Parallel and antiparallel angular momentum transfer of circularly polarized light to photoelectrons and Auger electrons at the Ni $L_3$ absorption threshold

We report the investigation of the angular momentum transfer of light to photoelectrons and Auger electrons at the Ni $L_3$ absorption threshold. Upon core-level excitation by circularly polarized light, the angular momentum of light, or helicity, is transferred to the emitted photoelectron. The angular momentum of the emitted photoelectron ($m_{\mathrm{f}}$) is the sum of the helicity ($\sigma$) and the orbital magnetic quantum number of the initial state ($m_{\mathrm{i}}$). Here the quantization axis was defined as the direction along the incident light. This can be measured by the parallax shift of the forward focusing peak (FFP) direction in the photoelectron intensity angular distribution. At the absorption threshold, the excited core-level electron is promoted to a conduction-band state and the angular momentum of the light is partially transferred to Auger electrons. We measured photoelectron and Auger electron intensity angular distributions from the Ni(111) surface at the $L_3$ absorption threshold. We observed a significant angular circular dichroism of the $\left[101\right]$ FFP for the $L_3{M}_{4,5}{M}_{4,5}$ Auger electrons. Furthermore, we discovered non-negligible reversal angular circular dichroism contrasts for the triplet components in the case of the $L_3{M}_{2,3}{M}_{4,5}$ Auger electrons, suggesting that the angular momentum of light was transferred to the emitted electron in the antiparallel way.

Figure 1

A resonant Ni $L_3{M}_{4,5}{M}_{4,5}$ Auger electron intensity angular distribution from the $h$-BN/Ni(111) surface at the kinetic energy ($E_{\mathrm{k}}$) of 840.5 eV excited by linearly polarized light with photon energy ($h\nu$) of 852.2 eV. Yellow lines indicate high-symmetry directions.

Figure 2

(a) Photoelectron spectrum excited by x rays with an energy of 852.2 eV (spectrum A: black line) at the $L_3$ absorption edge and 830 eV (spectrum B: red line) below the absorption edge. The top abscissa indicates the kinetic energy for spectrum A, while spectrum B was shifted by 22.2 eV. (b) Angle-resolved photoelectron intensity and (c) circular dichroism contrast for spectrum A. The bottom abscissa indicates the binding energy for both spectra and panels.

Figure 3

Azimuthal profiles of the [101] FFP intensity for photoelectron and Auger electrons. Intensity within the indicated kinetic energy range is integrated for each azimuthal profile. Angular values indicate the difference of the two Gaussian peak positions fitted for each FFP.

Figure 4

Azimuthal profiles of the [101] FFP intensity for Ni (a) $L_3{M}_{4,5}{M}_{4,5}$ Auger electron and (b) $3p$ photoelectron fitted with different angular momentum ($m_{\mathrm{f}}$) components. Mirror symmetrization operation was applied. (c) Fitting of the spectra for (b). The multiple-scattering components are removed.

Figure 5

Schematic diagram of an angular momentum transfer mechanism of x ray to emitted electrons for $L_3{M}_{2,3}{M}_{4,5}$.

Figure 6

(a) Circular dichroism contrast for the spectrum A shown in Fig. 2 before the offset correction. (b) Offset value for each kinetic energy obtained by averaging the circular dichroism over azimuthal angle. The top abscissa indicates the kinetic energy. The bottom abscissa indicates the binding energy for both spectra and panels.