Single and multiple Auger decay processes from the $\mathrm{N}{\mathrm{e}}^{+}1s^{-1}2p^{-1}np$ shake-up states studied with a multielectron coincidence method

The single, double, and triple Auger decay processes from the $1s^{-1}2p^{-1}np$ shake-up states of Ne have been studied with a multielectron coincidence method. It is revealed that the Rydberg electron promoted in the initial photoionization step generally acts as a spectator both in the single and double Auger decays. The single Auger decay predominantly produces three-hole one-particle final states while leaving the Rydberg electron virtually unchanged. In the cascade double Auger paths, the Rydberg electron is a spectator in the first step of the cascade process but is subsequently released in the second step, leading to the formation of three-hole final states. The direct paths of double Auger decays, which are much weaker than the cascade paths, produce preferably four-hole one-particle states due to the spectator behavior of the Rydberg electron. The distributions of the $\mathrm{N}{\mathrm{e}}^{4+}$ states populated by the triple Auger decays are also presented.

Figure 1

Energy level diagram for Ne ions. Different Auger decay paths from Ne $1s$ shake-up state are presented.

Figure 2

(a) Photoelectron spectrum derived from the total events in the coincidence dataset accumulated at a photon energy of 1006 eV for Ne. The locations of the $1s$ main and shake-up satellite states [13] are indicated with vertical bars. The four kinetic-energy ranges set to extract the curves in Figs. 3–3 are marked in red. (b) Two-dimensional map showing energy correlation between photoelectrons in the same range as in (a) and Auger electrons in the kinetic-energy range of 600–900 eV. Color represents number of coincidences found in a box of a 1 eV (vertical) × 0.37 eV (horizontal) size.

Figure 3

Auger electron spectra (black solid) obtained in coincidence with photoelectrons for the formations of (a) $1s^{-1}$, (b) $1s^{-1}2p^{-1}\left({}^{3}P\right)3p$, (c) $1s^{-1}2p^{-1}\left({}^{1}P\right)3p$, and (d) $1s^{-1}2p^{-1}\left({}^{1}P\right)4p$. Contributions from $1s^{-1}2p^{-1}\left({}^{3}P\right)4p$ and $1s^{-1}2p^{-1}\left({}^{3}P\right)np$ ($n\ge 5$) are included in (c,d), respectively. These spectra are plotted in the $\mathrm{N}{\mathrm{e}}^{2+}$ binding energy scale which is converted from Auger electron energy in the relation of $(\mathrm{N}{\mathrm{e}}^{2+}\mathrm{binding}\mathrm{energy})=$ (photon energy–photoelectron energy–Auger electron energy). The transition intensities from the $1s^{-1}$ state to the $2h$ levels, observed by conventional Auger spectroscopy [15, 16], are shown in (a) with red bars. The binding energies of the $2h$ and low-lying $3h1e$ levels, as well as the $\mathrm{N}{\mathrm{e}}^{3+}$ threshold, are indicated [19]. The shaded spectra show the contributions from the double Auger decays, which are obtained by the triple coincidences and are scaled by taking into account the known electron detection efficiency. The spectra essentially delineate the distributions of the $\mathrm{N}{\mathrm{e}}^{2+}$ states which are intermediately formed in the cascade double Auger paths, since the contributions from the direct double Auger paths are minor.

Figure 4

Two-dimensional map showing energy correlations of the two Auger electrons emitted in the double Auger decay from the $1s^{-1}2p^{-1}\left({}^{3}P\right)3p$ shake-up state. Color represents number of coincidences found in a box of a 1 eV (vertical) × 1.8 eV (horizontal) size. Intense structures around $\left(x,y\right)=\left(800\mathrm{eV},0–150\mathrm{eV}\right)$ are due to false coincidences including $1s$ photoelectrons and intense single Auger electrons from $1s^{-1}$. The diagonal lines (red broken) indicate the locations of the coincidence yields expected for the final formations of $3h$ states. A blind area arises around $y=100\mathrm{eV}$, because of the overlap with the photoelectron for the formation of the satellite state. The top panel shows the energy distribution of the fast Auger electron, obtained by projecting the coincidence yields on the two-dimensional map toward the $x$ axis. In this projection, the contribution from the false coincidences seen around $\left(x,y\right)=\left(800\mathrm{eV},0–150\mathrm{eV}\right)$ is excluded.

Figure 5

Spectra showing the populations of the $\mathrm{N}{\mathrm{e}}^{3+}$ states formed by the double Auger decays from (a) $1s^{-1}$, (b) $1s^{-1}2p^{-1}\left({}^{3}P\right)3p$, (c) $1s^{-1}2p^{-1}\left({}^{1}P\right)3p$, and (d) $1s^{-1}2p^{-1}\left({}^{1}P\right)4p$. Contributions from $1s^{-1}2p^{-1}\left({}^{3}P\right)4p$ and $1s^{-1}2p^{-1}\left({}^{3}P\right)np$ ($n\ge 5$) are included in (c,d), respectively. While the black solid curves are obtained for the whole processes of the double Auger decays, the red broken curves are deduced by eliminating the events including a slow electron with a kinetic energy below 50 eV and thus represent the $\mathrm{N}{\mathrm{e}}^{3+}$ populations resulting from the direct double Auger paths. These spectra are plotted in the $\mathrm{N}{\mathrm{e}}^{3+}$ binding energy scale which is converted from sum of the energies of two Auger electrons, in the relation of $(\mathrm{N}{\mathrm{e}}^{3+}\mathrm{binding}\mathrm{energy})=$ (photon energy)–(photoelectron energy)–(sum of the energies of two Auger electrons). The binding energies of the $3h$ and low-lying $4h1e$ levels, as well as the $\mathrm{N}{\mathrm{e}}^{4+}$ threshold, are indicated [19].

Figure 6

Spectra showing the populations of the $\mathrm{N}{\mathrm{e}}^{4+}$ states formed by the triple Auger decays from (a) $1s^{-1}$, (b) $1s^{-1}2p^{-1}\left({}^{3}P\right)3p$, (c) $1s^{-1}2p^{-1}\left({}^{1}P\right)3p$, and (d) $1s^{-1}2p^{-1}\left({}^{1}P\right)4p$. Contributions from $1s^{-1}2p^{-1}\left({}^{3}P\right)4p$ and $1s^{-1}2p^{-1}\left({}^{3}P\right)np$ ($n\ge 5$) are included in (c,d), respectively. These spectra are plotted in the $\mathrm{N}{\mathrm{e}}^{4+}$ binding energy scale which is converted from sum of the energies of two Auger electrons, in the relation of $(\mathrm{N}{\mathrm{e}}^{4+}\mathrm{binding}\mathrm{energy})=\left(\mathrm{photon}\mathrm{energy}\right)-\left(\mathrm{photoelectron}\mathrm{energy}\right)$–(sum of the energies of three Auger electrons). The binding energies of the $4h$ levels, as well as the $\mathrm{N}{\mathrm{e}}^{5+}$ threshold, are indicated [19].