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Optimisation of grazing-incidence monochromators
Nuclear Instruments and Methods 172 (1980) 143-148 © North-Holland Publishing Company
OPTIMISATION OF GRAZING-INCIDENCE MONOCHROMATORS Robert L. JOHNSON Max.Planek-Institut far Festkdrperforsehung, Heisenbergstr. 1, 7000 Stuttgart 80, Germany
The general characteristics of existing grazing-incidence monochromators are described and the present technological limitations discussed. A total system design approach is emphasized whereby the characteristics of a particular synchrotron light source can be optimally matched to an experimental technique. As an example FLIPPER II, an ultrahigh vacuum grazing-incidence monochromator for photoemission experiments built at the Max-Planck-Institut Stuttgart for use at DESY, Hamburg is briefly described.
1. Introduction
parallel. Only a simple rotation of the grating is required for wavelength scanning so it is comparatively easy to make an UHV compatible monochromator. It is necessary to use additional mirrors with the plane grating and the angles of incidence at the grating and mirror can be chosen to suppress higher orders over a limited wavelength range [7]. By linking a linear translation and rotation of a premirror to the grating rotation Kunz et al. [8] introduced a new monochromator mounting which provided suppression of higher orders over a wide wavelength range. The new mounting allowed the grating to operate in the blaze maximum for all wavelengths, the energy resolution was high and remained essentially constant, and monochromatic light was brought to an almost point-like focus by a paraboloidal mirror. The complicated linkage to translate and rotate the single premirror was abandoned for the UHV version of this monochromator and instead a series of six premirrors at different angles of incidence which could be individually placed in the incident beam were used. The angles of incidence of the mirrors and the mirror coatings were carefully selected to suppress higher orders over all the wavelength ranges. The resolution (E/zXE) varies from 5 6 0 0 - 1 2 5 0 over the energy range 1 3 - 2 5 0 eV and in practice is largely determined by the optical quality of the paraboloidal mirror and its accurate alignment. Photon fluxes of 3 X l0 s ph.s -a at 92 eV with a resolution 1400 have been reported for DORIS at 100 mA 2.2 GeV. For more details see refs. [9,10]. A number of plane grating monochrmators of varying complexity have been built for use with synchrotron radiation. One interesting variation is the
Grazing-incidence monochromators have recently been reviewed by Gudat and Kunz [1] and an earlier survey was given by Kunz [2]. Detailed descriptions of practically all existing monochromators have been published and much useful information is contained in refs. [3,4]. In this paper the general characteristics of grazing-incidence monochromators will be described, improvements to existing systems will be discussed and finally an optimised instrument will be mentioned. From the point of view of optical design it is convenient to classify grazing-incidence monochromators according to the type of diffraction grating used in the instrument. Thus monochromators using plane gratings form one category, those with spherical gratings a second category and those utilizing aspheric gratings form a third category. Monochromators which use two gratings have been constructed, however, owing to the loss in intensity caused by the second grating such systems have very limited application [5,6]. Successful monochromators of all three categories have been built and are in use at various synchrotron radiation facilities. Typical optical systems are shown in fig. 1.
2. Plane grating monochromators Monochromators in this category are appealing because of their inherent simplicity. Usually they exploit the small vertical divergence of synchrotron radiation and the incoming light is considered to be 143
IV. XUV MONOCHROMATOR SYSTEMS
144
R.L. Johnson/, Grazing-incidence monochromators PLANE GRATtNG
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monochromator used by Schmahl et al. [11] for their zone-plate X-ray microscope. In their monochromator the synchrotron radiation diffracted from a particularly large (20 cm) diffraction grating is brought to a focus on the sample by means of a condenser zone@ate.
3. Spherical grating monochromators High resolution spectroscopy in the grazing incidelace region has been traditionally performed with instruments using the Rowland circle mounting. Although the ReMand mounting provides excellent
R.L. Johnson / Grazing-incidence monochromators
resolution in its conventional form it has the disadvantage that the exit beam changes direction as the wavelength is changed. This problem was overcome by Codling and Mitchell [12] by using rotating mirror-slit combinations to change the direction of the incoming and outgoing beams. The entrance and exit slits positions remain fixed as the grating moves along the Rowland circle. This instrument produced slit-width limited resolution at HeI but has the disadvantage that as the wavelength is increased the grazing angles become smaller. Thus the geometry of the instrument is working against the blaze effect of the diffraction grating and this limits the usable wavelength range. The spectromonochromator developed by Jaegl4 and coworkers has many interesting improvements ref. [13] and refs. therein). For example, by simply replacing the plane premirror by a toroidal focussing mirror they increased the intensity by two orders of magnitude [14]. The instrument at LURE is extremely versatile. The angle between the entrance and exit slits can be freely chosen according to the type of experiment and wavelength region of interest. The ability to freely choose the angle of incidence is very important in obtaining optimum intensity since it has been shown that the experimental blaze maximum often differs from the theoretical value. Appropriate choice of the incidence angle Nso allows the higher order components to be reduced. However at wavelengths greater than 20 nm it becomes difficult to remove the contribution from higher orders. Because of the complicated motions required in the Rowland mounting it is difficult to make such a monochromator UHV compatible, however, this has been achieved by Brown et at. [15]. The "grasshopper" monochromator is based on the Vodar geometry. A l m Rowland circle rotates about the fixed exit slit to give a constant exit beam direction. The grating is linked to the mirror-slit and operates at a fixed grazing angle of 2 °. The mirror-slit and premirror translate parallel to the incoming beam on a linear air bearing. Using a Bausch and Lomb 600 lines/ram grating with 15 ~m entrance and exit slits the monochromator provides a spectral bandwidth of 0.15 A and range from 20 to 800 eV. Ahigh quality ultrasmooth cooled platinum coated copper mirror is used to collect and focus about 2 mr of the direct beam in the horizontal plane. Since the monochromator is well matched to the synchrotron light source high output fluxes are obtained (5 × 101° photons/s at 10 nm from SPEAR operating at 2.5 GeV 25 mA)
145
[16]. The short wavelength limit is primarily set by the mirror-slit combination which is particulady sensitive to carbon contamination because it is at 4 ° for 44 nm. Because of the small grazing angle higher orders can be a problem and stray light levels have been found to be rather high. However, this monochromator has certainly set new standards for constant deviation UHV grazing incidence monochromators.
4. Aspheric grating monochromators The first grazing-incidence monochromator for synchrotron radiation to use a toroidal diffraction grating was described by Madden and Ederer [17]. Although the monochromator having only a single optical element may be considered as the ideal system additional mirrors are usually necessary in practice. A toroidal grating system which also uses a toroidal premirror and a toroidal focussing mirror has been in operation on the ACO storage ring for some time [18]. This system provides high flux (~1011 phot/Jk s at 85 eV with 100 mA, 540 MeV in ACO), a resolution of ~0.7 A at 170 A with 100 #m slits and operates over the range 15-100 eV. The monochromator is mechanically simple because only a rotation of the grating is required. The good performance of this monochromator can be largely attributed to the premirror which accepts 5 mr of the light emitted by the ACO storage ring which also has excellent characteristics. Present literature [19] gives the impression that the astigmatism correction is due to the holographic grating whereas in fact the primary astigmatism correction is provided by the toroidal substrate. The fact that the grating is made holographically, i.e. with laser light, can be used to provide higher order corrections which are then, of course, wavelength dependent. The spectral range of simple rotation toroidal grating monochromators is restricted and the resolution depends strongly on the aperture. It is difficult to design a system in which the grating operates under optimum conditions at short wavelengths and contributions from higher orders can become significant at larger wavelengths. The polarization effects of the ACO TGM have been checked and little difference was measured between the horizontal and vertical orientations of the monochromator. In general it is not obvious what depolarising effects toroidal grating monochromator will produce - particularly when carbon contaminaIV. XUV MONOCHROMATOR SYSTEMS
146
RIL. Johnson / Grazing-incidence monochromators Hence it was demonstrated that it is possible to fabricate useable soft X-ray diffraction gratings on extremely curved substrates by using holographic techniques and that toroidal mirrors of sufficient quality for use at grazing incidence could be fabricated by conventional optical techniques. The techniques of holography combined with aspheric mirrors obviously opens up new design possibilities for grazing incidence. However, these possibilities have only been slightly explored.
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Over the past few years there have been significant improvements in the techniques used to fabricate mirrors and diffraction gratings designed for use at grazing incidence. Sophisticated methods have been developed for making large X-ray telescopes and these techniques can also be used for making high quality mirrors for synchrotron radiation applicatioms. The laser research programs have led to the development of mirrors capable of withstanding very high power and such mirrors also find applications in beamline optics. There are, unfortunately, unique problems associated with distortion and contamination of mirrors when used in high energy synchrotron radiation. There is no easy solution available at present and more research is necessary in this field. The diffraction grating is the central component of
WAVELENGTH ( n m ) Fig. 2. The variation of peak diffraction efficiency with wavelength for a laminar grating. The optimum grazing angles are also shown.
tion is present on the grating surface. It is possible to use toroidal gratings in Rowland circle monochromators, however, it is necessary to vary the angle of incidence with wavelength so that the condition for stigmation is fulfilled [20] (i.e. sin sin [3 = p/R where c~ and/3 are the grazing angles of incidence and diffraction, respectively, and p and R are the minor and major radii of the toms). Stigmatic images at wavelengths as short as 2.3 nm have been obtained from toroidal holographic gratings [21].
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Fig. 3. Summary of the best 24 diffraction efficiency measurements at 4.5 nm plotted against grazing angle arranged according to grating frequency.
R.L. Johnson / Grazing-incidence monochromators
a grazing incidence monochromator and it is the absolute diffraction efficiency of the grating which determines the photon flux at the exit slit. Hence, the easiest way to improve a monochromator is to use a better diffraction grating. Naturally the optimum grating has to be chosen according to the geometry and wavelength region of use. It is important to remember that all phase gratings have maximum efficiency at a particular angle of incidence for a given wavelength. The angle of incidence for maximum efficiency increases with wavelength. This can be seen in fig. 2 which shows the variation in peak diffraction efficiency (i.e. at the optimum angle) with wavelength for a laminar grating plotted from the data given by Haelbich [22]. The grating was designed for use at 10 nm by appropriately choosing the groove depth and indeed produces a very high diffraction efficiency at this wavelength. Theoretically laminar gratings should suppress all even orders, however, this is only true if the grating is operating in the correct geometry at the optimum wavelength. In practice laminar gratings can also have significant higher order efficiency. The properties of grazing incidence diffraction gratings have been discussed in a previous paper [23] and as a summary fig. 3 is reproduced from that paper. It can be seen that there is a significant reduction in efficiency as the line frequency increases, however, highly efficient 1200 lines/ram gratings are available. Fig. 3 shows only the results for the best 24 gratings out of over 100 measured at Imperial College, London. There is over an order of magnitude between the best and worst gratings so it is possible to gain an order of magnitude in flux by simply choosing the correct grating.
6. New possibilities and technological limitations We have seen that all existing grazing-incidence monochromators have various weaknesses. When choosing among existing designs it is necessary to carefully consider both the particular characteristics of the synchrotron light source to be used and also the types of experiments to be performed. For example there is no point in using a state-of-the-art prefocussing mirror in a monochromator without an entrance slit if the surface becomes distorted and eventually destroyed by high energy radiation. Of course low cost plane mirrors should be used in such situations but these are also part of the optical sys-
147
tern and will introduce aberrations if they become distorted. If we assume that we have the ideal synchrotron light source for VUV experiments then the ideal monochromator must accept the maximum solid angle to provide maximum luminosity, must be continuously tuneable throughout the VUV range, and must be capable of high resolution. These objectives can only be reached if a total design approach is used, starting from the characteristics of the electrons in the ring, through the beamline, monochromator and ending at the sample under investigation. There are, of course, many physical constraints and compromises have to be made to find a suitable balance. Most of the problems are of a purely technical nature relating to the fabrication of high quality mirrors and the mechanical realization of UHV compatible mechanisms. Recently developed testing techniques for grazing incidence mirrors should lead to an improvement in quality [24]. Hence, it should be possible to build a better monochromator with the improved optical components now available. One design which is consistent with the requirements of our ideal monochromator is that originally suggested by Kunz [8]. If this monochromator is combined with beamline optics which accept a large horizontal angle and produce a true parallel beam the optical system becomes equivalent to that of the Ebert-Fastie mounting using paraboloidal mirrors. Applying the principle that all rays must travel equal paths to form a perfect image it can be seen that this design is essentially aberration free when used with a synchrotron light source. In practice the aberrations of this design will be determined by the optical quality of the off-axis paraboloidal mirrors and the accuracy of their adjustment. It is relatively easy to make large high quality plane diffraction gratings and the monochromator grating-mirror combination ensures that the grating operates at near optimum conditions and effectively suppresses higher orders. The system is relatively simple mechanically and requires only a high precision rotation of the grating for the wavelength scan and a mechanism to bring the appropriate monochromator mirror into the beam. A second version of the FLIPPER [25] monochromator has been built at the Max-Planck-Institut ffir Festk6rperforschung, Stuttgart, for use at HASYLAB *. The monochromator is to be located 20 m from the tangent point on DORIS and focussing * HASYLAB - Hamburger Synchrotronstrahlungslabor. IV. XUV MONOCHROMATORSYSTEMS
148
R.L. Johnson /Grazing-incidence monochromators
beam line optics will be used. Contamination and distortion of the mirrors by the high energy components of the synchrotron radiation will most probably be the limiting factors in the performance. By using a state of the art diffraction grating and paraboloidal mirror it is hoped that a significant improvement over the existing FLIPPER monochromator will be achieved. I wish to thank Prof. C. Kunz, Dr. E.E. Koch and the DESY F41 group for their invaluable support and many helpful discussions.
References [1]W. Gudat and C. Kunz, in Synchrotron radiation, ed. C. Kunz, Topics in current physics (Springer-Verlag, Heidelberg, 1978). [2] C. Kunz, in Proc. Symp. for Synchrotron radiation users, eds. G.V. Marr and I.H. Munro (Daresbury Nucl. Phys. Lab. Report DNPL/R26, 1973). [31 E.E. Koch, R. Haensel and C. Kunz, eds., Vacuum ultraviolet radiation physics (Pergamon-Vieweg, Braunschweig, 1974). [4] Synchrotron Radiation Instrumentation and Developments, eds. F. Wuilleumier and Y. Farge, Nucl. Instr. and Meth. 152 (1978). [5] P. Jaegl~, P. Dhez and F. Wuilleumier, in ref. [3], p. 788. [6] C.H. Pruett, N.C. Lien and J.D. Steben, Proc. IIIrd Int.
Conf. Vacuum ultraviolet radiation physics, Tokyo (1971), p. 31. [7] K.P. Miyake, R. Kato and H. Yamashita, Sci. Light 18 (1969) 39. [8] C. Kunz, R. Haensel and B. Sonntag, J. Opt. Soc. Am. 58 (1968) 1415. [9] W. Eberhardt, Thesis, Universit~t Hamburg (1978). [ 10] G. Kalkoffen, Thesis, Universit~t Hamburg (1978). [11] G. Schmahl, D. Rudolph and B. Niemann, J. de Phys., Coll. C4 (suppl. no. 7) 39 (1978) 2(12. [12] K. Codling and P. Mitchell, J. Phys. E3 (1970) 685. [13] P. Dhez, P. Jaegl~, F. Wuilleumier, E. K~llne, V. Schmidt, M. Berland and A. Carillar, in ref. [4], p. 85. [14] M.Y. Adam, F. Wuilleumier, S. Krummacher, N. Sandner, V. Schmidt and W. Mehlhorn, J. Electr. Spectr. 15 (1979) 211. [15] F.C. Brown, R.Z. Bachrach, S.B.M. Hagstr6m, N. Lieu and C.H. Pruett, in ref. [ 3 ], p. 785. [16] F.C. Brown, R.Z. Bachrach, N. Lien, in ref. [4], p. 73. [17] R.P. Madden and D.L. Ederer, J. Opt. Soc. Am. 62 (1972) 722. [18] Y. Petroff, P. Thiry, R. Pinchaux and D. Lepere, Vth Int. Conf. Vacuum ultraviolet radiation physics, (abstracts), Montpellier (1977), p. 70. [ 19 ] Jobin-Yvon, Technical Information LHT 30. [20] H. Haber, J. Opt. Soc. Am. 40 (1950) 153. [21] R.J. Speer, D. Turner, R.L. Johnson, D. Rudolph and G. Schmahl, Appl. Opt. 13 (1974) 1258. [22] R.P. Haelbich, C. Kunz, D. Rudolph and G. Schmahl, in ref. [4], p. 127. [23] R.L. Johnson, in ref. [4], p. 117. [24] R.J. Speer, M. Chrisp, D. Turner, S. Mrowka and K. Tregidgo, Appl. Opt. 18 (1979). [25] W. Eberhardt, G. Kalkoffen and C. Kunz in ref. [4], p. 81.
OPTIMISATION OF GRAZING-INCIDENCE MONOCHROMATORS Robert L. JOHNSON Max.Planek-Institut far Festkdrperforsehung, Heisenbergstr. 1, 7000 Stuttgart 80, Germany
The general characteristics of existing grazing-incidence monochromators are described and the present technological limitations discussed. A total system design approach is emphasized whereby the characteristics of a particular synchrotron light source can be optimally matched to an experimental technique. As an example FLIPPER II, an ultrahigh vacuum grazing-incidence monochromator for photoemission experiments built at the Max-Planck-Institut Stuttgart for use at DESY, Hamburg is briefly described.
1. Introduction
parallel. Only a simple rotation of the grating is required for wavelength scanning so it is comparatively easy to make an UHV compatible monochromator. It is necessary to use additional mirrors with the plane grating and the angles of incidence at the grating and mirror can be chosen to suppress higher orders over a limited wavelength range [7]. By linking a linear translation and rotation of a premirror to the grating rotation Kunz et al. [8] introduced a new monochromator mounting which provided suppression of higher orders over a wide wavelength range. The new mounting allowed the grating to operate in the blaze maximum for all wavelengths, the energy resolution was high and remained essentially constant, and monochromatic light was brought to an almost point-like focus by a paraboloidal mirror. The complicated linkage to translate and rotate the single premirror was abandoned for the UHV version of this monochromator and instead a series of six premirrors at different angles of incidence which could be individually placed in the incident beam were used. The angles of incidence of the mirrors and the mirror coatings were carefully selected to suppress higher orders over all the wavelength ranges. The resolution (E/zXE) varies from 5 6 0 0 - 1 2 5 0 over the energy range 1 3 - 2 5 0 eV and in practice is largely determined by the optical quality of the paraboloidal mirror and its accurate alignment. Photon fluxes of 3 X l0 s ph.s -a at 92 eV with a resolution 1400 have been reported for DORIS at 100 mA 2.2 GeV. For more details see refs. [9,10]. A number of plane grating monochrmators of varying complexity have been built for use with synchrotron radiation. One interesting variation is the
Grazing-incidence monochromators have recently been reviewed by Gudat and Kunz [1] and an earlier survey was given by Kunz [2]. Detailed descriptions of practically all existing monochromators have been published and much useful information is contained in refs. [3,4]. In this paper the general characteristics of grazing-incidence monochromators will be described, improvements to existing systems will be discussed and finally an optimised instrument will be mentioned. From the point of view of optical design it is convenient to classify grazing-incidence monochromators according to the type of diffraction grating used in the instrument. Thus monochromators using plane gratings form one category, those with spherical gratings a second category and those utilizing aspheric gratings form a third category. Monochromators which use two gratings have been constructed, however, owing to the loss in intensity caused by the second grating such systems have very limited application [5,6]. Successful monochromators of all three categories have been built and are in use at various synchrotron radiation facilities. Typical optical systems are shown in fig. 1.
2. Plane grating monochromators Monochromators in this category are appealing because of their inherent simplicity. Usually they exploit the small vertical divergence of synchrotron radiation and the incoming light is considered to be 143
IV. XUV MONOCHROMATOR SYSTEMS
144
R.L. Johnson/, Grazing-incidence monochromators PLANE GRATtNG
-~1,' BEAM
-LINE
OPTICS
/ j=:::~__
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~
PAR MR I ROA~)LOI R OAL
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/Q, DOUBLE "q/. FOCUSI NG , "' EXIT MIRROR ENTRANCE / , ~ ' "" ". " , \ BEAM ~ APERTURE ,~ ~ " ' . ,\ ~'~ HORIZONTAL M (X) J'M2'" - . , 2' PHOTOEMISSION FOCUSING 2' ~.L" ~ ",L~'~AA~A~ORPTION PHO1 OTHER BEAM .... ~ 2 ~ EXPERIMENTS EXPERIMENTS MONOCH ROMATO R
DEFLECTOR , ,~=~-.~ " _2_
/r.ln~ / ~ / - ~ a" --..'~ ~ ~ GlX) ."~,~ ~ MI(O) GRATING b.: m mlw'W FROM vERTICAL FOCUSING SPEAR COLLECTION MIRROR
2°
. . . . .FILTERS . . EXIT VALVE
to toroidoi mirror 3:0
t"
.3m
~.3a m
t 5
roidol
m
toroidll 1
%-. J
Smrad
pho,o...,..ioo C Y ) experiment Fig. 1. Typical optical systems for grazing-incidence monochromators: (a) plane grating monochromator, (b) spherical grating monochromator, (c) toroidal grating monochromator.
monochromator used by Schmahl et al. [11] for their zone-plate X-ray microscope. In their monochromator the synchrotron radiation diffracted from a particularly large (20 cm) diffraction grating is brought to a focus on the sample by means of a condenser zone@ate.
3. Spherical grating monochromators High resolution spectroscopy in the grazing incidelace region has been traditionally performed with instruments using the Rowland circle mounting. Although the ReMand mounting provides excellent
R.L. Johnson / Grazing-incidence monochromators
resolution in its conventional form it has the disadvantage that the exit beam changes direction as the wavelength is changed. This problem was overcome by Codling and Mitchell [12] by using rotating mirror-slit combinations to change the direction of the incoming and outgoing beams. The entrance and exit slits positions remain fixed as the grating moves along the Rowland circle. This instrument produced slit-width limited resolution at HeI but has the disadvantage that as the wavelength is increased the grazing angles become smaller. Thus the geometry of the instrument is working against the blaze effect of the diffraction grating and this limits the usable wavelength range. The spectromonochromator developed by Jaegl4 and coworkers has many interesting improvements ref. [13] and refs. therein). For example, by simply replacing the plane premirror by a toroidal focussing mirror they increased the intensity by two orders of magnitude [14]. The instrument at LURE is extremely versatile. The angle between the entrance and exit slits can be freely chosen according to the type of experiment and wavelength region of interest. The ability to freely choose the angle of incidence is very important in obtaining optimum intensity since it has been shown that the experimental blaze maximum often differs from the theoretical value. Appropriate choice of the incidence angle Nso allows the higher order components to be reduced. However at wavelengths greater than 20 nm it becomes difficult to remove the contribution from higher orders. Because of the complicated motions required in the Rowland mounting it is difficult to make such a monochromator UHV compatible, however, this has been achieved by Brown et at. [15]. The "grasshopper" monochromator is based on the Vodar geometry. A l m Rowland circle rotates about the fixed exit slit to give a constant exit beam direction. The grating is linked to the mirror-slit and operates at a fixed grazing angle of 2 °. The mirror-slit and premirror translate parallel to the incoming beam on a linear air bearing. Using a Bausch and Lomb 600 lines/ram grating with 15 ~m entrance and exit slits the monochromator provides a spectral bandwidth of 0.15 A and range from 20 to 800 eV. Ahigh quality ultrasmooth cooled platinum coated copper mirror is used to collect and focus about 2 mr of the direct beam in the horizontal plane. Since the monochromator is well matched to the synchrotron light source high output fluxes are obtained (5 × 101° photons/s at 10 nm from SPEAR operating at 2.5 GeV 25 mA)
145
[16]. The short wavelength limit is primarily set by the mirror-slit combination which is particulady sensitive to carbon contamination because it is at 4 ° for 44 nm. Because of the small grazing angle higher orders can be a problem and stray light levels have been found to be rather high. However, this monochromator has certainly set new standards for constant deviation UHV grazing incidence monochromators.
4. Aspheric grating monochromators The first grazing-incidence monochromator for synchrotron radiation to use a toroidal diffraction grating was described by Madden and Ederer [17]. Although the monochromator having only a single optical element may be considered as the ideal system additional mirrors are usually necessary in practice. A toroidal grating system which also uses a toroidal premirror and a toroidal focussing mirror has been in operation on the ACO storage ring for some time [18]. This system provides high flux (~1011 phot/Jk s at 85 eV with 100 mA, 540 MeV in ACO), a resolution of ~0.7 A at 170 A with 100 #m slits and operates over the range 15-100 eV. The monochromator is mechanically simple because only a rotation of the grating is required. The good performance of this monochromator can be largely attributed to the premirror which accepts 5 mr of the light emitted by the ACO storage ring which also has excellent characteristics. Present literature [19] gives the impression that the astigmatism correction is due to the holographic grating whereas in fact the primary astigmatism correction is provided by the toroidal substrate. The fact that the grating is made holographically, i.e. with laser light, can be used to provide higher order corrections which are then, of course, wavelength dependent. The spectral range of simple rotation toroidal grating monochromators is restricted and the resolution depends strongly on the aperture. It is difficult to design a system in which the grating operates under optimum conditions at short wavelengths and contributions from higher orders can become significant at larger wavelengths. The polarization effects of the ACO TGM have been checked and little difference was measured between the horizontal and vertical orientations of the monochromator. In general it is not obvious what depolarising effects toroidal grating monochromator will produce - particularly when carbon contaminaIV. XUV MONOCHROMATOR SYSTEMS
146
RIL. Johnson / Grazing-incidence monochromators Hence it was demonstrated that it is possible to fabricate useable soft X-ray diffraction gratings on extremely curved substrates by using holographic techniques and that toroidal mirrors of sufficient quality for use at grazing incidence could be fabricated by conventional optical techniques. The techniques of holography combined with aspheric mirrors obviously opens up new design possibilities for grazing incidence. However, these possibilities have only been slightly explored.
2o
15
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z
LU
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5
9
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5. Possible improvements
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i
I
I
I
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Over the past few years there have been significant improvements in the techniques used to fabricate mirrors and diffraction gratings designed for use at grazing incidence. Sophisticated methods have been developed for making large X-ray telescopes and these techniques can also be used for making high quality mirrors for synchrotron radiation applicatioms. The laser research programs have led to the development of mirrors capable of withstanding very high power and such mirrors also find applications in beamline optics. There are, unfortunately, unique problems associated with distortion and contamination of mirrors when used in high energy synchrotron radiation. There is no easy solution available at present and more research is necessary in this field. The diffraction grating is the central component of
WAVELENGTH ( n m ) Fig. 2. The variation of peak diffraction efficiency with wavelength for a laminar grating. The optimum grazing angles are also shown.
tion is present on the grating surface. It is possible to use toroidal gratings in Rowland circle monochromators, however, it is necessary to vary the angle of incidence with wavelength so that the condition for stigmation is fulfilled [20] (i.e. sin sin [3 = p/R where c~ and/3 are the grazing angles of incidence and diffraction, respectively, and p and R are the minor and major radii of the toms). Stigmatic images at wavelengths as short as 2.3 nm have been obtained from toroidal holographic gratings [21].
300
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to existing m o n o c h r o -
mators
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1200
2400
3600
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i
i
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4 °
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i
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i
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Fig. 3. Summary of the best 24 diffraction efficiency measurements at 4.5 nm plotted against grazing angle arranged according to grating frequency.
R.L. Johnson / Grazing-incidence monochromators
a grazing incidence monochromator and it is the absolute diffraction efficiency of the grating which determines the photon flux at the exit slit. Hence, the easiest way to improve a monochromator is to use a better diffraction grating. Naturally the optimum grating has to be chosen according to the geometry and wavelength region of use. It is important to remember that all phase gratings have maximum efficiency at a particular angle of incidence for a given wavelength. The angle of incidence for maximum efficiency increases with wavelength. This can be seen in fig. 2 which shows the variation in peak diffraction efficiency (i.e. at the optimum angle) with wavelength for a laminar grating plotted from the data given by Haelbich [22]. The grating was designed for use at 10 nm by appropriately choosing the groove depth and indeed produces a very high diffraction efficiency at this wavelength. Theoretically laminar gratings should suppress all even orders, however, this is only true if the grating is operating in the correct geometry at the optimum wavelength. In practice laminar gratings can also have significant higher order efficiency. The properties of grazing incidence diffraction gratings have been discussed in a previous paper [23] and as a summary fig. 3 is reproduced from that paper. It can be seen that there is a significant reduction in efficiency as the line frequency increases, however, highly efficient 1200 lines/ram gratings are available. Fig. 3 shows only the results for the best 24 gratings out of over 100 measured at Imperial College, London. There is over an order of magnitude between the best and worst gratings so it is possible to gain an order of magnitude in flux by simply choosing the correct grating.
6. New possibilities and technological limitations We have seen that all existing grazing-incidence monochromators have various weaknesses. When choosing among existing designs it is necessary to carefully consider both the particular characteristics of the synchrotron light source to be used and also the types of experiments to be performed. For example there is no point in using a state-of-the-art prefocussing mirror in a monochromator without an entrance slit if the surface becomes distorted and eventually destroyed by high energy radiation. Of course low cost plane mirrors should be used in such situations but these are also part of the optical sys-
147
tern and will introduce aberrations if they become distorted. If we assume that we have the ideal synchrotron light source for VUV experiments then the ideal monochromator must accept the maximum solid angle to provide maximum luminosity, must be continuously tuneable throughout the VUV range, and must be capable of high resolution. These objectives can only be reached if a total design approach is used, starting from the characteristics of the electrons in the ring, through the beamline, monochromator and ending at the sample under investigation. There are, of course, many physical constraints and compromises have to be made to find a suitable balance. Most of the problems are of a purely technical nature relating to the fabrication of high quality mirrors and the mechanical realization of UHV compatible mechanisms. Recently developed testing techniques for grazing incidence mirrors should lead to an improvement in quality [24]. Hence, it should be possible to build a better monochromator with the improved optical components now available. One design which is consistent with the requirements of our ideal monochromator is that originally suggested by Kunz [8]. If this monochromator is combined with beamline optics which accept a large horizontal angle and produce a true parallel beam the optical system becomes equivalent to that of the Ebert-Fastie mounting using paraboloidal mirrors. Applying the principle that all rays must travel equal paths to form a perfect image it can be seen that this design is essentially aberration free when used with a synchrotron light source. In practice the aberrations of this design will be determined by the optical quality of the off-axis paraboloidal mirrors and the accuracy of their adjustment. It is relatively easy to make large high quality plane diffraction gratings and the monochromator grating-mirror combination ensures that the grating operates at near optimum conditions and effectively suppresses higher orders. The system is relatively simple mechanically and requires only a high precision rotation of the grating for the wavelength scan and a mechanism to bring the appropriate monochromator mirror into the beam. A second version of the FLIPPER [25] monochromator has been built at the Max-Planck-Institut ffir Festk6rperforschung, Stuttgart, for use at HASYLAB *. The monochromator is to be located 20 m from the tangent point on DORIS and focussing * HASYLAB - Hamburger Synchrotronstrahlungslabor. IV. XUV MONOCHROMATORSYSTEMS
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R.L. Johnson /Grazing-incidence monochromators
beam line optics will be used. Contamination and distortion of the mirrors by the high energy components of the synchrotron radiation will most probably be the limiting factors in the performance. By using a state of the art diffraction grating and paraboloidal mirror it is hoped that a significant improvement over the existing FLIPPER monochromator will be achieved. I wish to thank Prof. C. Kunz, Dr. E.E. Koch and the DESY F41 group for their invaluable support and many helpful discussions.
References [1]W. Gudat and C. Kunz, in Synchrotron radiation, ed. C. Kunz, Topics in current physics (Springer-Verlag, Heidelberg, 1978). [2] C. Kunz, in Proc. Symp. for Synchrotron radiation users, eds. G.V. Marr and I.H. Munro (Daresbury Nucl. Phys. Lab. Report DNPL/R26, 1973). [31 E.E. Koch, R. Haensel and C. Kunz, eds., Vacuum ultraviolet radiation physics (Pergamon-Vieweg, Braunschweig, 1974). [4] Synchrotron Radiation Instrumentation and Developments, eds. F. Wuilleumier and Y. Farge, Nucl. Instr. and Meth. 152 (1978). [5] P. Jaegl~, P. Dhez and F. Wuilleumier, in ref. [3], p. 788. [6] C.H. Pruett, N.C. Lien and J.D. Steben, Proc. IIIrd Int.
Conf. Vacuum ultraviolet radiation physics, Tokyo (1971), p. 31. [7] K.P. Miyake, R. Kato and H. Yamashita, Sci. Light 18 (1969) 39. [8] C. Kunz, R. Haensel and B. Sonntag, J. Opt. Soc. Am. 58 (1968) 1415. [9] W. Eberhardt, Thesis, Universit~t Hamburg (1978). [ 10] G. Kalkoffen, Thesis, Universit~t Hamburg (1978). [11] G. Schmahl, D. Rudolph and B. Niemann, J. de Phys., Coll. C4 (suppl. no. 7) 39 (1978) 2(12. [12] K. Codling and P. Mitchell, J. Phys. E3 (1970) 685. [13] P. Dhez, P. Jaegl~, F. Wuilleumier, E. K~llne, V. Schmidt, M. Berland and A. Carillar, in ref. [4], p. 85. [14] M.Y. Adam, F. Wuilleumier, S. Krummacher, N. Sandner, V. Schmidt and W. Mehlhorn, J. Electr. Spectr. 15 (1979) 211. [15] F.C. Brown, R.Z. Bachrach, S.B.M. Hagstr6m, N. Lieu and C.H. Pruett, in ref. [ 3 ], p. 785. [16] F.C. Brown, R.Z. Bachrach, N. Lien, in ref. [4], p. 73. [17] R.P. Madden and D.L. Ederer, J. Opt. Soc. Am. 62 (1972) 722. [18] Y. Petroff, P. Thiry, R. Pinchaux and D. Lepere, Vth Int. Conf. Vacuum ultraviolet radiation physics, (abstracts), Montpellier (1977), p. 70. [ 19 ] Jobin-Yvon, Technical Information LHT 30. [20] H. Haber, J. Opt. Soc. Am. 40 (1950) 153. [21] R.J. Speer, D. Turner, R.L. Johnson, D. Rudolph and G. Schmahl, Appl. Opt. 13 (1974) 1258. [22] R.P. Haelbich, C. Kunz, D. Rudolph and G. Schmahl, in ref. [4], p. 127. [23] R.L. Johnson, in ref. [4], p. 117. [24] R.J. Speer, M. Chrisp, D. Turner, S. Mrowka and K. Tregidgo, Appl. Opt. 18 (1979). [25] W. Eberhardt, G. Kalkoffen and C. Kunz in ref. [4], p. 81.