High-Precision Spectroscopy of CH5+: Demystifying the Ro-Vibrational Spectrum

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Benjamin McCall of the University of Illinois at Urbana-Champaig, is supported by an award from the Chemical Structure, Dynamics and Mechanisms program in the Chemistry division to implement two complementary experiments to study the high-resolution infrared spectrum of CH5+. This ion is the prototypical carbonium ion of the type so successfully used by George Olah and colleagues for superacid chemistry, and is also a critical intermediate in interstellar chemistry. A high-resolution ro-vibrational spectrum consisting of nearly 1000 lines was obtained over a decade ago, and remains completely unassigned. The first experiment aims to record the CH5+ spectrum at room temperature with sub-MHz precision and accuracy using cavity-enhanced velocity modulation spectroscopy, to enable the first experimental determination of ground-state energy level spacings using the method of four-line combination differences. Our second experiment aims to record the CH5+ spectrum using a novel fast ion beam spectrometer fed by a continuous supersonic expansion discharge source, to provide the first unambiguous, high-precision spectrum of CH5+ at low temperature; this in turn enables the experimental determination of the energies of the lowest rotational energy levels of CH5+. Taken together, these experiments should provide the first understanding of the ro-vibrational structure of this enigmatic molecule, and enable McCall and coworkers to construct an energy level diagram of the ground vibrational state of this ion for the first time. Ultimately, this new understanding should also enable the radio-astronomical detection of this important ion in dense interstellar clouds. This research, and associated activities, can be expected to have a broad impact in terms of enhancing interdisciplinary research, training undergraduate and graduate students, and integrating teaching and research. The overarching theme of these efforts is to bring together the chemistry, astronomy, and physics communities, and to encourage cross-fertilization between these three disciplines.