High-Precision Rovibrational Saturation Spectroscopy of H2, HD and HT

This thesis presents significant advancements in precision spectroscopy of molecular hydrogen (H₂) and its isotopologues HD and HT by implementing Noise-Immune Cavity-Enhanced Optical Heterodyne Molecular Spectroscopy (NICE-OHMS) combined with a high-finesse cryogenic cavity and frequency comb calibration. The research achieved saturation spectroscopy of extremely weak rovibrational transitions, including the first observation of saturated dipole transitions in HD, the first saturation of quadrupole transitions in H₂, and precision measurements of vibrational transitions in HT. These accomplishments push the boundaries of experimental molecular spectroscopy, providing highly accurate benchmark data for testing quantum electrodynamics and refining theoretical models. The innovations developed, such as the vibration-insensitive cryogenic cavity and the novel NICE-OHMS implementation, enable near-shot-noise-limited sensitivity, setting new standards for precision in molecular spectroscopy.