Photoionization of an autoionizing level of atomic oxygen

Direct evidence for the photoionization of an autoionizing level of atomic oxygen is presented. Atomic oxygen is produced in the 1${\mathit{s}}^2$2${\mathit{s}}^2$2${\mathit{p}}^4$ ${}_{}{}^1{}_{}{}^{}$ ${\mathit{D}}_2$ level by photodissociation of ${\mathrm{NO}}_2$ or ${\mathrm{N}}_2$O and excited to the ${(}^2$D°)3p ${}_{}{}^1{}_{}{}^{}$ ${\mathit{P}}_1$ and ${}_{}{}^1{}_{}{}^{}$ ${\mathit{F}}_3$ levels by two-photon absorption. These levels lie above the ${\mathrm{O}}^{+}$ ${}_{}{}^4{}_{}{}^{}$S${\mathrm{°}}_{3/2}$ ionization threshold but are forbidden to autoionize in LS coupling. Although the autoionization rates are thus very small, photoelectron spectra demonstrate that both levels do autoionize into the ${}_{}{}^4{}_{}{}^{}$S${\mathrm{°}}_{3/2}$ continuum. At laser intensities of ∼3×${10}^9$ W/${\mathrm{cm}}^2$, photoionization of the ${}_{}{}^1{}_{}{}^{}$ ${\mathit{F}}_3$ level competes effectively with autoionization. The implications of this work for high-intensity multiphoton ionization studies of above-threshold ionization and multiple ionization are discussed.