Resonant enhancement of pulsed-field ionization zero-kinetic-energy photoelectron spectra using microwave fields

In pulsed-field ionization zero-kinetic-energy spectroscopy (PFI-ZEKE), Rydberg states are typically field-ionized approximately 1 mu s after photoexcitation. During this waiting period optically accessible Rydberg states in zero field would decay due to autoionization or predissociation, however their lifetimes are lengthened by the electric fields due to ions or macroscopically applied fields. In this work, we experimentally demonstrate that the introduction of an appropriate microwave field can appreciably lengthen the lifetimes of Rydberg states of the NO molecule, which would otherwise decay by predissociation. The microwave field mixes optically accessible states with longer-lived noncore penetrating states. The resulting enhancement in field-ionization signal is seen at energies where the n, n + 1 spacing matches the microwave frequency. This method of suppressing predissociation introduces sharp, symmetric, resonances in the PFI-ZEKE spectra, at specific energies below the ionization thresholds, thus providing a new technique for the accurate determination of molecular ionization thresholds. (C) 2000 American Institute of Physics. [S0021-9606(00)01928-0].