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 wa s done via various rotational states of the A (2)Sigma(+) intermediate stat e (v(')=0) and with polarization parallel or perpendicular to the electric field. The splitting of the autoionizing Rydberg states into overlapping St ark manifolds is resolved for excitation above the saddlepoint created by t he field. We observe that the competing decay between photoionization and p redissociation can lead to an incorrect interpretation of threshold energy. The photoionization spectrum of NO Rydberg series attached to various rota tional thresholds is very similar owing to weak rotational coupling. The ex perimental results are accurately simulated by quantum calculations based o n multichannel quantum defect theory (MQDT). A more intuitive formulation o f the average behavior of the photoionization cross section is developed th at accounts for the suppression of ionization near the threshold due to com peting predissociating channels. (C) 1999 American Institute of Physics. [S 0021-9606(99)01128-9].