Sensitive and high-precision rovibrational spectroscopy of molecular ions relevant to astronomical and quantum chemistry

High-precision spectroscopy of molecular ions has enabled new discoveries in astronomy and provided valuable benchmarks for ab initio calculations of molecular structure. Astronomers and theorists require accurate and precise laboratory measurements of rotational and rovibrational transition frequencies, which is particularly challenging for molecular ions. This is because ions are generated in small quantities within laboratory discharges, which are easily obscured by the far more abundant neutral molecules. This dissertation has investigated ways to improve highly sensitive and selective techniques in ion spectroscopy to precisely and accurately determine rovibrational transition frequencies of a number of astronomically and fundamentally important molecular ions. The sub-Doppler technique Noise Immune Cavity Enhanced Optical Heterodyne Velocity Modulation Spectroscopy (NICE-OHVMS) combines the sensitivity of cavity-enhanced methods with the selectivity of velocity modulation. In this work, a NICE-OHVMS instrument which suffered from parasitic etalons and a sub-optimal mid-infrared (mid-IR) detector was significantly improved by developing methods for reducing the interference fringes and improving the detector performance. The changes enabled measurements which were previously unattainable. Throughout this work, the NICE-OHVMS instrument was used to investigate three molecular ions: OH+, H3+, and D2H+. All three ions are highly relevant to interstellar chemistry and were missing pure rotational transition frequencies that were within the coverage of current observatories. The new rovibrational data were used to predict accurate rotational transition frequencies for all three species, improving the chances of future astronomical detections. Additionally, H3+ is the simplest polyatomic molecule, making H3+ and D2H+ important benchmarks for ab initio calculations. By performing an extensive survey of fundamental, hot, and overtone band transitions, highly accurate absolute energy levels of H3+ were calculated for the first time. The final investigation utilized action spectroscopy to study CH2NH2+, another astronomically important molecule which lacked any rotationally resolved spectroscopic data. This was performed using the action spectroscopy technique Laser-Induced Inhibition of Complex Growth (LIICG), where vibrational excitation prevented helium from attaching to the ions which were collisionally cooled to cryogenic temperatures (10 K) in multipole ion trap. Measurements of rovibrational transitions were used to predict pure rotational transition frequencies, which were then measured with the first demonstration of millimeter-wave / mid-infrared double resonance using LIICG