Multiphoton optical and photoelectron spectroscopy of 4s-3d and 5s-4d Rydberg complexes of O2

The 4s-3d and 5s-4d Rydberg complexes of O₂ have been observed by (2+1) resonance-enhanced multiphoton ionization spectroscopy. All states show sharp rotational structure. Vibrational analysis (and rotational analyses of two bands) shows that all observed states are purely Rydberg in character with no detectable perturbations. Photoelectron spectra of the more intense peaks show the Δv=0 transition to be dominant, but large contributions from Δv≠0 transitions are observed and attributed to features in the ionization continuum. The Δv≠0 transitions are relatively weak for bands ascribed to excitation of a 3dδ Π_g state and provide a method for producing nearly pure vibrational preparations (v⁺=0-3) of O⁺₂ ions in the ground electronic state. Large amounts of O⁺ ions are observed and possible mechanisms of formation are discussed.