High-accuracy and high-precision infrared spectroscopy of fundamental molecular ions using noise immune cavity enhanced optical heterodyne velocity modulation spectroscopy

Solving the Schrodinger equation for molecules is anything but trivial given the multi-body nature of the problem, and exact solutions are not achievable ab initio. Therefore, quantum mechanical calculations on molecules require approximate treatment in order to calculate quantities such as energy levels and spectra. High accuracy calculations that achieve ``spectroscopic accuracy" (i.e. energy levels and spectra that are accurate to < 1 cm-1) are only possible for a select few molecular systems, and as such these systems are of great fundamental interest as they serve as test cases for state-of-the-art computational methods and strategies for solving the molecular Schrodinger equation. However, in order for these pioneering theoretical approaches to be validated, high-quality experimental spectroscopic data are required for the purpose of benchmarking the calculations. Many of these fundamentally interesting systems are molecular ions making spectroscopic investigations on them particularly challenging due to the difficulty in creating significant quantities of them. To overcome this obstacle, sensitive spectroscopic techniques are required. Achieving high spectroscopic sensitivity for the purpose of studying molecular ions requires minimizing technical noise while enhancing absorption signals and finding a way to discriminate between ionic and neutral signals. These goals can be accomplished by combining heterodyne modulation, cavity enhancement, and velocity modulation in a spectroscopic technique called Noise-Immune Cavity-Enhanced Optical Heterodyne Velocity Modulation Spectroscopy (NICE-OHVMS). This thesis will describe efforts to conduct high-resolution and high-precision infrared spectroscopic surveys of molecular ions using NICE-OHVMS. Two ions in particular, the HeH+ and H3+ ions which represent the simplest heteronuclear and simplest polyatomic molecules respectively, are the main focus of this work and record-breaking precision measurements of their infrared spectra are presented. In doing so, rovibrational transition rest frequencies with MHz-level level uncertainty (an improvement over previous measurements by two orders of magnitude) are presented. It is envisioned that these measurements will equip ab initio theorists with improved benchmark values that can be used to test the validity of new theoretical approaches aimed at calculating molecular potential energy surfaces and spectra