Laser Precision Spectroscopy on Hydrogen Isotopologues: Tritiated molecules and vibrationally excited H2

Hydrogen molecules plays an important role in research of fundamental physics as the subject of this thesis. H2 is the simplest neutral molecule consisting of two protons and two electrons. Because of its simplicity, it serves as a benchmark molecule for testing quantum chemical theories. Most of the precision measurements on molecular hydrogens are performed at the low lying ro-vibrational states of the stable isotopologues, H2, HD and D2. This thesis will present two distinct topics of precision spectroscopic measurement, 1) the heavier radioactive tritium-containing isotopologues of molecular hydrogen, HT, DT and T2 and 2) highly ro-vibrationally excited levels and quasi-bound state of molecular hydrogen to test theoretical calculations, in particular on the mass-dependent terms and the potential energy curves at large internuclear distances. In Chapter 1, a brief introduction of the theoretical framework to obtain the molecular hydrogen level energies is presented. The mass-dependent terms and the sensitivity of long-range potential energy curve at different ro-vibrational states will be discussed. Chapters 2 and 3 focus on the measurement of the vibrational interval in three tritium-containing isotopologues of molecular hydrogen. Spectroscopic studies on the static tritium-containing isotopologues gas samples with are performed using Coherent Anti-Stokes Raman Spectroscopy to measure the fundamental vibrational interval (v = 0 → 1). Several Q-branch lines (ΔJ = 0) of tritium-containing species were measured at 10^−4 cm^−1 uncertainty, presenting an over 100-times improvement. The experimental measurements on tritium-containing species were compared to results of non-adiabatic perturbation theory (NAPT) produced by Polish theorists. Chapters 4 - 7 cover the studies of highly vibrationally-excited and quasibound states of the ground electronic state of H2. The preparation of excited ro-vibrationally excited states in the ground electronic state of H2 is by the photolysis of H2S at 281.8 nm. In Chapter 5, the two highest vibrational levels v = 13 and v = 14 are probed through Doppler-free two-photon spectroscopy of F 1Σ+g - X 1Σ+g transitions. The assignments of the transitions are verified by recording excitation spectra from the populated F-states. The combination differences of transitions from v = 13 and v = 11 levels yield the vibrational intervals in the ground electronic state H2. A comparison with results from theoretical calculations produces good agreement. In Chapter 6 and 7, five quasi-bound states of H2 with lifetime in the range of 1 ns to 10 μs are probed through F - X transitions as well. The larger signal strength of these transitions compared to the other bound state transitions is explained by the enhanced Franck-Condon factor. Measurements of the F-X Q-branch and the S/O-branch (ΔJ = ±2) transitions connect all 5 observed quasi-bound resonances to give the X1Σ+gstate level intervals. The NAPT calculation scheme are extended to calculate the quasi-bound resonance energies. The calculated quasi-bound level intervals agree well with the observation.