SUB-DOPPLER ROVIBRATIONAL SPECTROSCOPY OF THE H3+ CATION AND ISOTOPOLOGUES

Molecular ions play a central role in the chemistry of the interstellar medium (ISM) and act as benchmarks for state of the art textit{ab initio} theory. The molecular ion H$_3^+$ initiates a chain of ion-neutral reactions which drives chemistry in the ISM, and observing it either directly or indirectly through its isotopologues is valuable for understanding interstellar chemistry. Improving the accuracy of laboratory measurements will assist future astronomical observations. H$_3^+$ is also one of a few systems whose rovibrational transitions can be predicted to spectroscopic accuracy ($<$1 cm$^{-1}$), and with careful treatment of adiabatic, nonadiabatic, and quantum electrodynamic corrections to the potential energy surface, predictions of low lying rovibrational states can rival the uncertainty of experimental measurementsfootnote{L. G. Diniz, J. R. Mohallem, A. Alijah, M. Pavanello, L. Adamowicz, O. L. Polyansky and J. Tennyson, textit{Phys. Rev. A} (2013), textbf{88}, 032506.}. New experimental data will be needed to benchmark future treatment of these corrections. _x000d_ _x000d_ Previously we have reported 26 transitions within the fundamental band of H$_3^+$ with MHz-level uncertaintiesfootnote{J. N. Hodges, A. J. Perry, P. A. Jenkins II, B. M. Siller, and B. J. McCall, textit{J. Chem. Phys.} (2013), textbf{139}, 164201.}footnote{A. J. Perry, J. N. Hodges, C. R. Markus, G. S. Kocheril, and B. J. McCall, textit{J. Mol. Spectrosc.} (2015), textbf{317}, 71--73.}footnote{A. J. Perry, C. R. Markus, J. N. Hodges, G. S. Kocheril, and B. J. McCall, textit{71st International Symposium on Molecular Spectroscopy} (2016), textbf{MH03}}. With recent improvements to our overall sensitivityfootnote{C. R. Markus, A. J. Perry, J. N. Hodges, and B. J. McCall, textit{Opt. Express} (2017), textbf{25}, 3709--3721.}, we have expanded this survey to include additional transitions within the fundamental band and the first hot band. These new data will ultimately be used to predict ground state rovibrational energy levels through combination differences which will act as benchmarks for textit{ab initio} theory and predict forbidden rotational transitions of H$_3^+$. We will also discuss progress in measuring rovibrational transitions of the isotopologues H$_2$D$^+$ and D$_2$H$^+$, which will be used to assist in future THz astronomical observations._x000d