Observation of a Four-Electron Auger Process in Near-$K$-Edge Photoionization of Singly Charged Carbon Ions

Single, double, and triple ionization of ${\mathrm{C}}^{1+}$ ions by single photons is investigated in the energy range of 286–326 eV, i.e., in the range from the lowest-energy $K$-vacancy resonances to well beyond the $K$-shell ionization threshold. Clear signatures of ${\mathrm{C}}^{1+}(1s2s^{2}2p^2{}^{2}D,{}^{2}P)$ resonances are found in the triple-ionization channel. The only possible mechanism producing ${\mathrm{C}}^{4+}(1s^2)$ via these resonances is direct triple-Auger decay, i.e., a four-electron process with simultaneous emission of three electrons.

Figure 1

Single- (a), double- (b), and triple-Auger (c) decay of a $K$-shell excited B-like atom.

Figure 2

Absolute cross sections for single and double ionization of ${\mathrm{C}}^{1+}$ ions by single photons. The photon energy axis has a break in the range where no resonances are expected. The experimental data are shown as small dots with statistical error bars. They are connected by a solid line with shading. The cross section data for single ionization in the high-energy region were multiplied by a factor of 10 and displayed again with a vertical offset. Important resonance groups are identified by their configurations.

Figure 3

Cross sections for triple ionization (shaded circles) of ${\mathrm{C}}^{1+}$ ions by single photons at energies around the $K$ edge. The energy resolution was 130 meV in this case. Also shown is the cross section (from Fig. 2) for double ionization multiplied by a factor of 0.009 (gray solid line with light shading). Members of the $1s2s2p^{2}nl$ Rydberg sequence are identified by their principal quantum number $n$. The lowest threshold for $K$-shell ionization (producing $1s2s^{2}2p\hspace{0.17em}{}^{3}P$ levels) was calculated using the cowan code [22]. It is shown as a (blue) vertical bar at 315.46 eV. The $1s2s2p^{2}nl$ Rydberg series converges to the series limit indicated by the (blue) vertical bar at 318.2 eV.

Figure 4

Cross sections for single, double, and triple ionization of ground-state ${\mathrm{C}}^{1+}$ ions by single photons with 92 meV bandwidth. The resonances seen in all the observed channels are associated with $K$-shell-excited ${\mathrm{C}}^{1+}(1s2s^{2}2p^2{}^{2}D,{}^{2}P)$ terms. The percentages given for the areas of the peaks indicate the branching factors for single-, double-, and triple-Auger decay.