Electric field effects in the near-threshold photoionization spectrum of nitric oxide

Zero-kinetic-energy–photoelectron spectroscopy (ZEKE-PES) and time-of-flight mass spectrometry are used to investigate the near-threshold, two-color photoionization of NO via the A 2Σ+, v=0 level. Pulsed-field ionization of Rydberg states within 20 cm−1 of the ionization threshold is shown to proceed via a diabatic mechanism. Particular emphasis is given to the effect of predissociation on the signal produced by delayed field ionization. A small (∼3 V/cm) dc electric field is shown to produce a dramatic decrease in the field ionization signal in both the NO+-ion and ZEKE-electron channels. This decrease is thought to be due to an increase in the predissociation rate caused by the dc electric field. The implications of these experiments for mass-analyzed threshold ionization are discussed.