15min:

JAMES N. HODGES, ADAM J. PERRY, BRIAN M. SILLER, Department of Chemistry, University of Illinois, Urbana, IL 61801; BENJAMIN J. MCCALL, Departments of Chemistry, Astronomy, and Physics, University of Illinois, Urbana, IL 61801.

Spectroscopy of H₃⁺ is of fundamental interest for advancing ab initio efforts to calculate spectra with high precision and accuracy. H₃⁺ 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. Increasing the precision of the calculations further will require a treatment of quantum electrodynamic effects, as has already been implemented for the diatomic case,\footnoteJ. Komasa, et al. J. Chem. Theor. Comp. (2011), \textbf7, 3105--3115. and testing these calculations will require higher-precision experimental data to guide ab initio calculations.

Noise Immune Cavity Enhanced Optical Heterodyne Velocity Modulation Spectroscopy, or NICE-OHVMS\footnoteB.~M. Siller, et al. Opt. Express (2011), \textbf19, 24822--7.\footnote K.~N. Crabtree, et al. Chem. Phys. Lett. (2012), \textbf551, 1--6., is a highly sensitive, highly precise technique that we have employed to observe transitions in the ₂ fundamental band of H₃⁺. 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 H₃⁺ transitions observed via NICE-OHVMS. Precision and accuracy of 1 MHz were achieved representing the most accurate and precise H₃⁺ line list that has been obtained to date.