Improved spectroscopy of molecular ions in the mid-infrared with up-conversion detection

Heterodyne detection, velocity modulation, and cavity enhancement are useful tools for observing rovibrational transitions of important molecular ions.\footnote{K.N. Crabtree, J.N. Hodges, B.M. Siller, A.J. Perry, J.E. Kelly, P.A. Jenkins II, and B.J. McCall, Chem. Phys. Lett. 551 (2012) 1-6.} We have utilized these methods to investigate a number of molecular ions, such as H$_3^+$, CH$_5^+$, HeH$^+$, and OH$^+$.\footnote{A.J. Perry, J.N. Hodges, C.R. Markus, G.S. Kocheril, and B.J. McCall, J. Mol. Spec. 317 (2015) 71-73.}\footnote{J.N. Hodges, A.J. Perry, P.A. Jenkins II, B.M. Siller, and B.J. McCall, J. Chem. Phys. 139 (2013) 164291.}\footnote{A.J. Perry, J.N. Hodges, C.R. Markus, G.S. Kocheril, and B.J. McCall. 2014, J. Chem. Phys. 141, 101101}\footnote{C.R. Markus, J.N. Hodges, A.J. Perry, G.S. Kocheril, H.S.P. M{\"u}ller, and B.J. McCall, Astrophys. J. 817 (2016) 138.} In the past, parasitic etalons and the lack of fast and sensitive detectors in the mid-infrared have limited the number of transitions we could measure with MHz-level precision. Recently, we have significantly reduced the amplitude of unwanted interference fringes with a Brewster-plate spoiler. We have also developed a detection scheme which up-converts the mid-infrared light with difference frequency generation which allows the use of a faster and more sensitive avalanche photodetector. The higher detection bandwidth allows for optimized heterodyne detection at higher modulation frequencies. The overall gain in signal-to-noise from both improvements will enable extensive high-precision line lists of molecular ions and searches for previously unobserved transitions