BL-16A: Variable Polarization Soft X-ray spectroscopy Station

Spokesperson: Kenta AMEMIYA


1. Outline

The soft X-ray beamline, BL-16A, provides circular and elliptical polarizations, as well as horizontal and vertical linear polarizations, by adopting APPLE-II type undulators and a variable-included-angle varied-line-spacing plane-grating monochromator [1]. One can apply the X-ray magnetic circular dichroism (XMCD), magnetic linear dichroism (XMLD) and X-ray resonant scattering (XRS) techniques in order to mainly investigate magnetic thin films. In addition, a real-time observation of surface chemical reaction is possible by using a wavelength-dispersive X-ray absorption spectroscopy (XAS), without the pump-and-probe technique [2].

Recently, a polarization switching between the circular polarizations, as well as the horizontal and vertical linear polarizations, has been achieved by using twin APPLE-II type undulators [3]. It is expected that the signal-to-noise ratio in the XMCD and XMLD measurements is significantly improved. In addition, a real-time observation of molecular orientation is also possible by combining polarization switching between horizontal and vertical polarizations with the wavelength-dispersive XAS [4].

Figure 1 shows schematic layout of the beamline optics. The beamtime is assigned for users by using three experimental ports, F1, F2, and F3 to efficiently utilize the beamtime.

Fig. 1. Schematic layout for BL-16A.

2. Performance

The available energy region depends on the polarization as listed below :
Circular polarizations: 297 - 1000 eV,
Horizontal linear polarization: 180 - 1500 eV,
Vertical linear polarization: 380 - 1500 eV,
Elliptical polarizations: 218 - 1500 eV.

One can change the undulator parameters including the polarizations at any time. A typical performance of the beamline is given in Figs. 2 and 3. A typical beam size at the experimental ports is ~0.1-0.2 and ~0.2-0.5 mm in the vertical and horizontal directions, respectively.

Fig. 2. Expected photon flux at fixed resolving powers, E/E.

  Fig. 3. Estimation of typical photon flux and resolving power by N K-edge spectrum.

3. Experimental apparatus

Figure 4 shows the alignment of the experimental stations. A superconducting-magnet XMCD apparatus is fixed at F2, while the wavelength-dispersive XAS measurement is carried out at F1. F3 is the gfree porth at which some apparatus are interchangeably placed.

Fig. 4. Geometrical layout of the experimental stations.

[List of experimental apparatus opened for users]
1. Superconducting-magnet XMCD apparatus
Magnetic field: up to 5 T, lowest sample temperature: 30 K, total electron yield (drain current) and fluorescence yield measurements (using a Silicon Drift Detector) are available.

2. Normal conducting-magnet XMCD apparatus
Magnetic field: up to 1.2 T, lowest sample temperature: 30 K, total electron yield (drain current) and total fluorescence yield (MCP) measurements are available.

3. Depth-resolved XMCD apparatus [5,6]
A depth-resolved XMCD measurement with an atomic-layer resolution is possible by using the detection-angle dependence of the probing depth of the electron-yield spectra. Magnetic field: up to 0.05 - 0.1 T (only before each measurement), lowest sample temperature: 100 K, total electron yield (drain current) measurement is also available.

4. References
[1] K.Amemiya et al., "Commissioning of a Soft X-ray Beamline PF-BL-16A with a Variable-Included-Angle Varied-Line-Spacing Grating Monochromator", AIP Conf. Proc. 1234 (2010) 295.

[2] K.Amemiya et al., "Real-time observation of CO oxidation reaction on Ir(111) surface at 33 ms resolution by means of wavelength-dispersive near-edge x-ray absorption fine structure spectroscopy", Appl. Phys. Lett. 99 (2011) 074104.

[3] K.Amemiya et al., "Fast polarization switching in the soft X-ray region at PF BL-16A", J. Phy.: Conf. Ser., in press.

[4] K.Amemiya et al, "Molecular orientation change during adsorption of NO and N2O on Ir(111) observed by real-time wavelength-dispersive x-ray absorption spectroscopy with polarization switching", Appl. Phys. Lett., submitted.

[5] K. Amemiya et al., "Direct observation of magnetic depth profiles of thin Fe films on Cu(100) and Ni/Cu(100) with the depth-resolved x-ray magnetic circular dichroism", Appl. Phys. Lett. 84 (2004) 936.

[6] K. Amemiya, "Sub-nm Resolution Depth Profiling of the Chemical state and Magnetic Structure of Thin Films by the Depth-Resolved X-ray Absorption Spectroscopy Technique", Phys. Chem. Chem. Phys. 14 (2012) 10477.