Nuclear Instruments and Methods 208 (1983) 303-305 303
North-Holland Publishing Company
A HIGH FLUX NORMAL INCIDENCE MONOCHROMATOR FOR CIRCULARLY POLARIZED
SYNCHROTRON RADIATION
A. EYERS, Ch. HECKENKAMP, F. SCHAFERS, G. SCHONHENSE and U. HEINZMANN
Fritz- Haber-lnstitut der Max-Planek-Gesellschaft, Faradayweg 4-6, 1000 Berlin 33, Germany
Design and performance of a 6.3 m normal incidence UHV-monochromator (see fig. l) of the Gillieson type for synchrotron
radiation are described. The monochromator will be used for simultaneously spin polarization and emission angle-resolved
photoelectron spectroscopy; it combines a high photon flux and a high degree of circular polarization of the monochromatized
radiation with a moderate resolution.
Special features of the design of the monochromator
are the following:
- Apertures movable in vertical direction allow the
selection of left-handed or right-handed circular polari-
zation (off-plane radiation).
- Apertures movable in horizontal direction may be
closed to improve the resolution (since no entrance slit
is used).
- The spherical mirror is adjustable to refocus the
beam onto the exit slit after each injection.
-Two different gratings interchangeable under
vacuum are used to cover the wavelength range from the
visible to the VUV.
- The wavelength scan is performed by a simulta-
neous rotation and, to maintain focusing, an indepen-
dent translation of the grating; both as well as the
motion of the vertical apertures are controlled by a
minicomputer.
- The monochromator has two exit slits, which will
be used for studies of atomic and molecular photoioni-
zation (in connection with a differential pumping stage)
and for surface photoemission studies, respectively.
The monochromator was built by Chelsea Instru-
ments Ltd., London U.K. (electronic hard and software),
by Bird and Tole Ltd., High Wycombe U.K. (mechani-
cal precision equipment) and by Vacuum Generators
Ltd., Hastings U.K. (vacuum vessel) and is now con-
nected with a beam line at BESSY, Berlin.
Technical data for the monochromator is listed in
table 1.
The resolution of the monochromator has been mea-
sured using grating 1 (1200 lines/mm) in connection
with a sodium D-lines resonance light source and a real
entrance slit (width 1 mm). For a 0.5 mm opened exit
slit the resolution measured by scanning the wavelength
scale of the monochromator and detecting the light
PLANE
(}RATING
t) 1200
i/ram
2)
3600
I/ram
---.....
BESSY
APERTURES
"'--
EXIT
SLIT
GAS PHASE
ONTAL V -~~
VERTICAL ~"~.- -~1~"~ HE R IE A L STUDIES
{ ÷ ~Qu, z,~. ~ .~ -- MIRROR
\o ÷-, o--, ~ -light) (R = 6,3m)
"wi~ SURFAC E
EXIT SLIT STUDIES
Fig. 1. Set-up of the Gillieson-type monochromator.
0167-5087/83/0000-0000/$03.00 © 1983 North-Holland IV. MIRRORS/GRATINGS/MONOCHROMATORS
304
A. Eyers et aL
/
Normal incidence monochromator
Table 1
Technical details of the monochromator
Spherical mirror (radius 6.3 m)
Acceptance
Plane grating
1) 1200 lines/mm MgF 2 +AI
coated
2) 3600 lines/mm Os coated
Dispersion
Absolute resolution expected for
electron beam and exit slit widths
of 1 mm
Expected circular polarization
VUV-intensity expected behind the
exit slit (electron current 500 mA
wavelength range and bandwidth
as in 2 above)
365X80X65 mm 3
AI + Os coated
54 mrad horizontal
10 mrad vertical
260× 140×60 mm 3
wavelength range
100-675 nm
wavelength range
35-130 nm
1) 5.4 mm/nm
2) 16.2 mm/nm
1) 0.33 nm
2) 0.12 nm
90%
> 10H photons/s
intensity behind the exit slit is shown in fig. 2, lower
and upper part, for horizontal apertures accepting 12
mrad and 54 mrad light divergence, respectively. The
resolution values measured shown in fig. 2 are in agree-
ment with the corresponding expected data mentioned
above.
The vacuum in the monochromator has been mea-
sured to be in the 10 -9 mbar after the monochromator
was installed at the BESSY beamline and was baked
out, after the optical components were built in and
during the full beamline and the monochromator is in
operation with synchrotron radiation.
9 ~
8-
7 ~
,,z
6 ~
o 5-
4 ¸
z 3 ¸
uJ
~, 2 ¸
1.
~'~ ~ 54 mrad
6~~3~ acceptance
o I ---
1'
~/'-T --~' " ~,-12 mrad
O L / k / ~horizonta[ acceptance
588.5 589.0 589.5 590.0 590.5
WAVELENGTH (nm}
Fig. 2. Resolution of the monochromator measured using a
sodium D-lines light source.
1
I
[erb.un.]
0.5
........ L.
-4
-2
0
2 L,
'I.'
.... ~... k,__
_t,-2 0 4
t~ [ mrad ]
I,,+I~
2
-/, -2 0 2 t,
Fig. 3. Intensity dependence of the radiation components
polarized horizontally (111) and vertically (1±) upon the vertical
angle + (_+0.25 mrad) the radiation is emitted with respect to
the BESSY plane. The intensities Ill' 1± and their sum have
been measured behind the monochromator exit slit at a wave-
length of 450_+ 0.2 nm.
1.0 , , , , , , , ,
0.8
i /
PL /1000 nm
0.6
Or,
-Z, -2 0 2 Z,
[ mrod ]
10
08
IPzI
06
O&
02
b
-z, -2 0 2
[ mred ]
Fig. 4. (a) Degree of linear and (b) circular polarization, PL and
Pz respectively, measured behind the monochromator exit slit
at a wavelength of 300_+0.2 nm as function of the vertical angle
~b the radiation is emitted with respect to the BESSY plane. The
experimental results are compared with calculated curves for
100 nm and 1000 nm wavelength (BESSY handbook, 1979).
A. Eyers et al. / Normal incidence monochromator 305
We have also measured the degrees of linear and
circular polarization PL and Pz of the monochromatized
radiation behind the exit slit by use of grating 1 (1200
lines/mm) in the near UV and visible spectral range.
The measurements have been performed by use of a
conventional Glan prism and a quarter wave plate and a
photodiode as the analyzer and the detector, respec-
tively. By moving both vertical apertures simultaneously
thus forming a horizontal slit of a width corresponding
to 0,5 mrad, intensity profiles of the BESSY radiation
have been measured. Fig. 3 shows the components Ill
and 11 of the radiation emitted, whose electric field
vector oscillates parallel and perpendicular with respect
to the orbital plane of the storage ring, and the total
intensity; they are functions of the vertical angle ~b the
radiation with a wavelength of 450 nm is emitted with
respect to the orbital plane of BESSY (q~=0). The
shapes and the halfwidths of the profiles are in good
agreement with the expected calculated values although
the actual position of the orbital plane which may
change slightly after each injection seems to be a little
bit too low.
Figs. 4(a) and (b) show the degree of linear and
circular polarization PL and Pz as function of the
vertical angle ~ the radiation is emitted with respect to
the BESSY plane. We have measured by use of the
quarter wave plate, that the radiation behind the mono-
chromator exit slit is completely polarized, i.e. P
= ~/(p2 + pz z ) = l, and that the circular polarization is
right-handed and left-handed for light emitted into di-
rections above and below the plane, respectively. The
experimental results, obtained at a wavelength of 300
nm are shown in fig. 4 as error crosses, where the
horizontal error bars are connected with the slit width
of the vertical apertures and the vertical error bars
determine the experimental uncertainty. The compari-
son with the calculated curves for wavelengths of 100
nm and 1000 nm based upon the Schwinger theory
shows a quantitative agreement.
The experimental results discussed in this paper
which are in good agreement with the calculated and
expected values show that BESSY is indeed a good
source of polarized radiation and that the monochroma-
tor described is well qualified for its application to
monochromatize the circularly polarized radiation
without a larger depolarization effect.
Advice and help by J. Wheaton and his colleagues of
Chelsea Instr. Ltd., by D. Tole and his colleagues of
Bird and Tole Ltd., by W. Peatman and his collegues of
BESSY and by U. Friess of Fritz-Haber-Institut is
gratefully acknowledged.
IV. M IRRORS/GRAT1NGS/MONOCHROMATORS