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 2SIGMA+, 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 (approximately 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.