Radiative Lifetimes of Some3pand3p′Levels of Oii

Abstract

The mean radiative lifetimes of several excited levels of O ii have been measured using ${\mathrm{O}}_{2}^{+}$ and ${\mathrm{O}}^{+}$ ion beams in the 20-30-keV energy range. The levels were populated by directing the beams through a differentially pumped collision cell containing gaseous helium at pressures of a few mTorr. The decays in the intensities of selected electronic transitions, arising from the levels of interest, were obtained as a function of the distance $x$ downstream from the exit aperture by using a \textonehalf{}-m grating monochromator equipped with a photomultiplier detector. These decay curves were fitted to an expression of the form $I=A{e}^{\ensuremath{-}\frac{x}{v{\ensuremath{\tau}}_{k}}}+\ensuremath{\Sigma}{j}^{}{B}_{j}{e}^{\ensuremath{-}\frac{x}{v{\ensuremath{\tau}}_{j}}}+C{e}^{\ensuremath{-}{k}^{\ensuremath{'}}x}$, where $A$, ${B}_{j}$, and $C$ are constants, $v$ is the speed of the ions, ${\ensuremath{\tau}}_{k}$ is the lifetime of the level $k$ of interest, ${\ensuremath{\tau}}_{j}$ is the lifetime of a level $j$ which cascades into level $k$, and ${k}^{\ensuremath{'}}$ is a decay constant associated with the drop in gas pressure downstream from the exit aperture of the collision chamber arising from gas streaming through the aperture. The intensity data and the computed fits were plotted either by hand or by a plotter connected to a computer. The number of exponentials in a given fit was adjusted until a good fit was obtained, as determined by visual inspection in conjunction with a ${\ensuremath{\chi}}^{2}$ test. Lifetimes (in nsec) obtained in this way are as follows: $\ensuremath{\tau}(3{p}^{4}{D}_{\frac{7}{2},\frac{1}{2}}^{0})=15.0\ifmmode\pm\else\textpm\fi{}0.8$, $\ensuremath{\tau}(3{p}^{4}{D}_{\frac{5}{2}}^{0})=14.9\ifmmode\pm\else\textpm\fi{}0.4$, $\ensuremath{\tau}(3{p}^{4}{D}_{\frac{3}{2}}^{0})=15.6\ifmmode\pm\else\textpm\fi{}0.5$, $\ensuremath{\tau}(3{p}^{4}{P}_{\frac{5}{2},\frac{3}{2}}^{0})=8.5\ifmmode\pm\else\textpm\fi{}0.6$, $\ensuremath{\tau}(3{p}^{2}{D}^{0})=11.6\ifmmode\pm\else\textpm\fi{}0.3$, $\ensuremath{\tau}(3{p}^{2}{P}^{0})=9.2\ifmmode\pm\else\textpm\fi{}0.1$, $\ensuremath{\tau}(3{p}^{\ensuremath{'}}^{2}F^{0})=12.1\ifmmode\pm\else\textpm\fi{}0.3$, and $\ensuremath{\tau}(3{p}^{\ensuremath{'}}^{2}D^{0})=9.2\ifmmode\pm\else\textpm\fi{}0.4$. These results are believed to be good to 10% or better. In addition to these results, measurements were also made on several other levels with lifetimes in the 6-nsec range. The reported values for the latter levels are upper limits to the actual lifetimes and are otherwise unreliable because of the possibility of significant errors arising from gas streaming. An extensive discussion of the treatment of possible errors from gas streaming, cascading, and blending of spectral lines is given.