Photon Factory Activity Report 2014 #32(2015) B

BL-13B/Beamline commissioning

In Situ Removal of Carbon Contamination from a Chromium-Coated Mirror:
Ideal Optics to Suppress Higher-Order Harmonics in the Carbon K-Edge Region
Akio Toyoshima,1 Takashi Kikuchi,1 Hirokazu Tanaka,1 Kazuhiko Mase,1,2,* and Kenta Amemiya1,2
1Institute for Materials Structure Science, High Energy Accelerator Research Organization 1-1 Oho,
Tsukuba 305-0801, Japan
2SOKENDAI (The Graduate University for Advanced Studies), 1-1 Oho, Tsukuba 305-0801, Japan
Carbon-free chromium-coated optics are ideal in the carbon K-edge region (280–330 eV) because thereflectivity of first-order light is larger than that of gold-coated optics while the second-orderharmonics (560–660 eV) are significantly suppressed by chromium L- and oxygen K-edge absorption.
We adopted chromium-, gold-, and nickel-coated mirrors in the vacuum ultraviolet and soft X-ray
branch beamline BL-13B at the Photon Factory. Carbon contamination on the chromium-coatedmirror was almost completely removed by exposure to oxygen at a pressure of 8 × 10−2 Pa for 1 h
under irradiation of non-monochromatized synchrotron radiation. The pressure in the chamberrecovered to the order of 10−7 Pa within a few hours. The reflectivity of the chromium-coated mirrorof 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 [1].
1 Introduction
Carbon contamination of optics is a serious issue insynchrotron radiation (SR) facilities because it decreases
the quality of experimental data such as near-edge X-ray
absorption fine structure (NEXAFS), resonant
photoemission, and resonant soft X-ray emission spectrain the carbon K-edge region. Recently, we developed anin situ method to remove carbon contamination fromgold-coated optics in a vacuum ultraviolet and soft X-ray(VSX) undulator beamline [2]. Chromium-coated optics
are better than gold-coated optics for the carbon K-edge
region (280–330 eV) because of the larger reflectivity andbecause second-order harmonics (560–660 eV) aresignificantly suppressed by chromium L- and oxygen Kedge absorption.
Here we report on carbon removal from chromiumcoated optics. Carbon contamination on a chromiumcoated mirror was almost completely removed byexposure to oxygen at a pressure of 8 × 10−2 Pa for 1 h
under irradiation of non-monochromatized SR. The
reflectivity of the chromium-coated mirror was observed
to be a factor of 1.3 larger than that of a gold-coatedmirror in the carbon K-edge region (280–330 eV). On theother hand, the reflectivity of the chromium-coated mirror
in the second-order harmonics region of the carbon Kedge (560–660 eV) was a factor of 0.1–0.48 smaller thanthat of the gold-coated mirror.
2 VSX Undulator Beamline, BL-13A/B
The VSX undulator beamline BL-13A at PF has a
Monk–Gillieson-type monochromator to cover a wide
photon-energy range at a high energy resolution [3].
Carbon contamination of the optics in BL-13A wasremoved almost completely by exposing the optics tooxygen gas at a pressure of 10−1–10−4 Pa for 17–20 h

under irradiation of non-monochromatized SR [2].
Recently, we developed a branch VSX beamline forsurface chemistry (BL-13B, Fig. 1). A branching planemirror (Mp) with a grazing angle of 2˚ is used to reflect
SR to BL-13B. The Mp substrate is coated with 1000-Åthick chromium, gold, and nickel layers as shown in Fig.
2. The chromium-, gold-, and nickel-coated mirrors canbe easily selected by adjusting the z-position of Mp.
Chromium-coated optics are ideal in the carbon K-edge
region (280–330 eV) because of the large reflectivity andsuppression of higher-order harmonics (Fig. 3). In
contrast, nickel-coated optics are suitable in the nitrogen
K-edge region (395 – 445 eV) because higher-orderharmonics are suppressed (Fig. 3). The Mp andM3B/M3B’ chambers are pumped with oil-freeturbomolecular pumps and non-evaporable getter (NEG)
pumps. No ion sputtering pumps are used because
carbon-contaminated ion sputtering pumps are thought toproduce hydrocarbons by collision with ionized residualhydrogen gas [4] Typical base pressures of the Mp andM3B/M3B’ chambers are 3 × 10−7 and 4 × 10−8 Pa,
respectively. Carbon contamination on the gold-coatedMp, M3B, and M3B’ were removed with oxygen
activated by non-monochromatized SR. Typical photon
intensities and energy resolutions (E/ΔE) of the BL13A/13B are shown in Fig. 4.

Fig. 1 Floor layout of BL-13A/B [1].

Photon Factory Activity Report 2014 #32 (2015) B

Fig. 2 Branching plane mirror to suppress higher-orderharmonics [1].

Fig. 3 Reflectivity of chromium-, gold-, and nickelcoated mirrors for the grazing angle of 2˚ and ppolarization calculated using the Center for X-Ray Optics
website[http://henke.lbl.gov/optical_constants/mirror2.html] [1].

Fig. 4 Typical photon intensity and photon energy
resolution (E/ΔE) of the BL-13A [5] and 13B in thephoton energy region of 30–1600 eV [1].
3 In Situ Carbon Removal from Chromium-CoatedMirrorCarbon contamination on the chromium-coatedmirror was removed by exposure to oxygen at a pressureof 8 × 10−2 Pa for 1 h under irradiation of nonmonochromatized SR. The pressure in the Mp chamberrecovered to 10−7 Pa within a few hours after the carbon
removal. Figure 5 shows photon intensity spectra of BL13B in the carbon K-edge region before and after in situ

carbon removal from the chromium-coated mirror. We
ascribed the small negative peak at 285.5 eV before thecarbon removal to the C 1s → * transition of graphitelike carbon. After the carbon removal, the peak at 285.5
eV disappeared and the photon intensity in the carbon Kedge region (280–330 eV) increased. These results
indicate that the carbon on the chromium-coated mirror
has been almost completely removed. Since the photon
intensity involves higher-order harmonics, the negative
peaks at photon energies of 266, 288, and 292 eV areascribed to oxygen K-, chromium LIII-, and chromium LIIedge absorption at 532, 576, and 584 eV, respectively.
The pressure in the chamber recovered to the order of
10−7 Pa within a few hours without baking. The beamline
can be used without additional commissioning.

Fig. 5 Photon intensity spectra of BL-13B in the carbon
K-edge region before and after in situ carbon removalfrom the chromium-coated mirror [1].
4 Comparison between Chromium- and Gold-CoatedMirrors
Figure 6 shows photon intensity spectra measured
with a chromium-, gold-, or nickel-coated mirror in thephoton energy regions of 250–330, 500–660, and 750–990 eV. The chromium LIII- and LII-edge absorption at 576
and 584 eV, respectively, are also observed in the spectra
measured with the gold- and nickel-coated mirrors (Fig. 6,dot-dashed lines) because chromium is used to improve
the adhesion property between the substrate and thecoating. Oxidized chromium seems to be responsible forthe oxygen K-edge absorption at a photon energy of 532
eV observed in the spectra measured with the gold- andnickel-coated mirrors (Fig. 6, dashed line). Since thephoton intensity involves higher-order harmonics, thepeak at a photon energy of 282 eV in the spectra
measured with the nickel- and gold-coated mirrors isassigned to the fifth-order undulator peak at 564 eV (Fig.
6, dotted line). The peaks at 282 and 564 eV arenegligible in the spectra measured with the chromiumcoated mirror, suggesting that 564-eV photons are mostlyabsorbed by oxidized chromium. The photon intensity inthe third-order harmonics region of the carbon K-edge(840–990 eV) is one order of magnitude smaller than that

Photon Factory Activity Report 2014 #32 (2015) B

in the second-order harmonics region (560–660 eV)
because the grazing angle is 2˚.

Fig. 6 Photon intensity spectra measured with achromium-, gold-, or nickel-coated mirror in the photon
energy regions of 250–330 (upper), 500–660 (middle),
and 750–990 eV (lower). Higher-order harmonics areinvolved [1].
Figure 7 shows the ratio of the photon intensity withthe chromium-coated mirror to that with the gold-coatedmirror. The measured ratio differs from the calculated
ratio because the surface chromium is oxidized andbecause the measured photon intensity involves higherorder harmonics. The photon intensity with thechromium-coated mirror is a factor of 1.3 larger than thatof the gold-coated mirror in the carbon K-edge region(280–330 eV), while the former is a factor of 0.1–0.48
smaller than the latter in the second-order harmonics
region (560–660 eV). These results demonstrate thatchromium-coated optics are more advantageous thangold-coated optics in the carbon K-edge region.
5 Conclusions
We constructed a branch VSX undulator beamline BL13B at the PF and opened it for users in October, 2013.
Plane mirrors coated with gold, chromium, or nickel areused to branch SR. Carbon contamination on thechromium-coated mirror was removed by exposure tooxygen at a pressure of 8 × 10−2 Pa for 1 h underirradiation of non-monochromatized SR. The basepressure of the beamline recovered to 10 −7 Pa in a few

hours without baking. The beamline can be used without
Fig. 7 Ratio of measured and calculated photon
intensities with a chromium-coated mirror to those with agold-coated mirror. Data in Figs. 3 and 6 are used. The
measured ratio is different from the calculated ratio
because the surface chromium is oxidized and because
the measured photon intensity involves higher-orderharmonics [1].
additional commissioning. The photon intensity with thechromium-coated mirror is a factor of 1.3 larger than thatof the gold-coated mirror in the carbon K-edge region(280–330 eV), while the former is a factor of 0.1–0.48
smaller than the latter in the second-order harmonics
region (560–660 eV). These results demonstrate thatchromium-coated optics are ideal for experiments inwhich intense SR without higher-order harmonics isrequired in the carbon K-edge region.
Acknowledgement
We express our sincere thanks to the staff of the PF fortheir valuable support during the course of theexperiments. This work was performed during thecommissioning of BL-13B.
References[1] A. Toyoshima et al., J. Synchrotron Rad., submitted.
[2] A. Toyoshima et al., J. Synchrotron Rad. 19, 722(2012).
[3] K. Amemiya and T. Ohta, J. Synchrotron Rad. 11,
171 (2004).
[4] R. Yamada et al., J. Nucl. Mater. 111–112, 744(1982).

Photon Factory Activity Report 2014 #32 (2015) B[5] Toyoshima et al., J. Phys.: Conf. Ser. 425, 152019(2013).
*mase@post.kek.jp