Photoelectron–Auger-electron angular-momentum transfer in core-ionized Ar: Beyond the standard post-collision-interaction model

Electron-ion coincidence experimental data obtained following argon $1s$ photoionization are reported. Slow photoelectrons were measured in coincidence with ${\mathrm{Ar}}^{+}$ and ${\mathrm{Ar}}^{2+}$ ions, and the $\beta$ angular distribution parameter was obtained. The measured beta parameter for the ${\mathrm{Ar}}^{2+}$-photoelectron coincidence measurements shows a significant deviation from the $\beta =2$ expected value. With the support of a quantum mechanical theory of post-collision interaction (PCI) which goes beyond the standard model, we attribute this deviation to angular-momentum exchange due to the interaction of the photoelectron with the Auger electron, while the role of the residual ion is negligible. The main mechanism of angular-momentum transfer and its effect on the asymmetry parameter $\beta$ near the photoionization threshold are considered.

Figure 1

Schematics of the coincidence setup: electron-ions coincidence setup. A: molecular jet; B: electrostatic lenses; C: ion time-of-flight spectrometer; D: electron time-of-flight spectrometer; E: 80 mm microchannel plates; SR: synchrotron radiation. See text for details.

Figure 2

(a) Projections of the $P_x$ and $P_y$ components of the photoelectron momentum. Left, at 2 eV of excess photon energy above threshold; center, at 4 eV of excess energy; right, at 10 eV of excess energy. Only the positive half of the projection along the $x$ axis is shown to reduce the size of the figure. The negative half is simply the symmetrical image. (b) Corresponding kinetic energy distributions.

Figure 3

(a) Photoelectron angular distribution measured in coincidence with the singly- charged ${\mathrm{Ar}}^{+}$ ion for excess photon energy $\mathrm{\Delta }E=$ 2 eV. Left: projection of the $P_x$ and $P_y$ components of the momentum. Right: number of counts as a function of the emission angle with respect to the polarization vector, and fit using the well-known dipole angular distribution formula (see text). (b) The same for photoelectrons measured in coincidence with the doubly charged ${\mathrm{Ar}}^{2+}$ ion.

Figure 4

Top: measured $\beta$ values for photoelectrons in coincidence with ${\mathrm{Ar}}^{+}$ and ${\mathrm{Ar}}^{2+}$ at 2, 4, and 10 eV of excess photon energy. Bottom: experimental and calculated values for $\mathrm{\Delta }\beta$, the deviation of $\beta$ from 2, as a function of excess energy.