Design of a High Resolution and High Flux Beamline for
VUV Angle-Resolved Photoemission at UVSOR-II
Shin-ichi Kimura, Takahiro Ito, Eiken Nakamura, Masahito Hosaka and MasahiroKatoh
UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585, JapanAbstract. A high-energy-resolution angle-resolved photoemission beamline in the vacuum-ultraviolet (VUV) region has
been designed for a 750 MeV synchrotron light source UVSOR-II. The beamline equips an APPLE-II-type undulator withthe horizontally/vertically linear and right/left circular polarizations, a modified Wadsworth-type monochromator and a highresolution photoelectron analyzer. The monochromator covers the photon energy range of 6 - 40 eV The energy resolution(hv/Ahv) and the photonfluxon samples are expected to be 2 x 104 and 1012 photons/sec at 10 eV, 4 x 104 and 5 x 1011
photons/sec at 20 eV, and 6 x 104 and 1011 photons/sec at 40 eV, respectively. The beamline provides the high-resolution
angle-resolved photoemission spectroscopy less than 1 meV in the whole VUV energy range.
Keywords: Wadsworth monochromator, APPLE-II undulator, high-resolution angle-resolved photoemission
PACS: 41.60.Ap,71.20.-b

INTRODUCTION
Recently, the energy resolution of photoemission experiments is dramatically improved by using a He discharge
lamp and a VUV laser [1]. Due to the improvement, the thermodynamical properties of solids can be understoodby the change of the electronic structure detected by photoemission. In the case of the angle-resolved photoemission
spectroscopy (ARPES) using such light sources, the wavenumber along the normal direction of sample surface (kz)cannot be set at a high symmetry point because the excitation photon energy that directly connects to kz cannot betuned. Then it is important to determine the three-dimensional band structure at the high-symmetry points by ARPES
with tunable photons to compare with the band structure calculation [2]. Since synchrotron radiation (SR) is suitable
for the use of the various photon energies, so many photoemission beamlines have been constructed in the world up tonow. Because of the low photon flux of SR, however, the energy-resolution in ARPES was lower than the other sources
mentioned above. Then we plan to construct a new undulator beamline in the vacuum-ultraviolet (VUV) region at a750 MeV synchrotron light source, UVSOR-II, Institute for Molecular Science, for the purpose of the high-energyresolution ARPES with the total energy resolution less than 1 meV in the excitation energy of 6 - 40 eV and with thevarious (vertically/horizontally linear and right/left circular) polarization of excitation photons. In 2003, UVSOR-II
has been upgraded to the lowest emittance (27 nm rad) ring among SRs with the acceleration energy below 1 GeV [3].
The electron beam size (2<j) was reduced to 0.04(H) x 1.2(H) mm2 in the 1-% coupling machine operation. To use thesmall beam size, we designed a various polarized APPLE-II-type undulator [4] and a VUV monochromator without an
entrance slit, so-called the Wadsworth monochromator [5, 6, 7], in which the grating moves not only rotationally but
also translationally to focus the monochromatized light on the exit slit. In this paper, the beamline design is reported.

DESIGN CONCEPT OF BL7U
The design concept of the VUV-ARPES beamline, BL7U, at UVSOR-II is as follows; (1) Both of the high photon
flux (Nph > 1011 photons/sec) and the high energy resolution (hv/Ahv > 2 x 104) are achieved simultaneously. (2)Various polarizations of vertically/horizontally linear and right/left circular ones are available because the symmetryof the electron orbital and spin directions can be selected. (3) The beamline space from the center of the undulator toa passage is limited to be about 12.5 m.
The schematic drawing of BL7U is shown in Fig. 1. To achieve the first two points of the design concept, we
employed the APPLE-II-type undulator with 3-m long, the periodic length Xu = 16 mm and the number of periods
TV = 38. Some types of undulators and multipole wigglers have been proposed to make various polarizations up toCP879, Synchrotron Radiation Instrumentation: Ninth International Conference,edited by Jae-Young Choi and Seungyu Rah© 2007 American Institute of Physics 978-0-7354-0373-4/07/$23.00
527
Downloaded 22 Feb 2007 to 133.48.165.19. Redistribution subject to AIP license or copyright, see http://proceedings.aip.org/proceedings/cpcr.jsp

//APPLE-llundulator(U7)

0I1I2I3m
I

FIGURE 1. The schematic figure of the high-resolution photoemission beamline BL7U that consists of the APPLE-II-typeundulator (U7), the modified Wadsworth-type monochromator (MO - S) and the high-resolution photoemission analyzer sitting
at Focal point. In the monochromator, there are five optical components, two plane mirrors (MO and Ml) with water cooling, onespherical grating (G), an exit slit (S) and one troidal refocusing mirror (M3). The spherical grating with the radius of 10 m is locatedat 22 m from the center of the undulator. There are no entrance slit and an exit slit (S) locating at 6.47 m from G. The second branch
is separated by a plane mirror (M2) located between G and S.

Ol0"
CQ10K

•

Linear Horizontal 1 st—c^- Linear Horizontal 3rd•m11

Linear Horizontal 5thCircular 1st10-Linear Vertical 1st-Linear Vertical 3rd I- Linear Vertical 5th100

Energy (eV)FIGURE 2. Brilliance spectra of the APPLE-II-type undulator of BL7U at UVSOR-II with the circular and linear polarizations
at the beam current of 500 mA. 1-% coupling machine operation is expected in the calculation.

now [4, 8, 9, 10, 11]. Among these undulators, only the APPLE-II-type one can make various polarizations of thehorizontally/vertically linear and right/left circular polarizations of light even though the large acceptance angle isadopted. The minimum gap is expected to be 24 mm and then the magnetic fields in the horizontal and verticaldirections are 0.498 and 0.764 T, respectively. The calculated brilliance with the linear and circular polarized light isshown in Fig. 2. The first order light with the horizontally linear and circular polarizations covers the photon energy
ranges of 4 - 50 eV and 7 - 50 eV, respectively. The peak brilliance of the circularly polarized light at around 35 eVreaches 1.7 x 10 17 photons/sec/mrad 2 /mm 2 /0.1%b.w. at 500-mA beam current, which is comparable to that of thirdgeneration SR sources.
We employed the Wadsworth-type monochromator, which has no entrance slit to obtain the high photon flux.
In this case, the light source size determines the energy resolution. Since the UVSOR-II is the lowest emittancering in the group of SRs with the acceleration energy below 1 GeV, we can use the very small beam size (2<j) of528
Downloaded 22 Feb 2007 to 133.48.165.19. Redistribution subject to AIP license or copyright, see http://proceedings.aip.org/proceedings/cpcr.jsp

TABLE 1. Optical components of the modified Wadsworth monochromator of BL7U.
Distancefrom emission
point (m)

Size

Shape

Horizontal
incident
angle

0.1, 0.2, 0.4, 1.0, 2.0,4.0, 8.0 mm0plane
plane-Cu

85 deg.
7.5 deg.

Ail on Si
Aii on Si, SiC,Si, Mo/Si multilayer(totalnine mirrors)
Aii on Si02

Pinhole (P)5.4-Mirror (MO)
Mirror (Ml)

5.0
11.0

100x30x20 mm325.4x25.4x10 mm3

Grating (G)22.0

30x30x10 mm3

spherical (r = 10 m,Line density = 1,200,2,400, 3,600 lines/mm)

1 deg.

Slit (S)
Mirror (M3)

28.67
30.67

3-200 iim
100x15x10 mm3--

troidal (r/ =7678.4 mm,rs = 231.5 mm)

80 deg.

Material

SUS/BeCu
Aii on Si02

0.04(V) x 1.2(H) mm2 in the 1-% coupling operation at the center of the undulator. The schematic drawing and theparameter of the optical components are shown in Fig. 1 and Table 1, respectively. The undulator light is guided tothe spherical gratings (G) by two plane mirrors (M0 and Ml) with water cooling. The heat load from the undulator
light is reduced by these two plane mirrors. Since the emission angle of the undulator is small, the distance from theemission point to G must be long to irradiate the wide area on the grating. To keep the polarization of the undulator
radiation, the incident angles of M0 and Ml are 85 and 7.5 degree, respectively. Normally incident mirrors lose photon
flux at higher energy regions because the reflectivity of all materials decreases with increasing photon energy. ThenMl equips nine plane mirrors, Si coated by Au, SiC, Si and a maximum of six Mo/Si multilayers. The mirrors can be
set on the light axis without breaking vacuum and be aligned by the pulse motors. The reflectivity of Au, SiC and Simirrors rapidly decreases with increasing photon energy above 25 eV. Then the Mo/Si multilayer mirrors cover above
25 eV [12]. One Mo/Si multilayer mirror has the peak reflectivity of 40 % and the band width of about 3 eV in theenergy range above 25 eV. Then five multilayer mirrors covering the energy ranges of 25 - 28 eV, 28 - 31 eV, 31 34 eV, 34 - 37 eV and 37 - 40 eV are employed.
The grating (G) is set at 11 m from Ml as shown in Fig. 1. G equips three spherical gratings of 10-m radius withthe constant line density of 1,200, 2,400 and 3,600 lines/mm optimized at the photon energies of 10, 20 and 33 eV,respectively. G moves not only rotationally for selecting photon energy but also for focusing the monochromatizedlight on the exit slit (S) locating at 6.47 m from G like an off-plane Eagle-type monochromator. The off-axis angle of
G is 1 degree to keep the similar focal length between the horizontal and vertical direction. The image of the selected
photon energies at around 40 eV with the grating of 3600 lines/mm and 1-% coupling machine operation at S isplotted in Fig. 3(a). The focus image at 39.999 eV (hv/Ahv = 4 x 104) can be separated from that at 40 eV by using
the 20-jim slit, but that at 40.0004 eV (hv/Ahv = 1 x 105) cannot be separated. The energy resolution is evaluatedto be about 6 x 104 in the 3,600 lines/mm region. After taking the same process, the energy resolution in the 1,200
and 2,400 lines/mm region is evaluated to be 2 x 104 and 4 x 104, respectively. The photon flux can be kept above
1011 photons/sec in the 0.01-% band width as shown in Fig. 3(b). Therefore the target performance of the beamline isachieved.
The beamline has three branches. The main end station is for ARPES in which a 200-mm-radius hemisphericalphotoelectron analyzer (MB Scientific AB, A-l analyzer) and a liquid-helium-cooled cryostat with a 6-axes pulse
motor control (A-VC Co. Ltd., i-GONIO) are installed. The main purpose is to determine the three-dimensional Fermi
surface of strongly correlated materials at low temperatures and its temperature dependence to reveal the origin of
the unconventional physical properties. The beam size on samples is reduced to 1/2 size of the image on the slit
shown in Fig. 3(a) by M3. Since the monochromator reduces the beam size to about 1/3, the total reduction ratio
from the electron beam size at the emission point is about 1/6. Then the beam size (2a) on samples is expected to be200(H) x 10(F) jUm2 in the 1-% coupling operation. The second is for photochemistry synchronized with lasers. The
second end station is located at the branch line separated from a plane mirror set at the M2 position. The last is fora micro-focus ARPES of solids using a multilayer-coated Schwarzschild mirror. The beamline is constructed in thesummer season of 2006 and the ARPES end station will be opened to users in FY2007.

529
Downloaded 22 Feb 2007 to 133.48.165.19. Redistribution subject to AIP license or copyright, see http://proceedings.aip.org/proceedings/cpcr.jsp

(a)

(b)

FIGURE 3. (a) Focus image at the exit slit (S) in Fig. 1. In this case, hv = 40 eV and 1-% coupling machine operation areassumed. Three images at hv = 39.999, 40, 40.0004 eV on the slit are plotted. The energy resolution (hv/Ahv) is evaluated to beabout 6 x 104 by using the 20 /im slit, (b) The evaluated photon flux after optical components. In the second mirror (Ml), thereflectivity below and above 25 eV originates from that of SiC and Au mirrors, respectively. In the grating (G), the efficiency ofgratings with 1,200, 2,400 and 3,600 lines/mm is used in the energy range of 5 - 13, 1 3 - 2 5 and 25 - 50 eV, respectively. When theMo/Si multilayer mirrors are used for Ml, the throughput intensities after Ml, G and on sample are expected to be several times
larger than those of the figure above 25 eV.

CONCLUSION
In conclusion, a new VUV beamline for angle-resolved photoemission at UVSOR-II was designed. The beamline
equips an APPLE-II-type undulator with various polarizations and a modified Wadsworth monochromator. The energy
resolution (hv/Ahv) is expected to be 2 x 10 4 , 4 x 1 0 4 a n d 6 x 10 4 in the photon energy ranges of 6 - 1 5 , 1 2 - 3 0
and 25 - 40 eV with the line density of 1,200, 2,400 and 3,600 lines/mm, respectively.
This work was a joint studies program of Institute for Molecular Science.

REFERENCES
1. T. Kiss, F. Kanetaka, T. Yokoya, T. Shimojima, K. Kanai, S. Shin, Y. Onuki, T. Togashi, C. Zhang, C. T. Chen, and S. Watanabe,
Phys. Rev. Lett. 94, 057001 (2005).
2. T. Ito, S. Kimura, H.J. Im, E. Nakamura, M. Sakai, T. Horigome, K. Soda, and T. Takeuchi, in these proceedings.
3. M. Katoh, M. Hosaka, A. Mochihashi, J. Yamazaki, K. Hayashi, Y Hori, T. Honda, K. Haga, Y Takashima, T. Koseki, S. Koda,
H. Kitamura, T. Hara, and T. Tanaka, AIP conf. Proc. 705, 49 (2004).
4. S. Sasaki, K. Miyata, and T. Takada, Jpn. J. Appl. Phys. 31, L1794 (1992).
5. J. Bisognano, M. Severson, M. A. Green, G. Rogers, M. Fisher, T. Kubala, and M. Bissen, Nucl. Instr. Meth. Phys. Res. A
467-468,492(2001).
6. C.R. Howie, S. Ali, R.P. Tuckett, D.A. Shaw, and J.B. West, Nucl. Instrum. Meth. Phys. Res. B 237, 656 (2005).
7. R.T. Wegh and A. Meijerink, Phys. Rev. B 60, 10820 (1999).
8. T. Tanaka and H. Kitamura, J. Synchrotron Rad. 3, 47 (1996).
9. S. Yamamoto, H. Kawata, H. Kitamura, M. Ando, N. Saki and N. Shiotani, Phys. Rev. Lett. 62, 2672 (1989).
10. H. Onuki, N. Saito, T. Saito, and M. Habu, Rev. Sci. Instrum. 60, 1838 (1989).
11. S. Kimura, M. Kamada, H. Hama, X.M. Marechal, T. Tanaka, and H. Kitamura, J. Electron Spec. Rel. Phenomen. 80, 437(1996).
12. T. Ejima, A. Yamazaki, T. Banse, K. Saito, Y Kondo, S. Ichimaru, and H. Takenaka, Appl. Opt. 44, 5446 (2005).

530
Downloaded 22 Feb 2007 to 133.48.165.19. Redistribution subject to AIP license or copyright, see http://proceedings.aip.org/proceedings/cpcr.jsp