Photoionization of an autoionizing level of atomic oxygen

Abstract

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}$${\mathit{D}}_{2}$ level by photodissociation of ${\mathrm{NO}}_{2}$ or ${\mathrm{N}}_{2}$O and excited to the ${(}^{2}$D\ifmmode^\circ\else\textdegree\fi{})3p $^{1}$${\mathit{P}}_{1}$ and $^{1}$${\mathit{F}}_{3}$ levels by two-photon absorption. These levels lie above the ${\mathrm{O}}^{+}$ $^{4}$S${\mathrm{\ifmmode^\circ\else\textdegree\fi{}}}_{3/2}$ ionization threshold but are forbidden to autoionize in LS coupling. Although the autoionization rates are thus very small, photoelectron spectra demonstrate that both levels do autoionize into the $^{4}$S${\mathrm{\ifmmode^\circ\else\textdegree\fi{}}}_{3/2}$ continuum. At laser intensities of \ensuremath{\sim}3\ifmmode\times\else\texttimes\fi{}${10}^{9}$ W/${\mathrm{cm}}^{2}$, photoionization of the $^{1}$${\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.