With this award from the Major Research Instrumentation (MRI) program Professor Arthur Suits from Wayne State University and colleague Robert Field from the Massachussetts Institute of Technology will develop a spectrometer that will join two powerful technologies: ultrabroad band chirped-pulse, Fourier-transform microwave spectroscopy and pulsed uniform supersonic flows obtained from a Laval (convergent-divergent) nozzle. This combination has the potential to offer a nearly universal detection method for near-nascent species thermalized at 30-50 K, that can deliver quantitative measurements of reaction intermediates, excited states, and vibrational distributions. The proposal is aimed at enhancing research training and education at all levels, especially in areas such as (a) detection of isomer and conformers, (b) quantitative detection and spectroscopic charaterization of unstable reaction intermediates, product vibration distributions, and molecular excited states, as well as (c) reaction dynamic, kinetics, combustion and atmospheric chemistry, physics and astrochemistry.
This unique spectrometer combines two powerful new techniques using sophisticated electronics developed for the communications industry. The combination results in something new: an instrument that can detect absolute numbers of trace amounts of unstable reaction products or excited states of molecules, and at the same time determine their structure: combining spectroscopy (giving structural information) and dynamics (revealing details of the reactive events). These unique attributes indicate that the new instrument will have broad applicability across many fields. During development, construction, testing and commissioning of the instrument many students will participate.
In this project we successfully developed a powerful new instrument that combines two emerging technologies: chirped-pulse microwave spectroscopy with a pulsed uniform supersonic flow. The former technique, pioneered by the Pate group at the University of Virginia, offered a means of recording spectra for a very broad range of molecules that yields detailed structural information hundreds to thousands of times faster than before. This suggested that the technique could be used to study the reactive properties of molecules and to characterize important reaction intermediates, if they could be prepared cold and at high density. The second new technique, uniform supersonic flows, offers an ideal environment for this purpose. Very low temperatures are needed at high density and high volume, and this is precisely what is given in these flows. This combination yields a nearly universal detection method that delivers isomer and conformer specific, quantitative detection and spectroscopic characterization of unstable reaction products and intermediates, product vibrational distributions, and molecular excited states. We refer to the new instrument as a CPUF (Chirped-Pulse, Uniform Flow) spectrometer. In the course of developing the new instrument combining these two technologies, we pursued several innovative strategies: We developed a new high-throughput fast pulsed valve that allowed us to achieve far lower temperatures than previously reported for pulsed uniform flows, and we designed a novel polycarbonate flow chamber to minimize microwave reflections. We documented the extraordinary performance of the flow system in one paper, and the capability of the combined system to perform photodissociation and reaction dynamics studies in another paper. To that end we showed that photodissociation of sulfur dioxide gave SO molecules whose vibrational distribution was recorded faithfully by the instument. In a second study, we showed that we could perform a chemical reaction, CN radical with acetylene, and detect the product directly in the flow using rotational spectroscopy. Other groups have been closely following our progress and are interested in replicating our successes. In developing and building this instrument we trained graduate students and post-docs in instrument development, in state-of-the-art microwave technology, in spectroscopy and in reaction dynamics. The work also involved exchanges between MIT and Wayne State, as well as informal collaboration with a group in France.