Resonant double Auger decay in carbon $\mathit{K}$-shell excitation of CO

We have studied double Auger decay after $\text{C}1s\to 2{\pi }^{\ast }$ photoexcitation in gas phase carbon monoxide. Two distinct processes, namely direct double Auger decay and cascade double Auger decay, are identified and studied in detail using multiple coincidence techniques. Cascade Auger decay is shown to be the overall dominant process. Decay channels involving the dissociation of the molecule followed by autoionization of the oxygen fragments are observed.

Figure 1

Sketch of the double-velocity spectrometer. A: supersonic molecular beam. B and ${\text{B}}^{\prime }$: uniform field electrostatic electrodes (prime refers to the ion side). C and ${\text{C}}^{\prime }$: nonuniform field focusing lenses. D and ${\text{D}}^{\prime }$: equipotential flight tube. E and ${\text{E}}^{\prime }$: position sensitive detector.

Figure 2

Sketch of the EPICEA setup dedicated to Auger electron-ion-ion coincidences. A: gas inlet system providing a diffuse gas jet. B: extraction plate. C: focusing electrostatic lens for electrons. D: double toroidal analyzer. E (${\text{E}}^{\prime }$ for ions): position sensitive detector. F: focusing lens for ions. G: field free flight tube.

Figure 3

Sketch of the magnetic bottle spectrometer. A: permanent conical magnet. B: hypodermic needle for sample gas introduction. C: 2.5-m-long solenoid. D: $\mu$-metal shielding. E: microchannel plates.

Figure 4

Schematic of the molecular energy levels for CO.

Figure 5

(Color online) (a) $E_e$-KER kinetic energy correlation diagram (KECD) of the $\left({\text{C}}^{+},{\text{O}}^{+},e^{-}\right)$ events; region I $\left(\text{KER}\le 5.25\text{eV}\right)$ and region II $\left(5.25\le \text{KER}\le 14\text{eV}\right)$ including structures $\alpha$, $\beta$, and $\gamma$ (see text); (b) total KER distribution, and (c) total electron energy distribution resulting from the projection of the KECD onto the horizontal and the vertical axis, respectively.

Figure 6

(Color online) Energy distributions extracted from the complete KECD corresponding to Fig. 5; intensities are in arbitrary but consistent units. Electron energy distributions for specific KER selections: (a) $0\le \text{KER}\le 5.25\text{eV}$, (b) $5\le \text{KER}\le 7.5\text{eV}$, and (c) $7.5\le \text{KER}\le 9.3\text{eV}$, with additional angular selection of electrons emitted about a plane perpendicular to the spectrometer axis $\mathit{z}$ (see text); KER distributions for specific energy selections: (d) $0\le E_e\le 1\text{eV}$, (e) $1\le E_e\le 3\text{eV}$, and (f) $5\le E_e\le 7\text{eV}$.

Figure 7

(Color online) Energy distribution of the Auger electrons in coincidence with a ${\text{C}}^{+}+{\text{O}}^{+}$ fragment.

Figure 8

(Color online) Electron energy spectrum corresponding to the production of ${\text{CO}}^{2+}$ bound states.

Figure 9

(Color online) KER and electron energy spectra for reaction ${\text{C}}^{2+}+\text{O}$ [panels (a) and (b)] and for reaction ${\text{O}}^{2+}+\text{C}$ [panels (c) and (d)].

Figure 10

Energy distribution spectrum of all electrons detected in coincidence, presented on a log scale. A filter was used to select electron pairs for which the sum of kinetic energies lies in the 220–250 eV range. The dip at the center of the distribution is an experimental artifact due to the detector dead time: two electrons arriving at the same time with the same energy are not counted in coincidence.

Figure 11

(Color online) Electron-electron correlation map obtained at $h\nu =287.4\text{eV}$ on the $\text{C}1s\to 2{\pi }^{\ast }$. This map shows the intensity versus the energy of the slow and the fast electron. The energy sharing between the two electrons appears as diagonal lines in the map. The two-dimensional distribution is discretized with a 1.2 eV step. Intensity is coded in a log scale.

Figure 12

(Color online) (a) Zoom of the electron-electron correlation map in Fig. 11, corresponding to energies of 0–6 eV for the slow electron and 227–255 eV for the fast electron (given in binding energy on the top of the graph). (b) On the top of the figure is shown (in blue) the energy distribution of the fast electron. In the 2D map the broad diagonal line indicates the ${\text{CO}}^{2+}$ ground state. (c) On the right (in red) is shown the energy distribution of the slow electron, for electron pairs selected in a 4 eV band around this line. Structures at 0.2, 0.7, 1.6, and 4 eV in the 2D map are due to false coincidences which have been subtracted in the 1D projection.

Figure 13

(Color online) Energy distributions obtained from electron-electron coincidence measurements of the double Auger decay in CO after $\text{C}1s\to 2{\pi }^{\ast }$ excitation at 287.4 eV. The sum of the two electron energies is shown in panel (A) on a binding energy scale and compared to the known thermochemical limits for fragments formation. Energy selections made are labeled (B), (C), and (D) and resulting energy distributions for the slow electrons are, respectively, displayed in panels (B)–(D). The lower panel displays the energy distribution of slow electrons selected in region T from Fig. 12—autoionizing states of atomic oxygen identified in the spectrum are indicated.

Figure 14

Energy distribution obtained for the fast Auger electrons in coincidence with the atomic lines observed in the 0.32–0.52 eV energy range. The two vertical lines indicate the ${\text{C}}^{+}+{\text{O}}^{+}$ threshold at 35.98 eV and the ${\text{CO}}^{2+}$ threshold at 41.325 eV.