Measurement of Absolute Cross Sections for Excitation of the \textbf{2s$^{2} \doub ^{1}S - 2s2p ^{1}P^{o}$ Transition in O$^{4+}$}

Experimental electron excitation cross sections are reported for the 2s$^{2 1}$S - 2s2p$^{ 1}$P$^{o}$ transitions in O$^{4+}$ located at 19.689 eV. The JPL electron-cyclotron resonance ion source is utilized [1], along with the electron energy loss method, in a merged electron-ion beams geometry[2]. The center-of-mass interaction energies for the measurements are in the range 18 eV (below threshold) to 30 eV. Data are compared with results of a 26-term \textbf{\textit{R}}-matrix calculation that includes fine structure explicitly \textit{via} the Breit-Pauli Hamiltonian [3]. There is good agreement with theoretical results and with previous electron energy-loss measurements [3]. Clear resonance enhancement is observed in both experiment and theoretical results near threshold for this $^{1}$S - $^{1}$P$^{o}$ transition. J. Lozano and N. Djuric acknowledge support through the NASA-NRC program. This work was carried out at JPL/Caltech and was supported by NASA. [1] J. B. Greenwood, S. J. Smith, A.Chutjian, and E. Pollack, Phys. Rev. A \textbf{59} 1348, (1999). [2] A. Chutjian, Physica Scripta \textbf{T110}, 203 (2004). [3] M. Bannister et al., Int.J. Mass Spectrometry \textbf{192}, 39 (1999).