Photoelectron recapture and reemission process associated with double Auger decay in Ar

Multielectron coincidence spectroscopy has been performed for Ar at a photon energy of only 0.2 eV above the $2p_{1/2}$ threshold. It is revealed that a postcollision interaction induced by double Auger decay leads to photoelectron recapture, followed by reemission of the captured electron, where the recapture of the slow photoelectron forms the $\mathrm{A}{\mathrm{r}}^{2+}$ Rydberg-excited states which subsequently undergo autoionization. The energy correlation of the emitted electrons discloses that both direct and cascade paths in the double Auger decay contribute to the photoelectron recapture.

Figure 1

Kinetic energy spectrum (solid black) of all electrons emitted from Ar ionized at a photon energy of 250.95 eV, which is only 2.3 and 0.2 eV above the $2p_{3/2}$ and $2p_{1/2}$ thresholds, respectively. The nominal kinetic energies of the $2p_{3/2}$ and $2p_{1/2}$ photoelectrons, expected in the absence of PCI, are indicated. The assignments of the photoelectron reemission structures (labeled as S1–S9) associated with the single Auger decay are summarized in Table I of Ref. [6]. Note that structure S2 overlaps the $2p_{3/2}$ photoelectron peak. The dashed red spectrum delineates the contribution from the normal double Auger decay after the $2p$ photoelectron emission, which was obtained in coincidence with the $2p$ photoelectrons in the kinetic energy range of 0–2.5 eV.

Figure 2

Energy correlation map plotting triple coincidence yields due to the photoelectron recapture and reemission process associated with the double Auger decay. The top panel shows the projection of the coincidence yields onto the horizontal axis, delineating the $\mathrm{A}{\mathrm{r}}^{2+}$ Rydberg states populated after the photoelectron recapture. The binding energies of $\mathrm{A}{\mathrm{r}}^{3+}$ levels [18] which correspond to the converging limits of these $\mathrm{A}{\mathrm{r}}^{2+}$ Rydberg states are indicated with dotted lines. Note that the $\mathrm{A}{\mathrm{r}}^{3+}$ states with the main configuration of $3p^{-4}3d$, lying densely in 105–115 eV, are not located, in order to avoid congestion.

Figure 3

Energy distributions of the slowest electrons emitted for the final formation of the three components of $\mathrm{A}{\mathrm{r}}^{3+}3p^{-3}$, which correspond to the intensity distributions along the diagonal lines in Fig. 2 for the formation of the individual $\mathrm{A}{\mathrm{r}}^{3+}$ states. The spectra delineate the energies of reemission electrons due to autoionization of the $\mathrm{A}{\mathrm{r}}^{2+}$ Rydberg states populated after the photoelectron recapture. The energies [18] of higher-lying $\mathrm{A}{\mathrm{r}}^{3+}$ levels with respect to the individual final $\mathrm{A}{\mathrm{r}}^{3+}$ states are indicated.

Figure 4

(a) Energy distribution of the two Auger electrons for the photoelectron recapture into the Rydberg states converging to $\mathrm{A}{\mathrm{r}}^{3+}3p^{-3}{}^{2}P$. The first 2.5-eV width spectral range and the corresponding last range are lost by masking the contribution from the double Auger decay of $2p^{-1}$ (see text). The blind range around the middle, where two Auger electrons have almost equal kinetic energies, arise from the detection dead time of 40 ns. (b) Enlargement of the low kinetic energy part of the distribution in (a).

Figure 5

Two-dimensional map of energy correlation between second faster and slowest electrons emitted for the final formation of $\mathrm{A}{\mathrm{r}}^{3+}3p^{-3}{}^{4}S$, through the photoelectron recapture into the Rydberg states converging to $\mathrm{A}{\mathrm{r}}^{3+}3p^{-3}{}^{2}P$. The second faster and slowest electrons correspond practically to the second-step Auger electron in the cascade double Auger decay and the reemission electron, respectively. The top and right panels show the replots of the corresponding ranges of the spectra in Figs. 4 and 3 (bottom), respectively.