_t = 1, 4_23- – 5_14- transition at 16439.93 MHz without significantly affecting other features in the spectrum.  Only transitions that share a common quantum state with the pumped transition (the n_t = 1, 5_14- state in this case) show an intensity modulation when the double-resonance pulse is applied.

To improve the signal-to-noise ratio of the propene microwave signal, we changed the polarizing pulse parameters and constructed a modified receiver circuit which allowed us to strike a balance between detection bandwidth and repetition rate.  Specifically, by reducing the polarizing pulse bandwidth from 5 GHz to 50 MHz and by judiciously choosing frequency sources and microwave filters to reduce the detection bandwidth from 5 to 1.25 GHz, we have been able to simultaneously quadruple the repetition rate and boost the propene signal by a factor of 10.  Combined, these modifications lead to a 20-fold enhancement of the signal-to-noise ratio of the propene transition in equal measurement time (Figure 3).

Figure 3.  Propene spectra acquired with a 5 GHz polarizing pulse, FID digitized at 10 GS/s (panel A) and with a 50 MHz polarizing pulse, FID digitized at 2.5 GS/s (panel B).  Both spectra represent 20 minutes of averaging; these two spectra represent extremes in the tradeoffs possible between spectral coverage (5 GHz vs. 50 MHz) and overall signal-to-noise ratio.

To address the light intensity issue, we will be using a 266 nm, Q-switched laser as our UV light source, courtesy of a colleague in physics, Prof. Mariana Sendova.  Over the next year, we plan to use this improved light source to make initial measurements on propene and extend them to 1-butene and isoprene.  Because hydrogen peroxide has very few rotational transitions in the frequency range of the spectrometer, it is difficult to follow its destruction with UV irradiation.  As such, in the coming year we also plan to look into slightly larger radical precursors that both dissociate under UV irradiation and have transitions that are easily monitored via their rotational spectra.