Knowledge Bank

Spectroscopy of H$\mathrm{3+}$ is of fundamental interest for advancing \textit{ab initio} efforts to calculate spectra with high precision and accuracy. H$\mathrm{3+}$ is the simplest polyatomic ion, which is why it is an excellent benchmark for theory. In order to perform calculations with spectroscopic accuracy, relativistic and non-adiabatic corrections to the Born-Oppenhiemer approximation must be included; calculations with these considerations agree to within hundredths of a wavenumber. (1999), \textbf{110}, 5056--5064.} Increasing the precision of the calculations further will require a treatment of quantum electrodynamic effects, as has already been implemented for the diatomic case, \emph{J. Chem. Theor. Comp.} (2011), \textbf{7}, 3105--3115.} and testing these calculations will require higher-precision experimental data to guide \textit{ab initio} calculations. \vspace{1em} Noise Immune Cavity Enhanced Optical Heterodyne Velocity Modulation Spectroscopy, or NICE-OHVMS \emph{Opt. Express} (2011), \textbf{19}, 24822--7.} \emph{Chem. Phys. Lett.} (2012), \textbf{551}, 1--6.}, is a highly sensitive, highly precise technique that we have employed to observe transitions in the $\nu 2$ fundamental band of H$\mathrm{3+}$. It combines the advantages of cavity enhancement and heterodyne detection with the ion-neutral discrimination afforded by velocity modulation. Combining a cavity with a high power mid-infrared light source, we can saturate rovibrational transitions. The resulting Lamb dips may be fit in order to determine line centers to a much higher precision than is possible for ordinary Doppler broadened profiles. Additionally, a frequency comb is used to surpass the limited accuracy and precision of a wavemeter. Here we present the results from comb calibrated H$\mathrm{3+}$ transitions observed via NICE-OHVMS. Precision and accuracy of $\sim$ 1 MHz were achieved representing the most accurate and precise H$\mathrm{3+}$ line list that has been obtained to date.