Threshold behavior in single-photon double ionization of atomic oxygen

The threshold behavior of the single-photon double-ionization cross section of atomic oxygen has been studied using vacuum uv radiation from a synchrotron storage ring. The double-ionization cross section appears to follow a power law ${\mathit{E}}^{\mathrm{\alpha }}$ (where E is the kinetic energy of the two electrons) from its threshold to about 2.0 eV above with an exponent α=1.08±0.03, which is consistent with Wannier’s theoretical value of 1.056. The cross section also shows the influence of the doubly excited 2s2${\mathit{p}}^3$${\mathit{nln}}^{\prime }$${\mathit{l}}^{\prime }$ (n,${\mathit{n}}^{\prime }$≳3) neutral states, among which the first three converge to the 2s2${\mathit{p}}^3$${(}^5$${\mathit{S}}^{\mathit{o}}$)4p${(}^6$${\mathit{P}}^{\mathit{o}}$) ionic state of oxygen.