Autoionizing Rydberg states of NO in strong electric fields
We report on an investigation on autoionization of Rydberg electrons of the nitric oxide molecule in strong, static electric fields. The excitation was done via various rotational states of the A 2S+intermediate state (v' = 0) and with polarization parallel or perpendicular to the electric field. The splitting of the autoionizing Rydberg states into overlapping Stark manifolds is resolved for excitation above the saddlepoint created by the field. We observe that the competing decay between photoionization and predissociation can lead to an incorrect interpretation of threshold energy. The photoionization spectrum of NO Rydberg series attached to various rotational thresholds is very similar owing to weak rotational coupling. The experimental results are accurately simulated by quantum calculations based on multichannel quantum defect theory (MQDT). A more intuitive formulation of the average behavior of the photoionization cross section is developed that accounts for the suppression of ionization near the threshold due to competing predissociating channels.