Journal of Electron Spectroscopy and Related Phenomena 101–103 (1999) 965–969

Design of the high-resolution BUS XUV-beamline for BESSY II

*M. Martins , G. Kaindl, N. Schwentner¨ ¨Freie Universitat Berlin, Institut f ur Experimentalphysik, Arnimallee 14, D-14195 Berlin, Germany

Abstract

We present the design characteristics of an XUV undulator beamline for BESSY II, based on a spherical-gratingmonochromator for the energy range from 35–500 eV. The monochromator will use a moveable pre-mirror and a moveableexit slit and has been optimized for highest resolution. For photon energies up to 200 eV a resolving power of 50 000 up to80 000 should be achievable, and for higher energies a resolving power of 20 000. All properties of the beamline have beenextracted from ray-tracing calculations taking slope errors as well as the characteristics of the source into account. 1999Elsevier Science B.V. All rights reserved.

Keywords: Monochromator; Synchrotron radiation; High-resolution

1. Introduction (slope) errors and the residual aberrations of theoptical components between the entrance and the exit

Using third-generation storage rings for synchrot- slit. An advantage of the Dragon design is theron radiation, like the ALS or BESSY II, ultra-high simplicity of the optical layout using the sphericalresolution with E /DE . 50 000 is accessable for the grating as the only optical element between thefirst time in the soft X-ray region. Current or planned entrance and the exit slits, i.e., only one opticalXUV beamlines for undulators are designed as element will contribute to figure errors. To track thespherical-grating monochromators [1–4] or plane- focus in this monochromator type, the exit slit mustgrating monochromators [5–7], and both designs are be moved over rather large distances (up to 1 m).suitable for very high energy resolution. The best This is not desirable for applications that require aresolving power in the XUV region so far is 64 000 small and spatially fixed focus. The latter disadvan-achieved at the ALS spherical-grating mono- tage can be overcome by an SGM design proposedchromator (SGM) beamline 9.0.1 for a photon first by Padmore [11], using an additional rotateableenergy of 64 eV by Kaindl et al. [8,9]. This shows pre-mirror similar to the SX700 [12]. In the Padmorethat spherical-grating monochromators are very design two optical elements contribute to the figurepromising for ultra-high resolution beamlines. Typi- errors, but this is still less than in a plane-gratingcal SGM setups are the Dragon [10] and the Pad- monochromator that requires at least three opticalmore design [11]. The maximum resolving power of elements. The most important aberration influencinga monochromator depends critically on the figure the resolution in both SGM types is the coma term of

the spherical grating, which vanishes only at oneenergy for each grating. To fulfill the Rowland circle*Corresponding author.condition for a whole energy region, a variation ofE-mail address: michael.martins@physik.fu-berlin.de (M. Mar-

tins) the positions of both, the exit slit and the pre-mirror

0368-2048/99/$ – see front matter 1999 Elsevier Science B.V. All rights reserved.PI I : S0368-2048( 98 )00382-X

966 M. Martins et al. / Journal of Electron Spectroscopy and Related Phenomena 101 –103 (1999) 965 –969

is a possible solution. Another design for a SGM have again the problem of a moveable exit slit, butwith a fixed exit slit was proposed by Senf et al. [2]. this is mechanically much simpler and thereforeInstead of moving the exit slit, in the constant length more precise than moving the whole pre-mirror /(CL) SGM the whole pre-mirror /grating unit has to grating chamber. All applications that require a smallbe moved, with a translation range similar to the and constant spotsize, but not the ultimate resolvingrange of a moving exit slit. power can use the monochromator with a fixed exit

The BUS-SGM beamline will be built and oper- slit in the Padmore mode.¨ated by the Berliner Universitatsverbund Synchrot- The optical layout of the SGM is shown in Fig. 1.

ronstrahlung (BUS), which is an association of The undulator radiation is focused onto the entrancepresently 14 groups from the three Berlin Univer- slit S by a horizontally deflecting water-cooled,1

¨sities, the Freie Universitat Berlin, the Humboldt sagittal cylindrical mirror M with demagnification1

¨ ¨Universitat zu Berlin, and the Technische Universitat 9:1. The monochromator will be equipped with threeBerlin. The setting up of the beamline will take place spherical gratings G with a radius of 41 m andin the second half of 1999 at the H3 position of the m5700, 1100, 2200 grooves /mm, to cover theBESSY II storage ring, and regular operation is energy range from 35–500 eV. The deflection angleplanned for the beginning of the year 2000. at the grating can be varied by rotating the pre-mirror

in the range from 11 to 168. The exit slit S has a2

moving range of 6500 mm. This allows zero comafor the energy regions 40–50, 70–90 and 150–1902. Designfor the three gratings, respectively. All optical com-ponents up to the grating, including the entrance slit,The beamline will be installed at the undulatorwill be water-cooled. The refocusing into threeU125/ II together with a 10-m normal incidencedifferent endstation positions is carried out by twomonochromator and an X-ray beamline. The un-exchangeable toroidal mirrors M . In the vertical4xdulator will have 32 periods with a length of 125 mmdirection they will image the exit slit onto the sampleeach, resulting in a total length of 4 m. In thewith a magnification of 1:1.5 and 1:2, respectively.undulator mode, the energy region from 10–600 eVIn the horizontal direction they image the sourceis usable. For the X-ray beamline, the undulator willwith a demagnification of 10:1 and 8:1 onto thework with large K values as a wiggler.sample. The deflection angles have been chosen asThe beamline is designed as a spherical-grating10 and 2208, respectively, with probe–mirror dis-monochromator with variable included angle and atances of 3 m and 4 m. The parameters of the opticalmoveable exit slit. In this configuration, the focusingcomponents are summarized in Table 1.condition [13]

2 2cos a cos a cos b cos bS D S D]] ]] ]] ]]2 1 2 5 0 (1)r9 R r R 3. Simulations

can be fulfilled either by moving the exit slitExtensive simulations of the monochromator have(Dragon type) or by varying the included angle

been made using the ray-tracing code SHADOW(Padmore type). a and b are the angle of incidence[14]. Figure (slope) errors and surface roughness ofand refraction, respectively, and r9 (r) the distancethe optical elements have been taken into account.entrance slit–grating (exit slit–grating); R stands forTo obtain a good statistics, all runs were made usingthe grating radius. As in the CL SGM design [2], the25 000 rays for each resolution point. Slope errors ofcoma term F300 the optical surfaces have been calculated using the

2 2 method described by del Rio and Marcelli [15]. Fig.cos a cos a sin a cos b cos b sin bS D S D]] ]] ]] ]] ]] ]]2 1 2 2 shows the calculated maximum resolving power ofr9 R r9 r R rthe beamline where both the pre-mirror and the exit5 F (2)300 slit are moved. The resolving power has been

vanishes for a whole energy region. In this design we calculated by tracing a monochromatic source

M. Martins et al. / Journal of Electron Spectroscopy and Related Phenomena 101 –103 (1999) 965 –969 967

Fig. 1. Optical layout of the spherical grating monochromator.

through the beamline and calculating the width of the RMS values given in Table 1 for the optical surfaces.energy distribution (FWHM) at the exit slit position. From these runs, the typical errors for the resolvingThe entrance slit was set to 5 mm and the appropriate power was estimated as 10%. Assuming slope errorssize of the exit slit was chosen. The upper part of of the pre-mirror and the grating better than 0.1 in.,Fig. 3 shows a typical image at the exit slit for the highest aberration-limited resolving power willphoton energies of 80 000 eV and 80 004 eV. The be about E /DE590 000 for a 5 mm entrance slit (1lower part shows the projection normal to the slit, in.52.54 cm). This ultra-high resolution will be onlywhich corresponds to the photon energy distribution, possible under very stable conditions and narrow slitand a fit of two Gaussian profiles. The width widths. A narrow entrance slit will be very important(FWHM) of the gaussian is ¯1.1 meV resulting in a to increase the diffraction-limited resolving powerresolving power of E /DE573 000. To estimate the given by the number of illuminated grating grooveserror of the calculated resolving power, ray-tracing N. On the other side, the diffraction will enlarge theruns of the beamline where made with several illuminated area of the spherical grating, which willdifferent randomly selected figure errors all with the increase the aberration of the grating, especially the

Table 1Parameters of the optical elements of the beamline

Optical Distance from Deflection Shape Radius Size Slope2element source (m) angle (8) (m) (mm ) error

M 17 9 Cylindrical 0.2 150320 1.001

S 18.91

M 21.2 10–16 Plane 300320 ,0.102

G 21.5 10–16 Spherical 41 160320 ,0.10

S 28.8–29.82

M 31.3 10 Toroidal 62.8 /0.21 150320 1.0041

M 31.3 20 Toroidal 40.8 /0.47 150320 1.0042

968 M. Martins et al. / Journal of Electron Spectroscopy and Related Phenomena 101 –103 (1999) 965 –969

Fig. 3. Image at the exit slit. The upper part shows the simulatedimage for two fixed photon energies 80.000 eV and 80.004 eV atthe position of the exit slit. In the lower part, the projectionnormal to the slit, which represents the photon energy distribution,

Fig. 2. Resolving power and flux of the monochromator. The is shown. The solid line represents the simulated data and theupper graph shows the maximum resolving power for a 5 mm dotted line is a fit of two Gaussian profiles to the data. Theentrance slit and varying widths and positions of the exit slit. The FWHM of the Gaussian is ¯1.1 meV, which leads to a resolvinglower part shows the calculated flux. The circled line represents power of E /DE573.000.the flux in case of maximum resolving power. The solid linerepresents the flux for a constant resolving power E /DE51.000.

2(g 5E /mc : electron energy; l : length of the un-0

most important coma aberration from Eq. 2. By dulator period; n: harmonic order). Tuning themoving the pre-mirror, the grating, as well as the exit undulator for the maximum flux will increase theslit, the coma aberration can be reduced to zero to flux by a factor of (2. Absolute values of theminimize this aberration in the given energy ranges, reflectivities of the optical elements have been takenwhich causes in the case of the ALS 9.0.1 beamline into account. The grating efficiency has been esti-the residual aberration, resulting in a broadening of mated to 10%, which is a reasonable value accordingthe experimental data [16]. to the suppliers. For the maximum resolving power

11The flux curves shown in the lower part of Fig. 2 shown in Fig. 2, a flux of (10 photons /s /100 mAhave been calculated with the SHADOW code to can be delivered. For a constant resolving power ofsimulate undulator radiation. For our simulations, E /DE51000 (0.1% bandwidth), the flux will be in

2 13taking into account a source size of 620342 mm the order of 10 photons /s /100 mA, which is a(FWHM) [17], with a beam energy of 1.9 GeV, we typical value for this type of beamline.have set the undulator wavelength l to the K value Due to the moveable exit slit, the spot size willof the smallest source size, given by [13] vary with the exit slit position. The minimum size

will be (60 mm vertically and (120 mm horizon-2l K0 tally for an exit slit – refocusing-mirror distance ofS D]] ]l 5 1 1 (3)2 22ng 2.1 m which corresponds to the middle position of

M. Martins et al. / Journal of Electron Spectroscopy and Related Phenomena 101 –103 (1999) 965 –969 969

the slit moving range. The direct, unfocused beam is Forschung und Technologie, project BMFT 05useable at a third endstation for experiments without SR8KE1 1.the need for small spot sizes. In this case typical spotsizes will be (2 mm horizontally and, depending onthe distance from the exit slit, between 1 mm and 7 Referencesmm vertically.

[1] P. Heimann et al., Nucl. Instrum. Methods A319 (1992) 106.[2] F. Senf, F. Eggenstein, W. Peatman, Rev. Sci. Instrum. 63

(1992) 1326.4. Conclusions[3] W. Peatman et al., Rev. Sci. Instrum. 66 (1995) 2801.[4] T. Warwick et al., Rev. Sci. Instrum. 66 (1995) 2037.

We described a spherical grating monochromator [5] R. Follath, F. Senf, Nucl. Instrum. Methods A390 (1997)for the BESSY II U125/ II undulator. The mono- 388.

[6] M. Koike, T. Hamioka, Rev. Sci. Instrum. 66 (1995) 2144.chromator is optimized for highest energy resolution[7] K. Sawhney et al., Nucl. Instrum. Methods A390 (1997) 395.using a variable included angle and a moveable exit[8] G. Kaindl et al., SRN 8 (1995) 29.

slit in order to reduce the contribution of the residual [9] K. Schulz et al., Phys. Rev. Lett. 77 (1996) 3086.coma aberration of the spherical grating. The results [10] C. Chen, F. Sette, Rev. Sci. Instrum. 60 (1989) 1616.of the ray-tracing calculations show a very high [11] H. Padmore, Rev. Sci. Instrum. 60 (1989) 1608.

[12] H. Petersen et al., Rev. Sci. Instrum. 66 (1995) 1.resolving power when both, the exit slit and the[13] W.B. Peatman, Gratings, mirrors and slits, Gordon andincluded angle are tuned.

Breach Science Publishers, Amsterdam, 1997.[14] B. Lai, F. Cerrina, Nucl. Instrum. Methods A246 (1986) 337.[15] M.S. del Rio, A. Marcelli, Nucl. Instrum. Methods A319

Acknowledgements (1992) 170.[16] H. Padmore, M. Howells, W. McKinney, in: J. Samson, D.

Ederer (Eds.), Techniques of vacuum ultraviolet physics,We acknowledge various helpful discussions withAcademic Press, Orlando, Fl, 1997, pp. 1–49.

F. Senf and M. Howells. This work is supported by [17] BESSY II Beamline Handbook, 1995.¨the Bundesminister fur Bildung, Wissenschaft,