Multiple photodetachment of oxygen anions via $K$-shell excitation and ionization: Direct double-detachment processes and subsequent deexcitation cascades

Experimental cross sections for $m$-fold photodetachment ($m=2–5$) of oxygen anions via $K$-shell excitation and ionization were measured in the photon-energy range of 525–1500 eV using the photon-ion merged-beams technique at a synchrotron light source. The measured cross sections exhibit clear signatures of direct double detachment, including double $K$-hole creation. The shapes of the double-detachment cross sections as a function of photon energy are in accord with Pattard's [J. Phys. B 35, L207 (2002)] empirical scaling law. We have also followed the complex de-excitation cascades that evolve subsequently to the initial double-detachment events by systematic large-scale cascade calculations. The resulting theoretical product charge-state distributions are in good agreement with the experimental findings.

Figure 1

Experimental cross sections for double (a), triple (b), and quadruple (c) photodetachment of ${\mathrm{O}}^{-}$ ions. The open symbols represent fine scans over a narrower energy range and the more widely spaced full symbols represent coarse scans that extend to higher energies of up to 1500 eV (see Fig. 3). The vertical dash-dotted lines at 529.6, 543.9, and 572.5 eV mark the thresholds for $1s$ SD, $1s+2p$ DD, and $1s+2s$ DD respectively. The full lines result from a simultaneous fit of empirical cross section formulas for these three ionization processes to the three experimental fine-scan data sets (see text) with the individual contributions being represented by the dashed lines as explained by the legend in panel (a).

Figure 2

Experimental (full circles) and theoretical (open diamonds) product-ion charge fractions $f_q$ [Eq. (4)]. The photon-energy ($E_{\mathrm{ph}}$) axis is compressed toward higher energies using $x={log}_{10}(E_{\mathrm{ph}}/\mathrm{eV}-400)$ for the abscissa.

Figure 3

Results from coarse photon-energy scans over a wide range. The differently colored data points represent the experimental cross sections ${\sigma }_m$ for multiple [$m$-fold, Eq. (1)] detachment of ${\mathrm{O}}^{-}$ ions for $m$=2 (olive), $m$=3 (blue), $m$=4 (magenta), and $m$=5 (orange). The error bars account only for the statistical experimental uncertainties. The full lines represent a power law with the exponent $-3.3\pm 0.1$ as obtained from a fit to ${\sigma }_3$ (see text).

Figure 4

Cross-section difference resulting from the subtraction of the scaled power-law fit from the measured cross section ${\sigma }_5$ (see Fig. 3). The full line results from a fit of the Pattard formula [Eq. (2)] to the data points. The fit results are listed in Table 5.