Double-resonance spectroscopy of transitions between autoionizing levels of atomic oxygen

Double-resonance spectroscopy is used to study transitions between autoionizing levels of atomic oxygen. The first laser is used to produce oxygen atoms in the 1s²2s²2p^{4 1}D₂ level by photodissociation of N₂O and to excite two-photon transitions from the ¹D₂ level to the long-lived (²D°)3p ¹F₃ autoionizing level. The second laser is used to probe transitions from the ¹F₃ level to higher-energy autoionizing levels as well as to the O^{+ 2}D° direct-ionization continuum. The transitions are detected by using a magnetic bottle electron spectrometer to monitor photoelectrons selectively with the kinetic energy specific to the process of interest. New transitions to the (²D°_5/2,3/2)ns' and nd' Rydberg-series members with n=16-31 are observed. The intensity dependence of these results has also been studied, and the implications of this relatively low-power measurement for above-threshold ionization are discussed.