Recent European advances in monochromator design for use with synchrotron radiation sources

Nuclear Instruments and Methods 172 (1980) 107-122 © North-Holland Publishing Company

Part I K X U V monochromator systems RECENT EUROPEAN ADVANCES IN MONOCHROMATOR DESIGN FOR USE WITH SYNCHROTRON RADIATION SOURCES K. CODLING

ZJ. Thomson Physical Laboratory, Reading RG6 2AF, England

The monochromators initially installed on synchrotron radiation sources throughout the world were not, generally speaking, designed to take full account of the characteristics of the electron beam cross section of the accelerator or the unique angular distribution of the synchrotron radiation. In the recent generation of monochromators, more care is being taken to match the characteristics of the monochromator with those of the source, in an attempts to optimise the product of throughput and resolution. One must, however, take account of the high photon flux in the new generation of high current storage rings in designing an integrated optical system. Moreover, there is a range of experiments where it is crucial to remove unwanted high spectral orders from the monochromator output. Such considerations may have a considerable bearing on the final design of a monochromator. The present report will outline the approach presently being taken at the European facilities at Bonn, Daresburry, DESY (Hamburg), FRASCATI and LURE (Paris).

m). At energies considerably lower than ~c, the important parameter when comparing the relative o u t p u t from each machine is the current, I. For most experiments, the radiation, which is emitted continuously as the electrons travel in their near-circular orbits must be monochromatised. The method by which this is done depends on the wavelength range o f interest. From 2 0 0 0 - 3 0 0 A ( 5 - 4 0 eV), normal incidence grating monochromators are used. From 4 0 0 - 2 0 A (30-600 eV) grazing incidence versions are required. Below 5 A, crystal monochromators are essential to achieve high resolution and efficiency. The difficult region between 20 and 5 • could in principle be c o v e r e d by either grating or crystal monochromators. In this report, only grating monochromators will be considered.

1. Introduction It is well known that a charged particle subjected to an accelerating field emits electromagnetic radiation. The centripetal acceleration of electrons in synchrotrons or storage rings produces radiation, called synchrotron radiation (SR), with the following characteristics [1]. It is a pure continuum. It is highly linearly polarised in the plane of the electron orbit but becomes progressively more circularly polarised at larger and larger angles to the horizontal plane. The radiation in the case of a storage ring is pulsed and may, therefore, be useful for lifetime experiments. The applications o f SR to many fields o f science has increased rapidly since the radiation was first used at the National Bureau of Standards, Washinton, in the early 1960's [2]. At the present time a number of storage rings are being built which will be dedicated to "non-nuclear physics" applications *. One of the first to be funded was the SRS at Daresbury, U.K.; new machines are presently being built in the U.S.A. (two at Brookhaven, one at Madison), Japan and Germany. The important parameters are the energy E and radius R, which determine the parameter ~c. ~c is a rough measure of the maximum unseable photon energy (~c ~ 2.2 Ea/R, where E is in GeV and R in

2. Source characteristics and their relevance to monochromator design Before discussing particular solutions to the problem of monochromatising the radiation, it is perhaps worth making a number of general comments about the unique properties of SR sources and how they influence the design and construction of monochromators.

2.1. Brightness o f the source * The situation with regard to the synchrotrons and storage rings in use as SR sources is shown in the introduction to Nucl. Instr. and Meth. 152 (1978) no. 1.

It is clearly the aim of a " t o t a l " optical system for SR (pre-mirror, monochromator, post-mirror) to have 107

IV. XUV MONOCHROMATOR SYSTEMS

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as high a flux of monochromatised radiation as possible incident at the site of a particular experiment. One of the major attributes of a SR source not mentioned specifically in the introduction is its inherent high brightness. This brightness is associated with the strongly directional nature of SR (the radiation is emitted in a forward cone of angle ~rms ~ moc2/E) and the small emittance of the electron beam in the accelerator.'This emittance is a product o f the transverse beam dimension and the corresponding angular spread; for example, the x and y emittances are given by ex = OxOx' and ey = ~yOy, where the prime indicates angular spread or derivative with respect to longitudinal distance. The coordinate system is shown in fig. 1. The vertical spread about the "optical axis" of the SR beam involves oy, and ~rms. Both are of the order of mrad and the net spread in the vertical direction is Ooy' = (~2 + o2,)1/2 y Corresponding to this is a vertical emittance Coy = o0y " Ooy' determined by the beam height aoy at the point of viewing (usually 1 mm or less). The effective horizontal emittance eox must include the beam line acceptance angle, since only a small fraction of the 2zr horizontal angle of total emission around the ring can be collected. The effective source emittances are transformed into quantities elx, ely by the beam line optics. It turns out that, in the vertically dispersing monochromator depicted in fig. 1, it is possible to accept a large fraction of the vertical angle aoy' but only a small fraction of the horizontal angle, 0 x. The appropriate input is then the vertically integrated source flux N(X) in photons s-1 mrad -1 in a certain bandwidth AX. The o u t p u t

flux Nout(X) is given by the expression: Nout(X ) = N(k) OxTBTM[(ax/elx)(ay/ely)] , where TB and TM are beam line and monochromator transmission coefficients and ax, c~y are the horizontal and vertical acceptances of the monochromator. The ratios a/el are always ~<1 even if they can formally exceed 1. Such considerations lead to the realisation that high current (>100 mA) in a storage ring is not necessarily, of itself, a desirable attribute (see section 2.3). A typical cross section of the recent generation of storage rings (DORIS, DCI, SRS) is a few mm 2. For example, the DORIS electron beam has a cross section approximately 1 mm (vertical) × 4 mm (horizontal). In attempting to collect all of the vertical distribution of radiation and fill the grating of the vertically dispersing monochromator, a premirror may reduce this "object" to 0.3 mm at the entrance slit. If high spectral resolution is required, a slit of only 0.1 mm may be necessary and hence a large loss of photon flux is inevitable. In such a case a storage ring of 10 mA but reduced vertical extent could be a positive advantage. 2.2. Polarisation o f the source

As has been pointed out in the introduction, SR is highly linearly polarised in the plane of the electron orbit. Indeed, one sees in fig. 2 that SR is theoretically 100% polarised in the orbital plane. There are a considerable number of experiments in atomic, molecular and solid state physics which

K. Codling ~Advances in monoehromator design '~w=10eV

vertically dispersing, and this orientation will obviously also ensure the maximum efficiency of the optical system. In certain instances it may be inconvenient to disperse vertically but at grazing angles of incidence the more conventional orientation may cause neither a great loss of flux nor a noticeable reduction in the degree of plane polarisation.

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require higldy polarised radiation. It is possible to maintain, or even enhance, the high degree of polarisation of SR by appropriate reflection geometry. That is, the mirrors and grating are orientated in such a way that the E-vector of the radiation is perpendicular to the plane of incidence. As an example, the reflection coefficients Rs (perpendicular) and Rp (parallel) are shown in fig. 3 for two different wavelengths. This means the monochromator will be

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With high energy (>1 GeV) synchrotrons or storage rings the emission of large numbers of X-rays creates various problems. The power level is quite high and as an example it is expected that a number of mirrors used in normal incidence at the SRS (Daresbury) will receive approximately 2 kW incident power over an area typically 15 c m × 5 cm. In a recent test at DORIS, when a glass mirror was inserted into the beam at a distance of 7 m from the tangent point at a beam current of 100 mA, the temperature rose to 600°C. When such high powers are incident on optical components, a number of things might happen. A change in the effective radius of curvature of the mirror may occur; more localised damage could occur due, for example, to large surface charge from photoemission causing surface cracking. If the mirror were situated in a relatively poor vacuum and pumped by a diffusion pump (or even an oil-bearing turbo-molecular pump), the inevitable hydrocarbon deposit on the mirror would be cracked by the intense SR. Another important origin of contamination is the cracking of carbon contained in residual gases. This graphite layer would cause a reduction in monochromator efficiency throughout the entire spectral range, but particularly around and above the carbon K-edge near 280 eV (44 A). At ACO, for example, the pre-mirror, PM, of the Jaegl6-Dhez monochromator (section 3.2.1) situated in a poor vacuum, required changing every day. Even the grating itself was changed every month. The requirements on the mirrors exposed directly to the SR from high energy machines are, therefore, unique and demanding. The mirrors must not thermally distort, since this would mean at best a loss of intensity and at worst a loss of resolution and a shift of the spectrum (in monochromators without entrance slit). The mirrors must have excellent optical figure and low light scattering. They must be compatible with a hydrocarbon-free, ultra-high vacuum IV. XUV MONOCHROMATOR SYSTEMS

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K. Codling '/Advances in monoehromator design

(UHV). They must have high reflectivity over a broad spectral range, preferably with minimal spectral structure. Stanford [3] have used thermo-electrically cooled copper mirrors over-coated with platinum and these have performed well, particularly a second smoother mirror with RMS roughness ~ 4 0 A. Silicon carbide mirrors, which are extremely smooth ( 4 - 1 5 A roughness), have been used successfully, but so far only small samples are available. DESY [4] have used a material 'Zerodur' in the "Flipper" monochromator (section 3.2.2) but with limited success. This is clearly a field where insufficient research has been performed, particularly in Europe. The approach at DORIS at the present time seems to be to ensure !hat the first component of any monochromator system will be an inexpensive plane silica mirror placed in grazing incidence and which is easily replaceable. This mirror will safeguard the more expensive toroidal or elliptical components that follow. The approach at NINA and more recently at BONN has been to use flat, triangular mirrors at grazing incidence bent into cylindrical shape so as to focus the SR at entrance slit or experiment. These inexpensive mirrors are cleaned and recoated with gold at regular intervals. 2.4. Continuum property o f the source The major advantage of a SR source when compared with the majority of conventional sources is that it is a pure continuum. In one important respect, however, this can be a decided disadvantage. The well-known grating equation is nX = d(sin a - sin/3), where n is the order, d is the groove spacing and a and/3 are the angles of incidence and diffraction. If a monochromator is set to transmit first order of wavelength X, it is clear that second order (of half the wavelength) can be transmitted. In the case of high energy accelerators, the intensity of second order radiation incident on the monochromator will be greater than that of first order and so the problem is exacerbated. In certain experiments, for example in the measurement of angular distribution of photoemitted electrons, this second order radiation is not a problem but in most other experiments it is. When measuring total photoabsorption cross sections, it is essential that second order be known or eliminated.

It is possible to remove second (or higher) order by a number of methods. The first involves changing the energy o f the accelerator so that second order is simply not emitted. This is not a viable method when other experiments are on line. In this context, it is important to realise that a monochromator which appears to have no order-sorting problems when used with a low energy accelerator may not adequately reject high orders when transferred to a high energy machine. The second method employed to remove high orders is to use thin film filters such as aluminium or tin. The range over which order-sorting occurs is restricted. One might even contemplate using a gas cell to absorb second order radiation. The third method, and the most relevant in the context of this report, is to build an order-sorting capability into the monochromator design. Here, one takes advantage of the fact that the minimum incidence angle at which a given wavelength can be efficiently reflected can be estimated from the simple theory of metals [5] O~min ~ Cos-l[3 X 10 -is 'Nea/~ • X ] ,

where Nef f iS the number of effectively free electrons per cm 3 and X is given in A. In this formula Nef f may be taken from sum rule plots. Using the full electron density overestimates the reflectance, particularly at the low energy end of the grazing incidence range. The elements irridium and osmium have the highest electron concentration and hence the highest ultimate reflectance. In practice, gold and platinum offer the non-trivial advantage of chemical stability. The property of total external reflection in the XUV is used in the following way. For a particular first order wavelength, the angle of incidence on the mirror or grating (or both) is so chosen that the first order is reflected but the second order is absorbed in the medium. Monochromators using this principle will be discussed in a later section (3.2); they must always be used in negative order (c~
K. Codling / Advances in monochromator design

111

2.5. Vacuum environment o f the source

The vacuum inside the toroid of a synchrotron is in the range 10 -6 - 10 .7 Torr; in a storage ring it is necessarily much better, 10 . 9 - 1 0 -10 Torr. Moreover, because of the low Nef f of oil (section 2.4) and the possibility of cracking of hydrocarbons on optical surfaces, it is essential that they should be avoided whenever possible. Such considerations have implications for monochromator design. Even if a beam line is built to act as a buffer between the UHV of the storage ring and the monochromator, it is nevertheless important to build the mon0chromator to UHV specifications. This leads to considerable complication and expense. All drives should be placed outside the vacuum system and should be as simple as possible. Each grating ought, in principle, to be an original, not a replica or one with any remaining photoresist (section 3.2.3). The grating ought to be bakeable to at least 100°C, preferably higher. The problems are most severe when experiments using atomic or molecular gases are coupled to the exit slit of a monochromator. The ambient pressure in a typical beam experiment is 5 × 10 -5 Torr, in which case a good deal of differential pumping will be required. High pumping speeds may be achieved with cryogenic pumping. In the latest generation of SRS (Daresbury) monochromators, the possibility of using the exit arm of the monochromator as a stage of differential pumping between grating and experiment is being explored.

3. Monochromators Although the majority of monochromators discussed below have been used on European facilities only recently, it is important to discuss their capabilities in relation to those that have been in use for some time. Therefore, wherever relevant, the principles of these earlier instruments will be described. The monochromators fall naturally into two categories, normal incidence and grazing incidence. 3.1. Normal incidence monochromators

The first monochromator to successfully use the unique properties of SR was the modified Wadsworth of Skibowskii and Steinmann [6]. This monochromator, shown in fig. 4, exploits the small divergence

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by Thimm at the Bonn synchrotron for use by Heinzmann and collaborators of the University of Manster. A schematic of the 10 m normal incidence monochromator is shown in fig. 6. Radiation is incident on a plane 4960 lines/mm holographic grating. The diffracted radiation is focussed by a concave, mirror onto the exit slit and the bandwidth of radiation through a 1.5 mm exit slit has been measured to

tion is incident on the grating. In fact beam splitters, two plane mirrors with 82.5 ° angle of incidence, provide 1 mrad X 1 mrad of radiation and prevent destruction of the grating. The instrument produces ~101° photons s -~ A -~ at the exit slit and a resolution of 1 - 2 A. Another monochromator which uses the electron beam as the entrance slit has recently been installed

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be 0.5 A. An aperture placed between the synchrotron and plane grating can be moved vertically to select either right or left circularly polarised radiation. The photon flux behind the exit slit has been estimated to be more than 109 photons s -1 for wavelengths from 4 0 0 - 1 8 0 0 A . The degree of circular polarisation has been found to be about 80%, see fig. 2. The rectangular aperture aiso serves the important purpose of removing the damaging X-rays. A Seya-Namioka, in use at the Bonn synchrotron at present, is shown schematically in fig. 7. It is a 1 m monochromator, built to UHV specifications, with in situ interchangeable gratings. Such a monochromator has an inherently poor-to-moderate resolution but the simple grating drive lends itself to UHV compatibility. Such a monochromator normally suffers from substantial astigmatism. This has been corrected by using two small mirrors, M, one in front of the entrance slit, the other behind the exit slit [8]. These cylindrical mirrors are simply made by bending flat

triangular mirrors, as is the large beam-splitter mirror, BM. The resolving power, which is supposed to be improved by this means, has yet to be checked but the radiation emerging from the exit slit is at least 99% plane polarised. At SR sources, monochromators based on conv e n t i o n a l m o u n t i n g s still predominate over newer optical designs involving asymmetrical mounts which have high products of throughput and resolution [9] or stigmatic mounts involving a simple rotation of a holographic grating [10]. An example of the latter is shown in fig. 8 and is in use at LURE [11]. Once again the source is the entrance slit and the solid angle of acceptance is 5 mrad (horizontal)X 2 mrad (vertical). To avoid damage, the platinum coated holographic grating, G, is illuminated by a plane mirror, M. The angle between entrance and exit arms is 13 °. A toroidai mirror, T, focusses the radiation at the experiment, E. Two sliding masks allow

Fig. 8. Schematic of a normal incidence monochromator at ACO. IV. XUV MONOCHROMATOR SYSTEMS

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K. Codling /Advances in monochromator design

the illuminated height of the grating to be varied in order to obtain more flux, or, alternatively, increased resolving power. The grating has an unusual shape, being longer (120 mm) than it is wide (30 mm). The recording parameters of the grating were determined by the usual method [12]. Three gratings are required to cover the wavelengths range of interest (1) 3500 lines/mm, 300 A < X < 1000 A; (2) 2400 lines/mm, 800 A < X < 2000 A; (3) 1900 lines/mm, 1200 A < X < 3000 A. With a source height (the electron beam) of 0.5 mm, the best band pass for the first grating is 0.3 A at all wavelengths. However, this worsens considerably, particularly at longer wavelengths, when the illuminated height of the rulings is increased from 10 to 50 mm. At 500 A, the incident flux of 2 X 1013 photons s -1 A -~ reduces to 7 X 109 photons s -1 A -1 at the experiment (an apparent efficiency of less than 0.1% near the blaze wavelength).

chromatic emission must remain unchanged as the photon energy varies. Any discussion of the relative merits of the various monochromators described below must not only include resolution but also the potential transmission or efficiency of the total optical system (the solid angle of collection of SR; the number of reflections; the vacuum environment which could ensure continued efficiency), the simplicity of mechanical realization of the monochromator (cost, reliability), the order-sorting capabilities and the wavelength coverage. Generally speaking the resolution of the Rowland circle instrument will be greatest, but at the expense of complexity. The plane grating monochromator can, however, match this resolution when the source size is sufficiently small. When great simplicity and high efficiency are required, at the expense of resolution and order-sorting capability, toroidal grating monochromator are to be preferred.

3.2. Grazing incidence monochromators 3.2.1. Concave grating monoehromators When the angle subtended between entrance and exit beams of a conventional Rowland circle geometry monochromator is small (c~+/3< 15°), wavelength scanning may be achieved by simply rotating the grating whilst translating it along the bisector of the two beams. This corrects adequately for the defocussing and under these conditions neither light source nor experiment need be moved. A number of commercial instruments are marketed which achieved high resolution by this means. A McPherson 3 m monochromator, modified for UHV, has been used successfully for a number of years at

When one wishes to work in the photon energy range above about 40 eV, poor normal incidence reflectivities lead one naturally into grazing incidence geometry (this report will not include the recent interesting work on multilayer films for enhancing reflectivity in the high energy regime [13]). By increasing the angle of incidence, abberations inevitably increase, thoughput is reduced and it becomes more crucial to place components in the correct location. Problems are increased by the requirement that the monochromator remains a constant (or zero) deviation instrument. That is, the direction of mono-

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DORIS [14] and has a capability of 0.03 A using a 0.01 mm slit (the slit-width-limited resolution for a 1200 lines/mm grating). In moving to grazing incidence (c~ + / 3 > 150 °) and maintaining Rowland circle geometry, such a motion as described above will clearly be inapplicable. A number of commercial grazing incidence monochromators are available but either the exit slit (and experiment) or, alternatively, the entrance slit (and light source) must move. Because of the unique requirements, of a SR source and because almost every SR facility has had its own ideas on how to tackle the problem, a considerable number of solutions has been suggested. Insufficient data are presently available to make critical comparisons. (a) The Pruett monochromator [15] is a twograting monochromator incorporating a "doubleVodar" concept. It has a complex scanning mechanism and would not be particularly UHV compatible. At least 4 reflections are involved and second order rejection was not as good as anticipated. No such monochromator is in use at a European facility. (b) The Brown monochromator [16] is a modified "single-Vodar" monochromator consisting basically of two mirrors and a grating, as shown in fig. 9. Mirror M~ moves parallel with the synchrotron beam as the monochromator scans. As shown, a 1 m Rowland circle rotates around the exit slit to give a constant beam direction. The grazing angles ( 2 - 3 °) ensure useful intensities above 500 eV. With an

entrance slit of 0.015 mm, a spectral bandwidth of 0.15 A is achieved. The photon flux at 1 0 0 A in this bandwidth is ~6 × 10 9 photon s -~ , giving an overall efficiency of 0.2%. Stray light with ruled replica gratings (particularly 2400 lines/mm) is quite high and higher orders are a problem above 140 A (below 90 eV). Such a monochromator is under construction for use at the ADONE facility. (c) The Codling monochromator [17] consists of two mirror-slits and a single grating. A UHV version was used at NINA for two years. It is now in BONN and is to be coupled to the 2.5 GeV synchrotron via a toroidal mirror. The instrument is compact, can be used in positive or negative order and achieves a slitwidth-limited resolution. (d) The Thimm monochromator [18] shown in fig. 10 has only two optical components. A cylindrical focussing mirror travels along the line of SR and simultaneously the mirror is rotated around an axis through its centre C. The angle of incidence on the grating is 88 °. The monochromator does not provide a constant direction of output: presumably a third reflection would be required to achieve this. It is an extremely bulky instrument, not UHV compatible and has only recently begun to be used at Bonn. It appears that it cannot be considered as a scanning instrument, but could be used at fixed, but variable wavelengths. The reason for this is that the cylindrical mirror does not, at the present time, focus the SR

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IV. X U V M O N O C H R O M A T O R SYSTEMS

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A number of plane grating monochromators are being employed on European facilities at the present time. In principle, they are high aperture instruments, since no entrance slit is employed. They lend themselves naturally to the principle of zero (or constant) deviation. (a) The Miyake monochromator [20] shown in fig. 12, consists basically of a plane grating, G, followed by a mirror, M, which focusses the almost parallel radiation on to the exit slit. The West [21] version of the instrument contains two mirrors M1 and M2 and exit slits $1 and $2. There are four focus conditions which provide order-sorting over a range from 3 5 - 5 0 0 A as shown below the schematic diagram. The monochromator was exposed directly to the full NINA X-ray beam and survived because original blazed and laminar gratings were used (see later). The Howells [22] version of the instrument was used at both NINA and TANTALUS and will be installed at the SRS with a toroidal pre-mirror. It has a 5% efficiency at the blaze wavelength and medium ( - 0 , 2 A) resolution. (b) The K u n z monochromator [23] shown in fig. 13 consists of a plane mirror, a plane grating and a paraboloid for focussing the radiation on the exit slit. The motion of the pre-mirror and grating,

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on the moving entrance slit to the required accuracy. (e) The JaeglO-Dhez monochromator [19] can be used in a single grating mode, as shown in fig. 11, or a two-grating mode. In the latter mode and extra concave mirror and grating are placed between G and $2 of the figure. Hence 5 reflections are required and a relatively poor efficiency must be expected. In this mode it is capable of good high-order and stray light rejection. The first grating is used in the negative first order, the second in the positive first order. It still relies on the selective reflection principle to ensure rejection of the ( - 2 , +2) spectrum, see section 2.4. This monochromator has been used successfully in the one-grating mode and by careful choice of angles of incidence, very good higher order rejection has been achieved.

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3G-75~,, 0.016~/step, e=83.3" 58-130~,,0,025A/step, ot :79 5" 96 - 270~,, 0.038~./step, ot =73.7" 170--500/~,, 0.058.&/step, = = 65.0"

Fig. 12. The Miyake-West monochromator.

£m

K. Codling /Advances in monoehromator design

117

PG SYNCHROTRONRADLATION

._..~o 2_____~e,5__

/ /

GRATING ARM .~

GRATING .

.

.

.

.

CM

PM

Fig. 13. The Kunz monochromator.

although simple in principle, is complex in realisation. As the plane mirror, PM, moves along the line of the SR, it rotates, as does the plane grating, PG. High order suppression is achieved since lower energies occur at the less grazing angles. The instrument is always "on blaze" and the energy resolution remains essentially constant as a function of wavelength. (c) The "Flipper" monochromator [4] is a UHV version of the previous instrument, capable of removing higher orders; it has been in use at DORIS for some time. The SR from DORIS is first deflected both horizontally and vertically by two plane mirrors made of "Zerodur" (manufactured by Zeiss). Zerodur is a special glass having low thermal expension but has a high probability of defect production. Cracks appear in the surface and silica is probably better from this point of view. The radiation is then incident through an entrance aperture upon the mirror magazine shown in fig. 14. This consists of six plane mirrors, each with two different coatings, and the plane grating. The mirrors can be pushed into the beam in turn quite reproducibly so that the radiation is deflected onto the grating. The wavelength is scanned by rotation of the plane grating. The instrument is reasonably efficient, producing ~3 X 109 photons s -1 A -1 at about 90 eV. It is a relatively high resolution instrument, giving a resolving power of 1400 at this energy. Flipper iII has been built at the Max-PlanckInstitut at Stuttgart. Using off-axis paraboloid mirrors before and after the grating, in a grazing incidence Ebert-Fastie mounting, it is hoped that a significant improvement over the earlier version will be achieved. 3. 2. 3. Toroidal grating monochromators When a concave grating monochromator is used in grazing incidence with a conventional light source, the loss of intensity due to astigmatism is large, becoming excessive as the angle is made more and more grazing. Because of the unique angular distribution of SR the losses incurred due to astigmatism are not quite so severe. Nevertheless. a considerable loss of intensity can occur.

Fig. 14. The "Flipper" mirror magazine. One way to minimise this loss is to use a toroidal rather than spherical surface. Speer and collaborators [24] have replaced spherical with toroidal gratings in conventional Rowland circle mountings, corrected astigmation over a limited wavelength range, and thereby great!y increased the luminosity of the monochromator [25]. Certain of the toroidal grating blanks used in their work had a small minor radius (5.65 mm) and it is inconceivable that these gratings could have been ruled by conventional mechanical means. However, by using holographic techniques, it is possible to form gratings On any substrate which can be coated with photoresist and exposed to the interference fringes formed by laser light. In fact, techniques have been developed at G6ttingen [26] for making all-metal gratings free from photoresist, an important aspect in the context of SR research (section 3.2.3.1). (a) The Madden monochromator [27] was the first toroidal grating monochromator to be used with SR. It consisted of a single optical element, a grating ruled on a toroidal surface. No entrance slit was used and wavelength scanning was achieved by simple rotation of the grating. The instrument was used in negative order. Details of the resolution and order-sorting capabilities have yet to be published but it must be an efficient, high aperture monochromator. It will clearly suffer from higher orders and the resolution is not expected to be good over more than a limited spectral range. A second exit arm is available to allow stigmatic focussing over a further wavelength range. (b) The Lepere-Petroff monochromator [28] is based on a toroidal holographic grating. The 0.3 m monochromator (the LIlT 30 of J o b i n - Y v o n ) w a s coupled to the ACO storage ring as shown in fig. 15. The monochromator has the same simplicity as the Seya-Namioka, w i t h a rotation of the grating and fixed entrance and exit slits. Radiation from ACO is focussed by the toroidal mirror on to the entrance slit and a toroidal mirror refocusses the monochromatised radiation on to the experiment. It is claimed that the IV. XUV MONOCHROMATOR SYSTEMS

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K. Codling /Advances in monochrornator design

toroidal mirror 3:1

5

toroidal grating "1.33 m

.3m

5mrod .Sm

experirnen~ Fig. ]5. Toroida! grating monochromator at ACO.

instrument is corrected for astigmatism and coma over the wavelength range 0 - 1 6 0 0 A. The toroidal mirror allowed 5 mrad o f the horizontal spread o f the SR to be used. Presumably, almost the whole o f the vertical distribution was collected. A larger version o f this instrument is presently installed at the ACO ring and has been operating successfully for over a year. The entrance arm is 2.9 m, the exit arm 3.4 m in length. Two values of included angle (a +/3) are possible, 150 ° and 166 °. The latter configuration has a 4.3 m exit arm length and has yet to be used. Considerably experience has been gained with the 3 m version. The toroidal mirror between storage ring and entrance slit demagnifies the 1 m m electron beam width by a factor of 3. It is suggested that attempts to demagnify by a larger factor than this at grazing angles of 5 ° would be pointless. The theoretical dispersion in the t 5 0 ° configuration is approximately 1.3 A / m m and the resolution achieved was of the order o f 0.15 A at 100 eV with 0.1 m m slits. Hence the instrument is performing close to expectation. The anticipated resolution on the 166 ° line is 0.03 )~ at similar slit widths. The photon flux at the experiment is estimated to be ~ 4 X 1012 photons s -1 A -1 at 110 eV. This is to be compared with the flux from " F l i p p e r " of 3 X 109 photons s -1 A -1 at 92 eV. Both were for a 100 m A beam current. The large factor improvement can be explained in terms of the larger angular acceptance o f the ACO instrument and the reduction in number o f reflection. The flux from the J a e g l 6 - D h e z monochromator with clean optics under similar circumstances is 1011 photons s -1 A -1 with an angular acceptance of 1.5 mrad (horizontal) of 3 mrad

(vertical). The order-sorting capability of the Petroff instrument has not been thoroughly tested, although it is clear that it will not match Flipper. In the 150 ° position, no higher orders were expected or observed above 100 eV. At around 35 eV (the L edge of aluminium in second order), the contribution from second order was 1 2 - 1 3 % - quite substantial. In the 166 ° configuration, higher orders are expected to be more significant. An earlier suggestion was to use a sheet o f metal to cut out the central part of the beam; this would mean a considerable loss of first order signal. A recent plan is to use a further reflecting plane mirror of variable angle to selectively reject higher order radiation. T h e efficiency of the monochromator at around 100 eV has not been seen to deteriorate significantly over a period o f a year. This is thought to be due to the large surface of the grating being illuminated. That does not mean to say that the efficiency near the carbon K edge (44 A) would not decrease. This energy cannot be reached with the 150 ° configuration. The vacuum in the region of the grating is claimed to be o f the order o f 5 × 10 -9 Tort, even though the grating is not an original, i.e. a certain amount of p h o t o r e s i s t remains on the toroidal surface. Irradiation with high flux o f synchrotron radiation seems not to degrade its performance, however. After the success of the Petroff monochromator, a number o f similar instruments have recently been purchased or ordered from J o b i n - Y v o n . Philips (Eindhoven) are building a "1.2 m " version for use at ACO. In the rest of Europe, Gudat has ordered a "1 m " version for use at DORIS. The included angle

K. Codling / Advances in monoehromator desQn

119

(a)

amplitude

(b)

shallow

(c)

laminar

blaze

0

,

,

•

. . . . . .

r--

h

Fig. 16. Principal types of grating for grazing incidence.

here is 146 °. With two gratings, the monochromator is expected to cover a wavelength range from 1 2 5 1995 A, with a resolving power of typically 5 0 0 1000. Sonntag is in the process of negotiating a 3 m vertically dispersing version of the instrument. The included angle will probably be 160 °. Jobin-Yvon provided a 3-grating solution but the present thinking is that the short wavelengths envisaged for the third grating will not be attainable due to the poor grazing angle and therefore 2 gratings will suffice. The pre-mirrors used in DORIS for the Sonntag instrument will not reduce the i mm high object (the electron beam) to below 0.3 ram. Hence in order to achieve high throughput and high resolution a large instrument is required. The SRS (Daresbury) have been funded to build a grazing incidence monochromator. It is hoped that it will cover a range from 100-1200 A. It will incorporate a toroidal grating and it is hoped that it will achieve a resolution of typically 0.03 eV. 3.2.3.1. Toroidal gratings: manufacture and testing There is undoubtedly a great interest at the present time in toroidal gratings for use in grazing incidence monochromators. Before discussing these gratings it is perhaps worthwhile mentioning the types of groove profile that are relevant for grazing incidence reflection gratings. Fig. 16 shows three of the types available. Amplitude (lightly ruled) gratings were developed by Siegbahn and clearly when a = b, then 50% of the light is lost even assuming 100% reflection efficiency from an ideal profile. Blazed gratings are well-known and gratings with small blaze angle ( ~ 1 - 2 ° ) , when used in grazing incidence, tend to blaze at or near the wavelength anticipated. Efficiencies can be high, but as the grazing angle decreases, shadowing occurs and the radiation is only diffracted from the top part of the

groove profile. It is in this region that it is difficult to control the quality of the rule surface. For a grating with a 1° blaze and 2400 lines/mm, the groove depth is a mere 7 0 • and surface roughness and other imperfections of the metallic coating may well exceed this figure. Scattered light becomes a real problem. Important advances in shallow blazed gratings have been made [29], the starting point being a blazed holographic grating formed in photoresist. The shallow blaze angle for XUV work cannot be made directly since it depends on the grating pitch and the wavelength of the laser radiation employed. By means of ion-etching, the saw-tooth profile from the photoresist is transferred into the substrate while reducing the blaze angle controllably by a factor up to 25. The smooth surface produced by this technique ensure high efficiency and low scattered light. In the case of laminar [30] gratings, radiation is diffracted from both the upper and lower surfaces and by careful choice of groove depth and angle of incidence, the beams can be made to interfere destructively in zero order and thus more energy is throWn into diffracted orders. By virtue of the fact that the whole grating contributes to the diffracted beam, the maximum geometrical first order diffraction efficiency is about 40%, a factor 4 greater than that for an amplitude grating. Once again, the smooth surfaces ensure high diffraction efficiencies. Since diffraction efficiencies depend also on path differences, it is clear that efficiencies must vary periodically with groove height, wavelength and incidence angle and it is possible to select the grating parameters to yield optimum performance over any given spectral range. At the small grazing angles used in the XUV shadowing of the grooves by the lands can become excessive and, ion-etched shallow blazed gratings ought to be more efficient. For completeness, this listing should include IV. XUV MONOCHROMATORSYSTEMS

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holographic gratings [31], defined here as those produced in photoresist and not subsequently modified (by ion-etching, for example). The profile of this type of grating, intermediate between the blazed and sinusoidal, derives from the process used in transforming the optical fringes, recorded in photoresist, into high reflectance metallised surfaces. Efficiencies are comparable with conventionally ruled gratings. The question arises as to the commercial availability of such toroidal gratings. It appears at the present that the only manufacturer of large size (the Sonntag grating is 164 mm X 24 mm) gratings is Jobin-Yvon. This company is prepared to quote on a UHV P e t r o f f type monochromator incorporating three or four different gratings to cover the wavelength range of interest at the required resolution. The gratings, which involve the same toroidal substrate but with holographic gratings of different numbers of lines per mm, can be interchanged under vacuum. The company is prepared to go through a ray tracing procedure to check the resolution of the monochromator at all wavelengths. This usually assumes an infinitely narrow entrance slit of a particular height and the ray tracing results in a profile at the exit slit that can be far from ideal. It is important to appreciate that any quoted FWHM resolution should, therefore, be treated with some caution. The gratings produced by J - Y are known to contain photoresist *, which could be a disadvantage if the grating is exposed to high flux from a storage ring. Although the experience of Petroff seems to be favourable, it is nevertheless desirable to have an "original" grating wherever possible. Both Rudolph and Schmahl [26] of G6ttingen and Franks and associates [32] of the National Physical Laboratory are capable of producing laminar gratings which are free from photoresist. NPL are also capable of producing shallow blazed gratings, but not so far on toroidal surfaces. The Rudolph and Schmahl laminar gratings are produced in a somewhat different fashion from those of NPL. In the former the photoresist is overexposed, developed, and where the E-field is strongest, the resist is removed. The silica and lands of photoresist are then overcoated with chromium. The photoresist plus coating is removed, leaving strips of chromium. Finally the chromium on silica is overcoated with a few hundred Angstroms of gold. In the NPL technique, after the lands of photoresist are left, ion-

* J-Y are now producing ion-etched original gratings.

..

etching removes the silica, to form the grooves. The grating is then gold-coated. It is fair to say that neither G6ttingen nor NPL produce these laminar gratings on a commercial basig at present. However, this should not deter interesteci parties from requesting information or even a quote for a particular requirement. Dr. R.J. Speer, Blackett Laboratory, Imperial College, London S.W. 7 can be approached with regard to the G6ttingen gratings; Mr. D.F. Paul of Ion Tech Lid, 2 Park Street, Teddington TW11 OLW deals with ion-etched grating requests. Ion Tech/NPL are currently producing their first laminar grating on a toroidal substrate, which will have a ruled area of 15 mm × 25 ram. Due to the limitations, which are not fundamental, on the size of their equipment, the maximum size of ruled area is about 60 mm at the present time. Problems would also exist with toroids of very small minor radii. These relate to photoresist deposition and the slope of surfaces with respect to fringes, a common problem. In the case of G6ttingen, there seems to be no particular limitation in size or shape. Both the G6ttingen and NPL gratings are "classical" in the sense that the grating is made by plane wave laser beams producing equi-spaced grooves. J o b i n - Y v o n , on the other hand, by careful choice of laser parameters can produce holographic gratings on toroidal surfaces which are corrected for aberrations over a range of wavelengths. The questions to be asked are "Is the resolution noticeably enhanced? .... Is the efficiency improved?" There seems to be a general agreement now amongst workers in the grazing incidence region that the major gain in efficiency using toroidal holographic gratings comes from the fact that a toroidai rather than spherical surface is used. In other words one attempts to gain as much as possible by geometrical optics and then hopes that physical optics (the holography) will improve matters even further. There is relatively little quantitative information on the improvements in resolution one might expect from careftfl holographic correction. Ray tracing has recently been performed [33], showing that the resolution of a TGM using a "straight groove" grating can equal that of an aberration corrected one. This means that gratings produced by the above etching techniques can be used, photoresist can l~e eliminated and irreversible degradation upon exposure to SR avoided. It is evident, then, that the major gain in such grazing incidence monochromators is via the correc-

K. Codling / Advances in monoehromator design

121

l_~ r c e SO M3

Sz

G SI

Fig. 17. Possible design of transmission grating monochromator. tion of primary astigmation through substrate curvature. These substrates (principally terics) are frequently characterized by widely different radii in orthogonal axes and are consequently very difficult to fabricate and test to the high tolerances required. Two methods are described here. At NPL a stylus is employed which makes contact with the aspheric surface. Measurement o f the figure is made interferometrically to an accuracy of better than 0.2 #m along the generators o f the curve. In the second method developed by the Imperial College Group for Optical Surfaces Ltd. [34] a Linnik [35,36] interferometer examines the surface optically b o t h for errors of wavefront during manufacture and to determine the o p t i m u m conjugates o f use. This latter technique is appropriate to users o f grazing incidence systems because although the asphere may not quite meet-one specification (say in the value of one o f the radii requested) it may nevertheless perform with indistinguishable results at slightly different object and image points. In view of the considerable costs involved in manufacture, a knowledge o f these conjugates can be of great use, provided a limited amount o f flexibility has been allowed for in the final locations o f the monochromator entrance and exit slits. 3.2.4. Transmission grating monochromators However well-designed the optical system for a grazing incidence m o n o c h r o m a t o r may be, the throughput is necessarily small, due in part to the poor reflectivities. Moreover, the scattered light contribution becomes a real problem at high energies, close to zero order. The development of large-area, free-standing transmission gratings [37] has opened up new possibilities for XUV monochrc.mators. The problem o f order-overlap can be partially overcome by choosing appropriate values for the grating mark/ space ratio. A possible experimental arrangement [38] is shown in fig. 17. It appears that, to achieve a resolving power o f

103 at high energies, an entrance slit, So, will be required. The distance between grating, G, and focussing mirror, M2, is about 19 m, the distance between M2 and exit slit is about 1 m. The monochromator can be scanned by simply rotating the grating. No such monochromator is at present under construction at a European facility. It is a pleasure to acknowledge the helpful discussions with Dr. R. Speer o f Imperial College, Dr. Y. Petroff of ACO, Dr. B. Sonntag o f DESY, Dr. A. Franks of NPL, Dr. J.B. West o f Daresbury Lab., and Dr. D. Lepere of J o b i n - Y v o n .

References [1] K. Codling, Rep. Progr. Phys. 36 (1973) 541. [2] R.P. Madden and K. Codling, Astron. Astrophys. J. 141 (1965) 364. [3] R.Z. Bachracli, S.A. Flodstrom, R.S. Bauer, V. Rehn and V.O. Jones, Nucl. Instr. and Meth. 152 (1978) 135. [4] W. Eberhardt, G. Kalkoffen and C. Kunz, Nucl. Instr. and Meth. 152 (1978) 81. [5] J.A.R. Samson, Techniques of vacuum ultraviolet spectroscopy (Wiley, New York, 1967). [6] M. Skibowskii and W. Steinmann, J. Opt. Soc. Am. 57 (1967) 112. [71 V. Saile, Nucl. Instr. and Meth. 152 (1978) 59. [8] N. Rehfeld, U. Gerhardt and E. Dietz, Appl. Phys. 1 (1973) 229. [9] M. Lavollee, in Vacuum ultraviolet radiation physics, ed. E.E. Koch (Pergamon, Braunschweig, 1974), p. 730. [10] M. Pouey, Appl. Opt. 13 (1974) 2739. [11] C. Depautex, M. Lavollee, G. Jezequel, J. Lemmonier and J. Thomas, Nucl. Instr. and Meth. 152 (1978) 69. [12] H. Noda, T. Namoika and M. Seya, J. Opt. Soc. Am. 64 (1974) 1043. [13] R.P. Haelbich and C. Kunz, Opt. Commun. 17 (1976) 287. [14] R. Frey, 1% Gotchev, O.F. Kalman, W.B. Peatman, H. Pollak and E.W. Schlag, Chem. Phys. 21 (1977) 89. [15] C.H. Pruett, N.C. Lien and J.D. Steven, IIIrd Int. Congress on Vacuum, Ultraviolet Radiation Physics, Tokyo (1971) 3/a A2-5. IV. XUV MONOCHROMATOR SYSTEMS

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[16] F.C. Brown, R.Z. Bachrach and N. Lien, Nucl. Instr. and Meth. !52 (1978) 73. [17] K. Codling and P. Mitchell, J. Phys E3 (1970) 685. [18] G. Puester and K. Thimm, Nucl. Instr. and Moth. 152 (1978) 95. [19] P. Jaegl~, P. Dhez and F. Wuilleumier, Roy. Sci. Instr. 48 (1977) 978. [20] K.P. Miyake, R. Kato and H. Yamashita, Sci. Light 18 (1969) 39. [21] J.B. West, K. Codling and G.V. Marr, J. Phys. E7 (1974) 137. [22] M.R. Howe!Is, D. Norman, G.P. Williams and J.B. West, J. Phys. E l l (1978) 199. [231 H. Dietrich and C. Kunz, Rev. Sci. Instr. 43 (1972) 434. [24] R.J. Speer, D. Turner, R.L. Johnson, D. Rudolph and G. Schmahl, Appl. Opt. 13 (1974) 1258. [25] R.L. Johnson, Nucl. Instr. and Meth. 152 (1978) 117. [261 D. Rudolph, G. Schmahl, R.L. Johnson and R.J. Speer, Appl. Opt. 12 (1973) 1731. [27] R.P. Madden and D.L. Ederer, J. Opt. Soc. Am. 62 (1972) 722. [28] C. Depautex, P. Thiry, R. Pinchaux, Y. Petroff, D.

[29] [30] [31] [32]

[33] [34] [35] [36] [37] [38]

kepere, G. Passereau and J. Flamand; Nuch Instr. and Meth. 152 (1978) 101. P.R. Stuart, M.C. Hutley and M. Stedman, Appl. Opt. 15 (1976) 2618. A. Franks, Sci. Progr. Oxf. 64 (1977) 371. E.W. Palmer, M.C. Hutley, A. Franks, T.F. Verrill and B. Gale, Rep. Progr. Phys. 38 (1975) 975. A. Franks, K. Lindsey, J.M. Bennett, R.J. Speer, D. Turner and D.J. Hunt, Phil. Trans. Roy. Soc. 277 (1975) 503. W.R. McKinney and M.R. Howells, these Proceedings, p. 149. Optical Surfaces Litd., Godstone Road, Kenley, Surrey CR2 5AA. R.N. Smartt and J. Strong, J. Opt. Soc. Am. 62 (1972) 737. R.J. Speer, M. Chrisp, D. Turner, S. Mrowka and K. Tregidgo, Appl. Opt. 18 (1979) 2003. J.H. Dijkstra and L.J. Lantwaard, Opt. Commun. 15 (1975) 300. J. Stohr, V. Rehn, I. Lindau and R.Z. Bachrach, Nucl. Instr. and Meth. 152 (1978) 43.