High Precision Energy Levels for the Simplest Polyatomic Molecule (H3+)

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H3+ is the simplest molecule with more than two atoms: it consists of three protons bound by two electrons. As such, it is an ideal benchmark system for testing state-of-the-art theoretical calculations of molecular energy levels. H3+ is significant not only for these fundamental reasons, but also because it has been widely observed in a variety of astronomical environments, including the atmospheres of Jupiter and Saturn and in clouds of gas between stars. These observations are made by precisely determining the colors of light that are emitted by the molecule, a technique called "spectroscopy". The latest calculations can reproduce the colors of H3+ to within the accuracy of current laboratory spectroscopy measurements, so further progress in theoretical development will require improvements in the laboratory instrumentation. The group supported by this grant plans to utilize a novel instrument that will allow them to measure H3+ colors to a precision that is 10,000 times better than previous work. The resulting dataset will serve as an enduring benchmark for high-level theoretical calculations, and will also assist astronomical observations of this important molecule. In the long run, it is anticipated that this work will help in the development of improved methods of controlling chemical reactions and thereby advance a significant fraction of the industrial sector.The work will use a recently-constructed instrument that allows routine sub-Doppler spectroscopy of molecular ions. This instrument will be upgraded to achieve a precision of approximately 30 kiloHertz, or one-millionth of a wavenumber, and then used to record high-precision spectra of H3+ in the mid-infrared. An analysis using combination differences and effective Hamiltonian fitting will be used to produce a set of experimentally determined energy levels of H3+, with a precision that is approximately four orders of magnitude better than previous work. These precise energy levels will permit the direct testing of non-adiabatic corrections to the Born-Oppenheimer approximation, various empirical corrections, and ultimately calculations of the Lamb shift for H3+.