Energy and Angular Distributions of Electrons Emitted by Direct Double Auger Decay

We have observed the direct $L_{2,3}MMM$ double Auger transition after photoionization of the $2p$ shell of argon by angle-resolved electron-electron coincidence spectroscopy. The process is responsible for about $20\%$ of the observed Auger electron intensity. In contrast to the normal Auger lines, the spectra in double Auger decay show a continuous intensity distribution. The energy and angular distributions of the emitted electrons allow one to obtain information on the electron correlations giving rise to the double Auger process as well as the symmetry of the associated two-electron continuum state.

Figure 1

A sketch of the experimental setup used for multiple time-of-flight electron-electron coincidence spectroscopy.

Figure 2

Two-dimensional electron-electron coincidence spectrum of Ar taken at $h\nu =270\mathrm{e}\mathrm{V}$ along with two noncoincident spectra. Structures due to normal Auger decay are marked by ${\mathrm{A}\mathrm{r}}^{2+}$. Diagonal stripes in which the sum of the kinetic energies is constant are caused by the double Auger electrons.

Figure 3

Summed coincidence spectrum of Ar taken at $h\nu =270\mathrm{e}\mathrm{V}$ showing different doubly and triply charged final states. The line thickness represents the statistical error. The energy positions of ${\mathrm{A}\mathrm{r}}^{3+}$ states obtained from 14, 15 are indicated as bars (Ar $2p_{3/2}$; full line: Ar $2p_{1/2}$; dashed line). Line groups of further interest (see Fig. 4) are grey shaded.

Figure 4

Coincident energy distribution of the double Auger intensity for three different groups (shaded regions in Fig. 3) of triply charged final states [$3s^{-1}3p^{-2}$: (A) total kinetic energy $\Sigma E_{\mathrm{k}\mathrm{i}\mathrm{n}}=135\pm 5\mathrm{e}\mathrm{V}$, dark grey line; (B) $\Sigma E_{\mathrm{k}\mathrm{i}\mathrm{n}}=147\pm 6\mathrm{e}\mathrm{V}$, light grey line. $3p^{-3}$: (C) $\Sigma E_{\mathrm{k}\mathrm{i}\mathrm{n}}=163\pm 8\mathrm{e}\mathrm{V}$, black line]. For the scaled curves only representative error bars are shown. The dashed lines indicate the contribution of direct double Auger emission. The grey shaded area marks the region where the Ar $2p$ lines interfere.

Figure 5

Angular correlation patterns summed over all Ar $3p^{-3}$ final ionic states (total kinetic energy $\Sigma E_{\mathrm{k}\mathrm{i}\mathrm{n}}=163\pm 8\mathrm{e}\mathrm{V}$). The slower electron $e_a$ is emitted in the $x$ direction indicated by an arrow. The numbers in brackets give the corresponding kinetic energy range of the slower electron. The curves are explained in the text.