- Email: [email protected]
Performance of a new plane-grating grazing-incidence UHV monochromator
80
Nuclear Instruments and Methods in Physics Research 222 (1984) 80-84 North-Holland, Amsterdam
PERFORMANCE OF A NEW PLANE-GRATING GRAZING-INCIDENCE UHV MONOCHROMATOR S. S U G A , M. T A N I G U C H I , S. S H I N 1), H. S A K A M O T O , M. Y A M A M O T O a n d M. SEKI
Synchrotron Radiation Laboratory, Institute for Solid State Physics, University of Tokyo, Tanashi, Tokyo 188, Japan Y. M U R A T A a n d H. D A I M O N 2)
Institute for Solid State Physics, University of Tokyo, Roppongi, Tokyo 106, Japan
A UHV monochromator with an ion-beam etched plane laminar grating (1200 grooves/mm) is designed, constructed and installed at the third beam line of SOR-RING (a 0.38 GeV electron storage ring). By using four interchangeable pre-reflecting plane mirrors (incidence angles set to 88, 75, 70 and 35° , coated with gold or aluminum) and two interchangeable spherical focusing mirrors (88 and 83°), one can cover the photon energy region from 8 to 400 eV with low content of the higher order light and the stray light. The performance of this monochromator is checked by reflectance, photoemission and photoelectron diffraction experiments.
1. Introduction
2. Design of the monochromator
Monochromators for synchrotron radiation spectroscopy in the soft X-ray region are required to satisfy such conditions as: 1) low content of the stray light, 2) suppression of the higher order light, 3) UHV compatible (bakable), 4) simple mechanism for scanning of the photon energy (by), 5) wide region of available photon energies, 6) variable resolution and 7) easy optical adjustment. Within these conditions, the Miyake-Kato type plane grating monochromator (PGM) satisfies the requirements of 2) and 4) [1]. Moreover, condition 5) is realized in the FLIPPER type PGM, by the use of several interchangeable pre-reflecting mirrors [2]. These constant deviation type monochromators with constant focal length are convenient for studies of very tiny samples, since the beam spot on the sample can be checked by eye by using zeroth order light. Meanwhile, conditions 1) and 3) are satisfied by employing the SiO2 laminar grating made by means of holographic exposure and reactive ion-beam etching [3,4]. In the present report, we briefly describe the design and performance of a new PGM monochromator installed at the third beam line of SOR-RING of the Institute for Solid State Physics.
According to the performance test of an ion-beam etched laminar grating in a modified Miyake-Kato type monochromator [3,4], we have designed a new PGM monochromator which is used for both photon- and photoelectron-spectroscopy. In order to facilitate the optical adjustment in the laboratory, whole optical components, except for the grating, are aligned in a horizontal plane defined by the synchrotron radiation as shown in fig. 1. They are placed on an optical channel which is supported from outside, independent of the vacuum chamber. The vacuum seal is provided by welded bellows. The axis of the grating holder is tightly supported by two high-precision ball bearings embedded in the side walls of the channel. On the other hand, mirror holders (with mirrors inside) loosely supported from outside are pressed to the correct position on the optical channel when they are selected for use. Thus the optical alignment of the monochromator once realized in the
1) Present address: Research Institute for Scientific Measurements, Tohoku University, Katahira, Sendai 980, Japan. 2) Present address: Department of Physics, The University of Tokyo, Hongo, Tokyo 113, Japan. 0167-5087/84/$03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
PGM MONOCHROMATOR entronce diophrogm
~l.6rn from the source point
exit slit
focusing mirrors
sornple
G
Fig. 1. Schematic layout of the SRL/ISSP PGM monochromatot.
S. Suga et aL / Plane- grating grazing- incidence UH V monochromator
atmosphere is not disturbed by the pumping of the whole monochromator. At present, four interchangeable mirrors (PM 1, PM 2, PM 3 and PM 5) are installed in the monochromator as the pre-reflecting mirror, in order to suppress the higher order light in different h~ regions. Table 1 provides details of the mirrors. The grating is rotated around the horizontal axis by a sine bar equipped with a high-precision ball bearing, whose outer wheel is in close contact with the smooth top surface of a vertically driven linear feedthrough. The linear motion of the feedthrough is provided from the outside of the welded bellows by a ball screw (with an accuracy better than 3 /xm). The linear motion of the ball screw is then measured by a magnescale (SONY Co. Ltd) with an accuracy of 1/~m. The diffracted light is focused onto the exit slit by the spherical focusing mirrors ( F M 1 or FM2). After passing through the exit slit, the monochromatic output light considerably diverges vertically, due to the small focal length employed in the present design. Besides, almost no horizontal focusing is supplied by this monochromator for the horizontal collection angle of 2.5 mrad. Since one of the main subjects expected for this monochromator is angle resolved photoemission spectroscopy, a small spot on the sample is highly desirable. Therefore, we have used two additional focusing mirrors after the exit slit, namely, a spherical mirror (R = 10000 mm) and a cylindrical mirror ( R v = 25 mm) with a horizontal axis. These two focusing mirrors are adjusted in the atmosphere so as to provide the smallest spot on the sample ( < 0.5 × 0.5 m m 2) for the zeroth order synchrotron radiation coming through a glass window set for the purpose of optical adjustment. Besides, the spherical mirror plays the role of horizontal focusing and the cylindrical mirror plays the role of vertical focusing. Both mirrors are used at an incidence angle between 84 and 85 ° .
ANALYZER CHAMBER
81
Table 1 Mirrors used in the PGM monochromator. Pre - reflecting mirrors
Plane mirrors
Incidence angle [deg]
Blank size [mm2]
Coat
PM] PM 2 PM 3 PM 4 PM 5
88 75 70 70 35
30 × 70 30 x 30 30 x 30 30 × 30 30 × 30
Au Au Au AI Al
Focusing mirrors
Spherical mirrors
Incidence angle [deg]
Radius [mm]
Blank size [mm2]
Coat
FM1 FM 2
88 83
5500 6000
30 x 70 30 × 30
Au Au
Versatile focusing mirrors
Spherical mirror Cylindrical mirror
Radius [mm]
Coat
R = 10000 mm R v = 25 mm
Au Au
The layout of the whole experimental setup is shown in fig. 2, where the synchrotron radiation (SR) comes from the right hand side. At the third beam line of S O R - R I N G , there is a strong limitation to the available area; namely, the distance from the source point to the wall is less than 4.5 m. Therefore, the first pre-reflecting mirror (PM1) is placed at around 1.6 m from the source point. Though the source size is evaluated as ox = 0.75 m m and oy = 0.2 mm for the low stored current, the vertical source size considerably increases for higher stored current (100-500 mA) due to the x - y coupling.
PGM MONOCHROMATOR
S,~ CF
Fig. 2. PGM monochromator and experimental setup at the third beam line of SOR-RING.
III. VUV/SOFT X-RAYS
82
S. Suga et al. / Plane-grating grazing-incidence UHV monochromator
Assuming that the fwhm (full width at half maximum) of the electron beam distribution (corresponding to 2.350) is 1 mm, the fwhm of the focused beam on the exit slit is evaluated as 30/~m for FM 1 and 140/zm for FM 2, neglecting the astigmatism. By taking the grating equation for the 1200 grooves/mm grating for each pair of PM and FM mirrors into account, the source-size limited resolution is evaluated as 0.35 ,~ for PM~/FM 1, and 0.95 ,~ for PM1/FM 2. The ray tracing analysis has provided an equivalent result. In order to improve the resolution, one is required to diminish the source size or to place the whole monochromator at rather a distant place from the source point. These two methods are, however, very difficult in our case. One can improve the resolution by use of a higher density grating. In the present setup, the direct synchrotron radiation can be guided to the analyzer chamber when the mirrors are pulled out from the optical path and both the entrance diaphragm and the exit slit are opened. The direct synchrotron radiation is sometimes useful for such special experiments as white light diffraction and radiation damage.
thick Cr film and a 500 A thick Au film for both gratings. We now briefly describe the method of optical adjustment. First, all the optical components are put on the designed positions of the optical channel. Mirrors and the grating are carefully adjusted by use of a laser light, which is set to pass through the center of both the entrance diaphragm and the exit slit. The vacuum chamber is installed at the beam line, with the center of the entrance and exit flanges just at the position of the synchrotron radiation. Then the optical channel is put into the vacuum chamber and supported from outside, making the center of the entrance diaphragm and the exit slit lie on the synchrotron radiation. Finally, mirrors are pressed to the optical channel and the exit slit is moved along the exit beam, in order to find out the position for the best resolution. When this optical adjustment is completed, the whole monochromator is pumped out.
4. Results and discussion This monochromator is equipped with a 128 I/s sputter ion pump, a Ti sublimation pump and a 110 1/s turbo-molecular pump (TMP). The bake-out of the
3. Experimental In the case of the plane laminar grating, it is well known that groove parameters should be properly chosen, in order to maximize the diffraction efficiency [5] in the particular wavelength region. We have employed two laminar gratings, HLG10 and H L G l l shown in fig. 3, for this new PGM monochromator. The fabrication method of these gratings (by means of holographic exposure and reactive ion-beam etching) is already reported in a previous paper [3]. The groove is engraved in a SiO2 substrate by the CHF3 reactive ion-beam etching using the Au pattern as a mask. HLG10 and 11 are obtained by etching for 20 and 45 s, respectively, under the following conditions: CHF3 pressure of 0.8 x 10 4 Torr, accelerating voltage of 500 V, current density of 0.4 m A / c m 2. The inclination angle of the land edge is set to around 78 ° through this procedure. The surface of the original SiO2 grating is then covered with a 100 ~,
1000
PGM ISSP HLG 10
d 100 I/) C
.E
lo
~o
~oo
200 300 4b0 ~;o photon energy (eV)
100C
-.
& E=2.TeV
o.6sev/p~ ~
=
PGM ISSP 83.7
>, IOC
ION ETCHED LAMINAR GRATING 1200/mm
d=8333A
HLG10
a/b=1:2.3
10
h=120~, ,L,h
/////~'//////r////////////~,/,~///////////////
HLG11
PM1/FM1
$~PM1/FM2 /FM1 PM3/FM2
'
100
2~)0
3~)0
~0
5~)0
6~)0
photon energy(eV) HLG11
a/b=1:1.5
h=280.~,
/////~//////////////~/////// Fig. 3. Profile of two laminar gratings (HLG10 and 11).
Fig. 4. Output spectra o f the P G M monochromator measured for a constant slit width by the yield of a Au photocathode: a) result for HLG10 in June 1982, b) result for H L G l l in July 1983.
S. Suga et aL /Plane-grating grazing-incidence UHV monochromator
monochromator is carried out for 24 h at 200°C, while it is pumped down by the TMP. An ultrahigh vacuum better than 2 × 10 -1° Torr is realized afterwards by using the sputter ion pump. The output spectra of the monochromator have been measured by the photoelectron yield of the Au photocathode. Figs. 4a and b show the results measured for the HLG10 and H L G l l . When we normalize the result by the resolution (AE), we notice that the relative efficiency of this monochromator is rather high in the highest energy region with hp > 100 eV, as is the case of SX-700 at BESSY [6], for the combination of PM1/FM r This result is partly due to the large incidence angle (88 °) for the PM~ and FM~, in contrast to the result of FLIPPER [2]. As for the resolution of the monochromator, it is experimentally found to be 0.45 ,~ ( P M ] / F M 1), 0,8 ,~, (PM1/FM2), 3.4 ,~ (PM2/FM1) and - 3 •A(PMa/FM2), as shown in fig. 4b, for a constant slit width of about 30 /xm. Further reduction of the slit width has provided almost no improvement of the resolution. As for the content of the second order light, it is evaluated at h~, = 70 eV as 5% for the setup PM1/FM 2. It is well known for a laminar grating that the profile parameter ( b / d ) = 0.5 provides the maximum efficiency for near normal incidence. For grazing incidence, however, maximum efficiency is expected for b / d larger than 0.5, due to the shadowing effect of the lands on the grooves. Based on an electromagnetic theory, optimized grating parameters have been calculated by Neviere et al. [5]. According to their results for the laminar grating with 1200 grooves/mm, HLG10 is expected to provide the maximum efficiency around h u = 400 eV for P M ] / F M 1 and around hu = 200 eV for PM1/FM 2. On the other hand, H L G l l hardly satisfies the optimum condition for P M ] / F M r But is is expected that H L G l l provides the maximum efficiency around h ~, = 40 eV for P M 3 / F M 2. The results shown in figs. 4a and b are not inconsistent with these predictions. We will further comment on the performance in the lowest energy region (not shown in fig. 4). For P M s / F M 2, the grating HLG10 provides strong photon flux between 18 and 8 eV. Although the photon flux is very low above h~, = 18 eV as designed, we have observed remarkable oscillatory structures there. On the other hand, the grating H L G l l has shown only oscillatory structures for P M s / F M 2 in the whole region. According to a simplified model for interference, such an oscillation is a function of ~rh(cos ct +cos fl)/)~, where ct and fl are the incidence and diffraction angles of the light on the grating, h is the groove depth and )~ is the wavelength [7]. Therefore, ther oscillatory region is expanded into the longer wavelength region, when the groove depth is enlarged. The result for H L G l l with h = 280 ,~ is thus qualitatively understood in comparison with that for HLG10 with h = 120 ,~. As a whole, the laminar grating HLG10 is more appropriate than
83
H L G l l for this PGM monochromator, in order to realize the wide working range of h ~ as well as the high efficiency in the highest energy region. One may recognize in fig. 4 that the C (carbon) ls edge structure is rather obviously observed for H L G l l , whereas the structure is not remarkable for HLG10. We consider that this is mainly due to the degradation of the PM 1 mirror, since slight carbon contamination was observed on PM 1 when we replaced the grating HLG10 by H L G l l in May 1983. The degradation of PM 1 may have been induced by a rather poor vacuum ( - 1 0 -8 Torr) during test measurements for wavelength calibration.
5. Application to spectroscopy The solid curves in fig. 5 show the reflectance spectra of VSe 2 and VO 2 in the vanadium 3p core excitation region, where 3p ~ 3d excitation is dominant. VSe 2 is a layer type crystal, which shows metallic conductivity. On the contrary, VO 2 is well known to show the metal-insulator transition at T¢ = 340 K (metallic above To). The spectrum of VO 2 in fig. 5 is for the metallic phase. The spectrum in the insulator phase is, however, hardly different from that in the metallic phase, in contrast to our expectation based on a screening of the 3p-3d interaction by the free carriers in the metallic phase. Then we consider that the 3p --* 3d excitation in VO 2 is rather localized in both insulator and metallic phases. The remarkable difference of the V 3p core excitation between VO 2 and VSe 2 is then assigned to the different degree of hybridization of the V 3d orbitals
"7 [
/theoretical
:1
E2
/i B
.=
VO2(M)
A1
2
, ...,r..
40
50 60 photon energy(eV)
Fig. 5. Reflectance spectra of VSe2 and VO2 in the metal phase (solid curves) measured in the region of the V 3p core excitation. The dashed curves show results of the theoretical calculation of c2 based on the localized excitation model [8]. III. VUV/SOFT X-RAYS
84
S. Suga et al. / Plane-grating grazing-incidence UHV monochromator
with anion p orbitals. Since the vanadium ion is considered to be tetravalent in these compounds (namely, one 3d electron occupied in the ground state (m = 1)), the spectrum is expected to be similar to that of the Ti 3+ 3p core excitation [8]. By taking the above-mentioned hybridization into account, we have analyzed the experimental result on the 3p53d " ÷ t localized excitation model as shown by the dashed curves in fig. 5. Assuming the cubic crystal field lODq = 3.3 eV, the reduction factors for the S l a t e r - C o n d o n parameters are evaluated as x F = 0.6, gG = 0.5 for VO 2 and xv = 0.35 and leg = 0.25 for VSe 2. The small reduction factors for the 3 p - 3 d Coulomb and exchange interactions in VSe 2 demonstrate the high degree of hybridization of the V 3d orbitals. A similar experiment has been carried out for polytypes of TaSe 2. As another example of its application in the higher photon energy region, we have checked the photoelectron diffraction for the S absorbed on Ni(001), by use of a hemispherical electron energy analyzer. Diffraction of the photoelectron was measured for the S 2p core ( E b = 162 eV) electrons of the c(2 × 2)S-Ni(001). The sample was prepared in the U H V sample preparation chamber by introducing H2S gas onto a Ni(001) clean surface obtained by cycles of Ar ion bombardment and annealing at 800°C by electron bombardment. The c(2 x 2) structure was examined by LEED. Fig. 6 shows the result of the normal photoelectron diffraction as a function of h~, (open circles). This result is in qualitative agreement with that of Rosenblatt et al. [9], elucidating that the present P G M monochromator installed at the 0.38 GeV storage ring is providing effective photon flux up to hu = 400 eV. Since we did this experiment before installing the focusing mirrors behind the exit slit, the signal to noise ratio was not satisfactory. By use of the additional focusing mirrors mentioned above, the beam
"~',
I t'-
-
Ni(O01)-c( 2x2)S
/
0.5
g~
;
,-,.
I
t
"4
#
>
o
/
nr"
0
250
i
,
,
i
300 350 Photon Energy (eV)
Fig. 6. Normal emission photoelectron diffraction for the S 2p core level (Eb=162 eV) of c(2X2)S-Ni(001) (circles). The dashed curve shows the result of Rosenblatt et al. [9].
is focused into a small spot (0.5 x 0.5 mm 2) on the sample. As for the intensity in this case, one can have a rough idea from the following experimental result: 1000 c o u n t s / s for the angle resolved photoemission ( + 1 ° and A E = 0.1 eV) from the 2p core levels of S i ( l l l ) excited at hv = 150 eV (Ahp < 1 eV) at a stored current of 150 mA.
6. Summary A plane grating monochromator with an ion-beam etched laminar grating is constructed and installed at the third beam line of S O R - R I N G . Since the entire monochromator is bakable, a U H V better than 2 x 10 -~° Torr is easily attained after a bake at 200°C for 24 h. By using a grating with 1200 g r o o v e s / m m , the photon energy region from 8 to 400 eV is covered with a rather low content of the stray and higher order light. A small beam size ( < 0 . 5 x 0 . 5 mm 2) is realized on the sample, which facilitates the angle resolved photoemission spectroscopy as well as optical studies of various samples of small dimensions. The authors acknowledge Professor H. Aritome, Professor S. N a m b a and Dr. S. Matsui for providing us ion-beam etched laminar gratings. They are grateful to Professor H. Kanzaki and colleagues of the synchrotron radiation laboratory, for their continuous support. They are also much obliged to Mr. T. Uchibori, Mr. M. Mineta and Mr. M. Yorimoto of the workshop of our institute for their skilful production of parts of the monochromator.
References [1] K. Miyake, R. Kato and H. Yamashita, Sci. Light 18 (1969) 39. [2] W. Eberhardt, G. Kalkoffen and C. Kunz, Nucl. Instr. and Meth. 152 (1978) 81. [3] S. Matsui, K. Moriwaki, H. Aritome, S. Namba, S. Shin and S. Suga, Appl. Opt. 21 (1982) 2787. [4] H. Aritome, S. Matsui, K. Moriwaki, H. Aoki, S. Namba, S. Suga, A. Mikuni, M. Seki and M. Taniguchi, Nucl. Instr. and Meth. 208 (1983) 233. [5] M. Neviere, J. Flamand and J.M. Lerner, Nucl. Instr. and Meth. 195 (1982) 183. [6] H. Petersen, J. Haase, A. Puschmann, A. Reimer and R. Treichler, Proc. Synchr. Rad. Users Workshop, May 1983. [7] A. Franks, K. Lindsey, J.M. Bennett, R.J. Speer, D. Turner and D.J. Hunt, Trans. R. Soc. London A277 (1975) 503. [8] T. Yamaguchi, S. Shibuya, S. Suga and S. Shin, J. Phys. C15 (1982) 2641. [9] D.H. Rosenblatt, J.G. Tobin, M.G. Mason, R.F. Davis, S.D. Kevan and D.A. Shirley, Phys. Rev. B23 (1981) 3828.
Nuclear Instruments and Methods in Physics Research 222 (1984) 80-84 North-Holland, Amsterdam
PERFORMANCE OF A NEW PLANE-GRATING GRAZING-INCIDENCE UHV MONOCHROMATOR S. S U G A , M. T A N I G U C H I , S. S H I N 1), H. S A K A M O T O , M. Y A M A M O T O a n d M. SEKI
Synchrotron Radiation Laboratory, Institute for Solid State Physics, University of Tokyo, Tanashi, Tokyo 188, Japan Y. M U R A T A a n d H. D A I M O N 2)
Institute for Solid State Physics, University of Tokyo, Roppongi, Tokyo 106, Japan
A UHV monochromator with an ion-beam etched plane laminar grating (1200 grooves/mm) is designed, constructed and installed at the third beam line of SOR-RING (a 0.38 GeV electron storage ring). By using four interchangeable pre-reflecting plane mirrors (incidence angles set to 88, 75, 70 and 35° , coated with gold or aluminum) and two interchangeable spherical focusing mirrors (88 and 83°), one can cover the photon energy region from 8 to 400 eV with low content of the higher order light and the stray light. The performance of this monochromator is checked by reflectance, photoemission and photoelectron diffraction experiments.
1. Introduction
2. Design of the monochromator
Monochromators for synchrotron radiation spectroscopy in the soft X-ray region are required to satisfy such conditions as: 1) low content of the stray light, 2) suppression of the higher order light, 3) UHV compatible (bakable), 4) simple mechanism for scanning of the photon energy (by), 5) wide region of available photon energies, 6) variable resolution and 7) easy optical adjustment. Within these conditions, the Miyake-Kato type plane grating monochromator (PGM) satisfies the requirements of 2) and 4) [1]. Moreover, condition 5) is realized in the FLIPPER type PGM, by the use of several interchangeable pre-reflecting mirrors [2]. These constant deviation type monochromators with constant focal length are convenient for studies of very tiny samples, since the beam spot on the sample can be checked by eye by using zeroth order light. Meanwhile, conditions 1) and 3) are satisfied by employing the SiO2 laminar grating made by means of holographic exposure and reactive ion-beam etching [3,4]. In the present report, we briefly describe the design and performance of a new PGM monochromator installed at the third beam line of SOR-RING of the Institute for Solid State Physics.
According to the performance test of an ion-beam etched laminar grating in a modified Miyake-Kato type monochromator [3,4], we have designed a new PGM monochromator which is used for both photon- and photoelectron-spectroscopy. In order to facilitate the optical adjustment in the laboratory, whole optical components, except for the grating, are aligned in a horizontal plane defined by the synchrotron radiation as shown in fig. 1. They are placed on an optical channel which is supported from outside, independent of the vacuum chamber. The vacuum seal is provided by welded bellows. The axis of the grating holder is tightly supported by two high-precision ball bearings embedded in the side walls of the channel. On the other hand, mirror holders (with mirrors inside) loosely supported from outside are pressed to the correct position on the optical channel when they are selected for use. Thus the optical alignment of the monochromator once realized in the
1) Present address: Research Institute for Scientific Measurements, Tohoku University, Katahira, Sendai 980, Japan. 2) Present address: Department of Physics, The University of Tokyo, Hongo, Tokyo 113, Japan. 0167-5087/84/$03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
PGM MONOCHROMATOR entronce diophrogm
~l.6rn from the source point
exit slit
focusing mirrors
sornple
G
Fig. 1. Schematic layout of the SRL/ISSP PGM monochromatot.
S. Suga et aL / Plane- grating grazing- incidence UH V monochromator
atmosphere is not disturbed by the pumping of the whole monochromator. At present, four interchangeable mirrors (PM 1, PM 2, PM 3 and PM 5) are installed in the monochromator as the pre-reflecting mirror, in order to suppress the higher order light in different h~ regions. Table 1 provides details of the mirrors. The grating is rotated around the horizontal axis by a sine bar equipped with a high-precision ball bearing, whose outer wheel is in close contact with the smooth top surface of a vertically driven linear feedthrough. The linear motion of the feedthrough is provided from the outside of the welded bellows by a ball screw (with an accuracy better than 3 /xm). The linear motion of the ball screw is then measured by a magnescale (SONY Co. Ltd) with an accuracy of 1/~m. The diffracted light is focused onto the exit slit by the spherical focusing mirrors ( F M 1 or FM2). After passing through the exit slit, the monochromatic output light considerably diverges vertically, due to the small focal length employed in the present design. Besides, almost no horizontal focusing is supplied by this monochromator for the horizontal collection angle of 2.5 mrad. Since one of the main subjects expected for this monochromator is angle resolved photoemission spectroscopy, a small spot on the sample is highly desirable. Therefore, we have used two additional focusing mirrors after the exit slit, namely, a spherical mirror (R = 10000 mm) and a cylindrical mirror ( R v = 25 mm) with a horizontal axis. These two focusing mirrors are adjusted in the atmosphere so as to provide the smallest spot on the sample ( < 0.5 × 0.5 m m 2) for the zeroth order synchrotron radiation coming through a glass window set for the purpose of optical adjustment. Besides, the spherical mirror plays the role of horizontal focusing and the cylindrical mirror plays the role of vertical focusing. Both mirrors are used at an incidence angle between 84 and 85 ° .
ANALYZER CHAMBER
81
Table 1 Mirrors used in the PGM monochromator. Pre - reflecting mirrors
Plane mirrors
Incidence angle [deg]
Blank size [mm2]
Coat
PM] PM 2 PM 3 PM 4 PM 5
88 75 70 70 35
30 × 70 30 x 30 30 x 30 30 × 30 30 × 30
Au Au Au AI Al
Focusing mirrors
Spherical mirrors
Incidence angle [deg]
Radius [mm]
Blank size [mm2]
Coat
FM1 FM 2
88 83
5500 6000
30 x 70 30 × 30
Au Au
Versatile focusing mirrors
Spherical mirror Cylindrical mirror
Radius [mm]
Coat
R = 10000 mm R v = 25 mm
Au Au
The layout of the whole experimental setup is shown in fig. 2, where the synchrotron radiation (SR) comes from the right hand side. At the third beam line of S O R - R I N G , there is a strong limitation to the available area; namely, the distance from the source point to the wall is less than 4.5 m. Therefore, the first pre-reflecting mirror (PM1) is placed at around 1.6 m from the source point. Though the source size is evaluated as ox = 0.75 m m and oy = 0.2 mm for the low stored current, the vertical source size considerably increases for higher stored current (100-500 mA) due to the x - y coupling.
PGM MONOCHROMATOR
S,~ CF
Fig. 2. PGM monochromator and experimental setup at the third beam line of SOR-RING.
III. VUV/SOFT X-RAYS
82
S. Suga et al. / Plane-grating grazing-incidence UHV monochromator
Assuming that the fwhm (full width at half maximum) of the electron beam distribution (corresponding to 2.350) is 1 mm, the fwhm of the focused beam on the exit slit is evaluated as 30/~m for FM 1 and 140/zm for FM 2, neglecting the astigmatism. By taking the grating equation for the 1200 grooves/mm grating for each pair of PM and FM mirrors into account, the source-size limited resolution is evaluated as 0.35 ,~ for PM~/FM 1, and 0.95 ,~ for PM1/FM 2. The ray tracing analysis has provided an equivalent result. In order to improve the resolution, one is required to diminish the source size or to place the whole monochromator at rather a distant place from the source point. These two methods are, however, very difficult in our case. One can improve the resolution by use of a higher density grating. In the present setup, the direct synchrotron radiation can be guided to the analyzer chamber when the mirrors are pulled out from the optical path and both the entrance diaphragm and the exit slit are opened. The direct synchrotron radiation is sometimes useful for such special experiments as white light diffraction and radiation damage.
thick Cr film and a 500 A thick Au film for both gratings. We now briefly describe the method of optical adjustment. First, all the optical components are put on the designed positions of the optical channel. Mirrors and the grating are carefully adjusted by use of a laser light, which is set to pass through the center of both the entrance diaphragm and the exit slit. The vacuum chamber is installed at the beam line, with the center of the entrance and exit flanges just at the position of the synchrotron radiation. Then the optical channel is put into the vacuum chamber and supported from outside, making the center of the entrance diaphragm and the exit slit lie on the synchrotron radiation. Finally, mirrors are pressed to the optical channel and the exit slit is moved along the exit beam, in order to find out the position for the best resolution. When this optical adjustment is completed, the whole monochromator is pumped out.
4. Results and discussion This monochromator is equipped with a 128 I/s sputter ion pump, a Ti sublimation pump and a 110 1/s turbo-molecular pump (TMP). The bake-out of the
3. Experimental In the case of the plane laminar grating, it is well known that groove parameters should be properly chosen, in order to maximize the diffraction efficiency [5] in the particular wavelength region. We have employed two laminar gratings, HLG10 and H L G l l shown in fig. 3, for this new PGM monochromator. The fabrication method of these gratings (by means of holographic exposure and reactive ion-beam etching) is already reported in a previous paper [3]. The groove is engraved in a SiO2 substrate by the CHF3 reactive ion-beam etching using the Au pattern as a mask. HLG10 and 11 are obtained by etching for 20 and 45 s, respectively, under the following conditions: CHF3 pressure of 0.8 x 10 4 Torr, accelerating voltage of 500 V, current density of 0.4 m A / c m 2. The inclination angle of the land edge is set to around 78 ° through this procedure. The surface of the original SiO2 grating is then covered with a 100 ~,
1000
PGM ISSP HLG 10
d 100 I/) C
.E
lo
~o
~oo
200 300 4b0 ~;o photon energy (eV)
100C
-.
& E=2.TeV
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PGM ISSP 83.7
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ION ETCHED LAMINAR GRATING 1200/mm
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HLG10
a/b=1:2.3
10
h=120~, ,L,h
/////~'//////r////////////~,/,~///////////////
HLG11
PM1/FM1
$~PM1/FM2 /FM1 PM3/FM2
'
100
2~)0
3~)0
~0
5~)0
6~)0
photon energy(eV) HLG11
a/b=1:1.5
h=280.~,
/////~//////////////~/////// Fig. 3. Profile of two laminar gratings (HLG10 and 11).
Fig. 4. Output spectra o f the P G M monochromator measured for a constant slit width by the yield of a Au photocathode: a) result for HLG10 in June 1982, b) result for H L G l l in July 1983.
S. Suga et aL /Plane-grating grazing-incidence UHV monochromator
monochromator is carried out for 24 h at 200°C, while it is pumped down by the TMP. An ultrahigh vacuum better than 2 × 10 -1° Torr is realized afterwards by using the sputter ion pump. The output spectra of the monochromator have been measured by the photoelectron yield of the Au photocathode. Figs. 4a and b show the results measured for the HLG10 and H L G l l . When we normalize the result by the resolution (AE), we notice that the relative efficiency of this monochromator is rather high in the highest energy region with hp > 100 eV, as is the case of SX-700 at BESSY [6], for the combination of PM1/FM r This result is partly due to the large incidence angle (88 °) for the PM~ and FM~, in contrast to the result of FLIPPER [2]. As for the resolution of the monochromator, it is experimentally found to be 0.45 ,~ ( P M ] / F M 1), 0,8 ,~, (PM1/FM2), 3.4 ,~ (PM2/FM1) and - 3 •A(PMa/FM2), as shown in fig. 4b, for a constant slit width of about 30 /xm. Further reduction of the slit width has provided almost no improvement of the resolution. As for the content of the second order light, it is evaluated at h~, = 70 eV as 5% for the setup PM1/FM 2. It is well known for a laminar grating that the profile parameter ( b / d ) = 0.5 provides the maximum efficiency for near normal incidence. For grazing incidence, however, maximum efficiency is expected for b / d larger than 0.5, due to the shadowing effect of the lands on the grooves. Based on an electromagnetic theory, optimized grating parameters have been calculated by Neviere et al. [5]. According to their results for the laminar grating with 1200 grooves/mm, HLG10 is expected to provide the maximum efficiency around h u = 400 eV for P M ] / F M 1 and around hu = 200 eV for PM1/FM 2. On the other hand, H L G l l hardly satisfies the optimum condition for P M ] / F M r But is is expected that H L G l l provides the maximum efficiency around h ~, = 40 eV for P M 3 / F M 2. The results shown in figs. 4a and b are not inconsistent with these predictions. We will further comment on the performance in the lowest energy region (not shown in fig. 4). For P M s / F M 2, the grating HLG10 provides strong photon flux between 18 and 8 eV. Although the photon flux is very low above h~, = 18 eV as designed, we have observed remarkable oscillatory structures there. On the other hand, the grating H L G l l has shown only oscillatory structures for P M s / F M 2 in the whole region. According to a simplified model for interference, such an oscillation is a function of ~rh(cos ct +cos fl)/)~, where ct and fl are the incidence and diffraction angles of the light on the grating, h is the groove depth and )~ is the wavelength [7]. Therefore, ther oscillatory region is expanded into the longer wavelength region, when the groove depth is enlarged. The result for H L G l l with h = 280 ,~ is thus qualitatively understood in comparison with that for HLG10 with h = 120 ,~. As a whole, the laminar grating HLG10 is more appropriate than
83
H L G l l for this PGM monochromator, in order to realize the wide working range of h ~ as well as the high efficiency in the highest energy region. One may recognize in fig. 4 that the C (carbon) ls edge structure is rather obviously observed for H L G l l , whereas the structure is not remarkable for HLG10. We consider that this is mainly due to the degradation of the PM 1 mirror, since slight carbon contamination was observed on PM 1 when we replaced the grating HLG10 by H L G l l in May 1983. The degradation of PM 1 may have been induced by a rather poor vacuum ( - 1 0 -8 Torr) during test measurements for wavelength calibration.
5. Application to spectroscopy The solid curves in fig. 5 show the reflectance spectra of VSe 2 and VO 2 in the vanadium 3p core excitation region, where 3p ~ 3d excitation is dominant. VSe 2 is a layer type crystal, which shows metallic conductivity. On the contrary, VO 2 is well known to show the metal-insulator transition at T¢ = 340 K (metallic above To). The spectrum of VO 2 in fig. 5 is for the metallic phase. The spectrum in the insulator phase is, however, hardly different from that in the metallic phase, in contrast to our expectation based on a screening of the 3p-3d interaction by the free carriers in the metallic phase. Then we consider that the 3p --* 3d excitation in VO 2 is rather localized in both insulator and metallic phases. The remarkable difference of the V 3p core excitation between VO 2 and VSe 2 is then assigned to the different degree of hybridization of the V 3d orbitals
"7 [
/theoretical
:1
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/i B
.=
VO2(M)
A1
2
, ...,r..
40
50 60 photon energy(eV)
Fig. 5. Reflectance spectra of VSe2 and VO2 in the metal phase (solid curves) measured in the region of the V 3p core excitation. The dashed curves show results of the theoretical calculation of c2 based on the localized excitation model [8]. III. VUV/SOFT X-RAYS
84
S. Suga et al. / Plane-grating grazing-incidence UHV monochromator
with anion p orbitals. Since the vanadium ion is considered to be tetravalent in these compounds (namely, one 3d electron occupied in the ground state (m = 1)), the spectrum is expected to be similar to that of the Ti 3+ 3p core excitation [8]. By taking the above-mentioned hybridization into account, we have analyzed the experimental result on the 3p53d " ÷ t localized excitation model as shown by the dashed curves in fig. 5. Assuming the cubic crystal field lODq = 3.3 eV, the reduction factors for the S l a t e r - C o n d o n parameters are evaluated as x F = 0.6, gG = 0.5 for VO 2 and xv = 0.35 and leg = 0.25 for VSe 2. The small reduction factors for the 3 p - 3 d Coulomb and exchange interactions in VSe 2 demonstrate the high degree of hybridization of the V 3d orbitals. A similar experiment has been carried out for polytypes of TaSe 2. As another example of its application in the higher photon energy region, we have checked the photoelectron diffraction for the S absorbed on Ni(001), by use of a hemispherical electron energy analyzer. Diffraction of the photoelectron was measured for the S 2p core ( E b = 162 eV) electrons of the c(2 × 2)S-Ni(001). The sample was prepared in the U H V sample preparation chamber by introducing H2S gas onto a Ni(001) clean surface obtained by cycles of Ar ion bombardment and annealing at 800°C by electron bombardment. The c(2 x 2) structure was examined by LEED. Fig. 6 shows the result of the normal photoelectron diffraction as a function of h~, (open circles). This result is in qualitative agreement with that of Rosenblatt et al. [9], elucidating that the present P G M monochromator installed at the 0.38 GeV storage ring is providing effective photon flux up to hu = 400 eV. Since we did this experiment before installing the focusing mirrors behind the exit slit, the signal to noise ratio was not satisfactory. By use of the additional focusing mirrors mentioned above, the beam
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250
i
,
,
i
300 350 Photon Energy (eV)
Fig. 6. Normal emission photoelectron diffraction for the S 2p core level (Eb=162 eV) of c(2X2)S-Ni(001) (circles). The dashed curve shows the result of Rosenblatt et al. [9].
is focused into a small spot (0.5 x 0.5 mm 2) on the sample. As for the intensity in this case, one can have a rough idea from the following experimental result: 1000 c o u n t s / s for the angle resolved photoemission ( + 1 ° and A E = 0.1 eV) from the 2p core levels of S i ( l l l ) excited at hv = 150 eV (Ahp < 1 eV) at a stored current of 150 mA.
6. Summary A plane grating monochromator with an ion-beam etched laminar grating is constructed and installed at the third beam line of S O R - R I N G . Since the entire monochromator is bakable, a U H V better than 2 x 10 -~° Torr is easily attained after a bake at 200°C for 24 h. By using a grating with 1200 g r o o v e s / m m , the photon energy region from 8 to 400 eV is covered with a rather low content of the stray and higher order light. A small beam size ( < 0 . 5 x 0 . 5 mm 2) is realized on the sample, which facilitates the angle resolved photoemission spectroscopy as well as optical studies of various samples of small dimensions. The authors acknowledge Professor H. Aritome, Professor S. N a m b a and Dr. S. Matsui for providing us ion-beam etched laminar gratings. They are grateful to Professor H. Kanzaki and colleagues of the synchrotron radiation laboratory, for their continuous support. They are also much obliged to Mr. T. Uchibori, Mr. M. Mineta and Mr. M. Yorimoto of the workshop of our institute for their skilful production of parts of the monochromator.
References [1] K. Miyake, R. Kato and H. Yamashita, Sci. Light 18 (1969) 39. [2] W. Eberhardt, G. Kalkoffen and C. Kunz, Nucl. Instr. and Meth. 152 (1978) 81. [3] S. Matsui, K. Moriwaki, H. Aritome, S. Namba, S. Shin and S. Suga, Appl. Opt. 21 (1982) 2787. [4] H. Aritome, S. Matsui, K. Moriwaki, H. Aoki, S. Namba, S. Suga, A. Mikuni, M. Seki and M. Taniguchi, Nucl. Instr. and Meth. 208 (1983) 233. [5] M. Neviere, J. Flamand and J.M. Lerner, Nucl. Instr. and Meth. 195 (1982) 183. [6] H. Petersen, J. Haase, A. Puschmann, A. Reimer and R. Treichler, Proc. Synchr. Rad. Users Workshop, May 1983. [7] A. Franks, K. Lindsey, J.M. Bennett, R.J. Speer, D. Turner and D.J. Hunt, Trans. R. Soc. London A277 (1975) 503. [8] T. Yamaguchi, S. Shibuya, S. Suga and S. Shin, J. Phys. C15 (1982) 2641. [9] D.H. Rosenblatt, J.G. Tobin, M.G. Mason, R.F. Davis, S.D. Kevan and D.A. Shirley, Phys. Rev. B23 (1981) 3828.