Development of a versatile micro-focused angle-resolved photoemission spectroscopy system with Kirkpatrick-Baez mirror optics.

Angle-resolved photoemission spectroscopy using a micro-focused beam spot [micro-angle-resolved photoemission spectroscopy (ARPES)] is becoming a powerful tool to elucidate key electronic states of exotic quantum materials. We have developed a versatile micro-ARPES system based on the synchrotron radiation beam focused with a Kirkpatrick-Baez mirror optics. The mirrors are monolithically installed on a stage, which is driven with five-axis motion, and are vibrationally separated from the ARPES measurement system. Spatial mapping of the Au photolithography pattern on Si signifies the beam spot size of 10 µm (horizontal) × 12 µm (vertical) at the sample position, which is well suited to resolve the fine structure in local electronic states. Utilization of the micro-beam and the high precision sample motion system enables the accurate spatially resolved band-structure mapping, as demonstrated by the observation of a small band anomaly associated with tiny sample bending near the edge of a cleaved topological insulator single crystal.

Beamline commissioning for microscopic measurements with ultraviolet and soft X-ray beam at the upgraded beamline BL-13B of the Photon Factory.

Beamline 13 of the Photon Factory has been in operation since 2010 as a vacuum ultraviolet and soft X-ray undulator beamline for X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), and angle-resolved photoelectron spectroscopy (ARPES) experiments. The beamline and the end-station at branch B have been recently upgraded, enabling microscopic XPS, XAS, and ARPES measurements to be performed. In 2015, a planar undulator insertion device was replaced with an APPLE-II (advanced planar polarized light emitter II) undulator. This replacement allows use of linear, circular, and elliptical polarized light between 48 and 2000 eV with photon intensities of 109-1013 photons s-1. For microscopic measurements, a toroidal post-mirror was renewed to have more focused beam with profile sizes of 78 µm (horizontal) × 15 µm (vertical) and 84 µm × 11 µm at photon energies of 100 and 400 eV, respectively. A high-precision sample manipulator composed of an XYZ translator, a rotary feedthrough, and a newly developed goniometer, which is essential for microscopic measurements, has been used to control a sample specimen in six degrees of freedom, i.e. translation in the X, Y, and Z directions and rotation in the polar, azimuthal, and tilt directions. To demonstrate the performance of the focused beams, one- and two-dimensional XPS and XAS scan measurements of a copper grid have been performed. It was indicated from analysis of XPS and XAS intensity maps that the actual spatial resolution can be determined by the beam size.

Photoelectron shield for the first mirror of a soft X-ray beamline.

At a soft X-ray beamline with an undulator source, significant heat generation at the first-mirror chamber and light emission at the viewport were found, which can be explained by photoelectrons from the mirror. The chamber temperature increases up to approximately 50°C over a period of several hours. A photoelectron shield consisting of thin copper plates not only prevents the heat generation and light emission but also improves the pressure of the vacuum chamber, if a voltage of a few tens of V is applied to the shield. The total electron yield of the shield reached as much as 58 mA under high heat-load conditions, indicating the emission of numerous photoelectrons from the first mirror. Heat-balance analyses suggest that approximately 30% or more of the heat load on the first mirror is transferred to the surroundings.

In situ removal of carbon contamination from a chromium-coated mirror: ideal optics to suppress higher-order harmonics in the carbon K-edge region.

Carbon-free chromium-coated optics are ideal in the carbon K-edge region (280-330 eV) because the reflectivity of first-order light is larger than that of gold-coated optics while the second-order harmonics (560-660 eV) are significantly suppressed by chromium L-edge and oxygen K-edge absorption. Here, chromium-, gold- and nickel-coated mirrors have been adopted in the vacuum ultraviolet and soft X-ray branch beamline BL-13B at the Photon Factory in Tsukuba, Japan. Carbon contamination on the chromium-coated mirror was almost completely removed by exposure to oxygen at a pressure of 8 × 10(-2) Pa for 1 h under irradiation of non-monochromated synchrotron radiation. The pressure in the chamber recovered to the order of 10(-7) Pa within a few hours. The reflectivity of the chromium-coated mirror of the second-order harmonics in the carbon K-edge region (560-660 eV) was found to be a factor of 0.1-0.48 smaller than that of the gold-coated mirror.

In situ removal of carbon contamination from optics in a vacuum ultraviolet and soft X-ray undulator beamline using oxygen activated by zeroth-order synchrotron radiation.

Carbon contamination of optics is a serious issue in all soft X-ray beamlines because it decreases the quality of experimental data, such as near-edge X-ray absorption fine structure, resonant photoemission and resonant soft X-ray emission spectra in the carbon K-edge region. Here an in situ method involving the use of oxygen activated by zeroth-order synchrotron radiation was used to clean the optics in a vacuum ultraviolet and soft X-ray undulator beamline, BL-13A at the Photon Factory in Tsukuba, Japan. The carbon contamination of the optics was removed by exposing them to oxygen at a pressure of 10(-1)-10(-4) Pa for 17-20 h and simultaneously irradiating them with zeroth-order synchrotron radiation. After the cleaning, the decrease in the photon intensity in the carbon K-edge region reduced to 2-5%. The base pressure of the beamline recovered to 10(-7)-10(-8) Pa in one day without baking. The beamline can be used without additional commissioning.