Zeeman effect in the Rydberg states of NO, probed by resonantly enhanced multiphoton ionisation

This work is dedicated to the study of the magnetic field effects on the electronic structure of Rydberg states of a diatomic molecule, nitric oxide (NO). Rydberg states are probed by one and two color resonantly enhanced multiphoton ionisation, with the help of tunable pulsed dye lasers. The strength of the coupling between the Rydberg electron and the molecular core follows scaling laws with principal quantum number (n) and we have been able to observe within a constant magnetic field, weak, intermediate and 'strong' field regime, by probing higher lying Rydberg states. Thus the spin-orbit coupling dominates the structure of the ground state. In non penetrating Rydberg states, the electronic fine structure is determined by long range interactions, due to quadrupole field and ionic core polarizability. These anisotropic terms vanish rapidly with n and we observe for the first time the Pashen-Back effect for the electronic angular momentum in molecules. The competition between intra-molecular coupling and external field interaction is analysed - with the help of a perturbation treatment. Furthermore, a model based on the multichannel quantum defect theory is proposed, which can be applied to a larger energy scale. Finally, diamagnetic Zeeman effect is observed at high resolution for the first time in a molecule. The results are interpreted in terms of level shifts and mixing between nf and np Rydberg series by the quadratic Zeeman effect. (author)