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First results from the high resolution XUV undulator beamline BW3 at HASYLAB
Nuclear Instruments and Methods in Physics Research A 337 (1994) 603-608 North-Holland
NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH Section A
First results from the high resolution XUV undulator beamline BW3 at HASYLAB C.U.S. Larsson b,1, A. Bevtler a, O. Björneholm b, F. Federmann b, U. Hahn b, A. Rieck b, S. Verbin a,2, T. Möller b, II Inst. für Experimentalphysik, Universität Hamburg, Luruper Chaussee 149, 22761 Hamburg, Germany
b Hamburger Synchrotronstrahlungslabor HASYLAB at Deutsches Elektronensynchrotron DESY (Hamburg), Notkestr 85, 22603 Hamburg, Germany
(Received 16 June 1993 ; revised form received 6 September 1993) Initial results from the XUV undulator beamline at the 4.5 GeV storage ring DORIS are reported . The beamline consists of three different undulators mounted on a revolver type device and a modified SX-700 plane grating monochromator with a spherical focusing mirror. It covers the spectral range from 50-2000 eV with excellent spectral resolution and high photon flux. In the energy range 400 eV to 1 keV a spectral resolution estimated to 4500-9000 is obtained which is slightly better than the best values previously reported . In the energy range 50-1700 eV the photon flux is of the order of 10 11 -1013 /s for a resolving power of a thousand normalized to a ring current of 100 mA . A study of the photofragmentation of H2O following K-shell excitation energetically close to the oxygen K-edge at - 540 eV demonstrates the excellent performance of the beamline, in particular for the investigation of secondary processes where a high flux is necessary. 1. Introduction In the past years several innovative designs for grating monochromators working in the soft X-ray range (XUV, 50-2000 eV) have been proposed and constructed . Several of the instruments were planned to achieve a spectral resolution better than 100 meV, i.e . smaller than the natural linewidth of C, N, O K-edges. The performance of some of these instruments however is severely limited by figure errors of the optical elements [1,2]. Bright undulator sources which emit radiation in a narrow cone allow one to make use of new optical concepts [2,3]. On the other hand, the collimated radiation of undulator sources may lead to heat load problems which may deteriorate the spectral resolution or reduce the photon flux . Therefore, the design of a monochromator and the undulator are strongly coupled. For the design of the monochromator it is of considerable importance that plane or spherical surfaces can be manufactured with Now with the Swedish Defence Research Establishment, Department of Information Technology, Link6ping, Sweden. 2 Home address: St . Petersburg University, Department of Physics, St . Petersburg, Russian Federation . * Corresponding author. 1
much better accuracy than aspherical ones . At present, two different concepts seem to be the most promising for achieving high flux and high resolving power simultaneously: (i) spherical grating monochromators (SGM) of the DRAGON type [4] or related designs [5,6], (ü) monochromators based on the early concepts of plane grating monochromators (PGM) [7] like the SX-700 [8-101 . In both cases, special emphasis has to be put on the construction of the pre-optics, e.g . efficient cooling systems. Preliminary results from the SGM at Brookhaven [11] indeed show the influence of heat load problems . In the following, we present initial results from the modified SX-700 monochromator with a spherical focusing mirror behind an undulator at HASYLAB. Making use of graphite coated with SiC and a triple-undulator being optimized for a total emitted power less than 100 W in the most frequently used energy range, the heat load problems were reduced to an acceptable level [12,13]. During the whole beamline commissioning period we had no indications of heat load problems. First tests show that the measured photon flux is close to the expected values . The spectral resolution obtained so far is even better than specified [9]. An estimated resolving power of approximately 7000-11000 at the N K-edge at 400 eV is ob-
0168-9002/94/$07 .00 © 1994 - Elsevier Science B.V. All rights reserved SSDI0168-9002(94)E0868-S
604
C.U. S. Larsson et al /Nucl. Instr. and Meth . in Phys. Res. A 337 (1994) 603-608
Side View
M3
full aperture of the focusing mirror . According to the SX-700 design the position of the exit slit is fixed by keeping c = cos /3/cos a = const, where a and /3 are the angle of incidence and diffraction at the plane
M2 ßm1 Fig. 1 Optical layout of the SX-700 plane grating monochromator S- undulator source ; MO and Ml . plar.- mirrors, G: grating, M2 : spherical focusing mirror; ES : exit slit ; M3 : toroidal refocusing mirror
S
grating, respectively, [8]. If r, and r2 are the distances
between the source and the focusing mirror and between the mirror and the exit slit, respectively, the
vertical spot size o-v* at the exit slit plane is given by [8,y] o-,* =o-,r,/r,c,
tained . At
the Ne K-edge at 870 eV the resolving
where o- is the vertical source size . In the standard
obtained by making use of a special high resolution mode of the SX-700 monochromator . Basically, the
design a value of c = 2.25 was adopted in view of grating efficiencies in the XUV regime . By choosing larger values for c the resolving power can be consider-
moved to larger distances from the monochromator.
limiting the resolution . The gain in resolving power is
parameter c in the SX-700 monochromator. The flux obtained so far should allow one to analyse secondary
result of the defocusing of the plane grating. This leads
power is approximately 4500 . These values could be
virtual source which limits the spectral resolution is
ably enhanced as long as the vertical source size is
This can be simply done by changing the fixed focus
due to an increase in the virtual source distance as a
processes, e.g . with time-of-flight spectroscopy, photoemission or XUV-fluorescence with a spectral resolu-
to a much larger demagnification of the source [9].
This feature is particularly useful for a monochromator
without an entrance slit where usually the limit for the
tion comparable to the natural linewidth .
resolution, given by the vertical source size, cannot be
2. Description of the beamline
refocusing mirror . The spot size at the sample is less than 1 mm 2.
overcome .
The last optical element in the beamline is a toroidal
The optical layout of the beamline is presented in Fig. 1. In order to cover a broad photon energy range a configuration of three 4 m long undulators (total scan-
3. Results
length of the different structures is 9, 12 and 18.6 cm, respectively. The total power delivered from the undu-
the exit slit for the three undulators and the different
ning range 15-2000 eV in the first harmonic) mounted on a revolver type device was adopted. The period
lators is kept below 100 W for the most frequently used energy range (200-2000 eV) [12-14]. However, if the
gap is closed down to 30 mm the first harmonic moves to 15, 90 and 150 eV, respectively, for the three structures while the totally emitted power increases up to
2000 W as a result of the high energy of DORIS III
Photon flux measurements were performed behind
mirror coatings . In the energy range 50-1000 eV a photon flux in the order of 10 12 up to 1 X 10 13 (photons/0 .1% bandwidth normalized
to
100 mA) was
measured in the first harmonic for the best undulator/
coating combinations . This is close to the theoretical
values using the calculated flux from the undulator [16], the grating efficiencies and the reflectivities of the
(4 .5 GeV) . The first optical elements (premirror MO in
mirrors. The effect of surface roughness is not included in the calculation .
for the horizontally deflecting mirror MO, the premirror M1 and the grating are made of graphite coated
the refocusing mirror M3 at the sample position. Fig. 2
shows a comparison between measured and calculated
to the working range of the different undulators . The
coating. The experimental values are somewhat lower than the calculated ones . Whether the losses in flux are
the ring plane mirror MI and grating G, see Fig. 1) are water cooled to withstand the heat load . The substrates
with SiC. Three different coatings (SiC, SiO, and Au) are available on the first mirror optimized with respect main modification in the new design with respect to the conventional SX-700 is the replacement of the refocusing ellipsoidal mirror which limits the resolving power in the original one [15] by a spherical one (0.19
arc sec slope error). Spherical aberrations which might limit the spectral resolution are reduced to an accept-
able value by using a focal length of 6000 mm . This allows one to obtain high spectral resolution with the
Further flux measurements were performed behind
photon flux for undulator I using the first mirror's gold
due to small misalignments or due to a reduced reflectivity as
a result of surface roughness is
an
open
question . The photon flux at 600 eV could be considerably increased up to approximately 3 X 10 12 photons/
0.1% (for 100 mA) with the undulator III using a
smaller gap and the SiC coating. It turned out that the useful working range of the beamline is 50-1800 eV . Below 50 eV the contribution of light from higher
C.U.S. Larsson et al. I Nucl. Instr. and Meth . in Phys. Res. A 337 (1994) 603-608
orders becomes troublesome . Above 1.8 keV the measured flux drastically decreases in accordance with the scanning range of undulator III (see Fig. 2, insert). It should be pointed out that the flux measured at 1 .7 keV using undulator III in the first harmonic is approximately four to six times larger than the flux obtained with the structures I or II in the third harmonic. The resolving power was tested directly by measuring the absorption of Ar, Ne and NZ gas. In addition, partial and total ion yields were recorded with a small time-of-flight mass spectrometer and an effusive molecular jet. Initial measurements, using the first order of diffraction of the monochromator, were performed in a small triple-cross chamber supplied by the Physics Department of University of Uppsala. The determination of the actual resolving power in the XUV regime is a severe problem since the natural linewidth of even the sharpest lines is comparable or larger than the bandpass energies of the best monochromators [11,17]. Generally numbers given in recent papers [15,17] are obtained from a fitting procedure of the measured absorption spectra using natural linewidths obtained from electron-energy-loss-spectroscopy or photoemission data . For the sake of com-
*lo lt 15 9 '0 .
10
by
5
ó rn
Undulator 1 150 mm gap measured 6
i 500
1000
1500
Photon Energy(eV) Fig. 2. Comparison between measured (bottom, GaAs photodiode, Hamamatsu) and calculated (top) photon flux (Undulator 1, gold coating of the first mirror) normalized to an electron current of 100 mA . The scanning range of the first harmonic of the different undulators is given in the insert . For small magnetic gaps with a totally emitted power greater than 100 W the curves are dashed.
605
e) first order
b) second order
01 399
I
400
r
I
401
Energy [eV]
I-
402
.
I
403
Fig . 3. Vibrationally resolved K-shell absorption of N Z recorded by measuring the total ion yield using a high resolu tion mode in the first and second order of diffraction. Parameters: c = 6, 20 win exit slit . parison we give numbers relying on similar procedures although it is quite clear that the natural linewidths entering into the determination are not sufficiently precise . A standard test for high resolution is vibrational resolution in the NZ IS 'Tr * transition at - 401 eV . An estimated resolution of 150 meV. has been obtained in the standard SX-700 mode (c = 2.25) with an 80 ltm exit slit, which is close to the calculated value of 135 meV [9]. The resolving power of this instrument without an entrance slit is mainly limited by the source size (vertical height 0.8 mm (FWHM)). The design of the beamline (a spherically focusing mirror and a fixed exit slit which can be set to different positions for the different values for c) allows us to increase the resolution by using larger values for c. With c = 6 and a 20 win exit slit excellent spectral resolution is obtained (see Fig. 3) . To obtain the ultimate resolution it was necessary to take only the central part of the undulator cone (corresponding - 50% of the optical active surface) . This reduces the spherical aberration by a factor 4 [9]. The valley to peak ratio between the first valley at 400.95 eV and the third peak at 401 .31 eV, respectively is a sensitive measure of the spectral resolution [17] . We obtained a value 0.75 (first order) and 0.70 (second order), respectively . The value obtained in the second order of diffraction is even better than the best value of 0.73 reported so far for the Brookhaven XIB beamline [11] while for other high resolution monochromators the value ranges be-
606
CU. S. Larsson et al. /Nucl. Instr. and Meth . in Phys. Res. A 337 (1994) 603-608
tween 0.85 and 1 [15,17,18] . The quoted spectral resolution of these instruments ranging between 40 and 74 meV shows the uncertainties in the determination of the resolution . We estimate that our spectral resolution measured in the first order is similar to that at Brookhaven X1B which is suggested to be below 50 meV [11] . Therefore, a conservative estimate gives a resolving power (first order) of 5000-8000 at 400 eV . Using the second order the resolution could be increased to approximately 7000-11000. These values are based on the results obtained in the first order and the ratio between the dispersion in the first and second orders . According to the dispersion of the instrument [8,15] the resolving power in the second order should be V~2 times higher than in the first order. Since the natural linewidth of the N is - "rr * transition (130 meV [15]) is much smaller than the bandpass of the monochromator the shape of the lines measured in the first and second order differs only slightly (see Figs . 3a and 3b). Measurements at the Ne K-shell at 867 eV are a more critical test because the resolving power generally decreases with increasing energy. For the Ne is-3p transition (see Fig. 4) a width of 350 meV (FWHM) is obtained which is better than the best values of 400 meV [15] and 450 meV [19], respectively, reported so far. By using the second order (see Fig. 4b) the width reduces to 290 meV. Using the relation between the resolving power in the first and second orders the natural linewidth and the spectral resolution can be
m . G 5
Fig. 4. Total ion yield of neon gas following K-shell excitation . The lines are due to transitions mto 3p, 4p and 5p states. Parameters : c = 6, 20 Wm exit slit .
H'
c
C:2 0
U
532
542 537 Energy [eV] Fig. 5. Partial ion yield curves of H2O measured with a time-of-flight mass spectrometer, spectral resolution 300 meV. estimated from the measured widths of the transition is - 3p . For the natural linewidth we obtained a value of 215 meV. This value is slightly lower than the values of 230 meV obtained from photoemission data [20] and 310 meV from EELS measurements [21] . For the bandpass of the monochromator we obtained 270 meV (first order) and 195 meV (second order), respectively . This corresponds to a spectral resolution of 3200 (first order) to 4500 (second order) at 870 eV . These values seem to be somewhat low in view of the quoted resolving power, estimated to be 4000 for 450 meV [19] measured with the 10 m monochromator at the Photon Factory and 3300 for 400 meV [15] width obtained with the SX700/11 at Bessy. Presumably, the natural linewidth of 310 meV assumed for the determination of the resolution is too large while our value of 215 meV is probably slightly too small . Another advantage of the monochromator is that photons of the second diffraction order can be sufficiently suppressed by using a small value for c, e.g . c = 1 .6. We note in passing that high spectral resolution can also be obtained in this higher order suppression mode when the position of the exit slit is adjusted . (For a constant value of c the position of the exit slit is fixed for all energies, however, the distance from the focusing mirror is different for different values of c.) First scientific results are presented in Fig. 5 as an example of the performance of the beamline . An im-
CU. S . Larsson et al. I Nucl. Instr. and Meth . in Phys. Res. A 337 (1994) 603-608
portant feature of the new beamline is that the gap of the undulator can be changed simultaneously while scanning the monochromator, and Fig. 5 shows partial ion yield curves of H2O gas excited close to the oxygen K-edge are recorded while simultaneously changing the undulator gap in steps of 0.05 mm, corresponding to 0.5 eV in the first harmonic . The time structure of the synchrotron radiation in the 4 bunch mode (240 ns separation) is directly used to start and stop a short time-of-flight mass spectrometer (10 mm total length of the flight tube allowing detection of particles up to 1600 atomic mass units in single bunch mode). Partial ion yield counting rates up to 5 x 10 4 cps are obtained at a spectral resolution of 300 meV which is smaller than the intrinsic linewidth of the absorption bands of H 2O. The partial ion yield curves exhibit some structure [22] previously not seen in absorption [23] or H+ yield curves [24] . Furthermore, the measurements show that the fragmentation pattern depends on the primary excited state. Very recently, these measurements have been extended to small (H 20) and Arn clusters [25] . Further improvements to the beamline are in progress. Ray-tracing calculations for minimizing the spot size at the sample position have been performed for two different optical concepts . A small spot size is highly desirable in view of the scientific program at the beamline, e.g . XUV fluorescence, time-of-flight spectroscopy on atoms, molecules and clusters. At present, the spot size is limited by aberrations of the toroidal refocusing mirror due to the fact, that this mirror is the only horizontally focusing element in the beamline . A vertical spot size of 100 ltm or less should be possible by using a combination of two toroidal mirrors to compensate for aberrations. Alternatively, a special ellipsoidal mirror with three different axis lengths should also give a rather small spot size while the reflectivity losses would be smaller. After this work was completed we became aware of recent results obtained with a plane grating monochromator at Bessy. Similar to our optical layout this monochromator makes use of a spherical focusing mirror [26] . As a result excellent spectral resolution with values only slightly lower than those reported here is obtained. This underlines again the usefulness of this concept. 4. Conclusion The first successful tests show the excellent performance of the XUV beamline at the HASYLAB wiggler laboratory . The ultimate resolving power obtained at the nitrogen and neon K-edge is superior to that obtained with other XUV-monochromators . In particular, if the time structure of the DORIS III storage ring at HASYLAB (1-5 MHz repetition rate) is considered this beamline will have advantages over similar beam-
607
lines at third generation machines in the domain of time-resolved measurements . We also draw attention to the excellent performance of the beamline in the high energy range 1-1.8 keV, an energy range which can only be covered with great difficulties by undulator beamlines at the new machines like the ALS, ELETTRA and BESSY II . Acknowledgement A fruitful cooperation with R. Reininger (SRC, Wisconsin) is kindly acknowledged . We are also grateful to P. Giirtler, who performed numerous calculations for the flux of the undulator and Th . Kracht who wrote the software for running the experiment . Special thanks also to D. Mancini and N. Wassdahl (University of Uppsala) for support in the first beamtime period . Furthermore, we are indebted to the Fritz-Haber-Institut (U . Becker), the Kernforschungsanlage Jiilich (W . Eberhardt) and the University of Uppsala (J . Nordgren) who have also taken part in funding the beamline . References [1] G.P . Williams, Nucl. Instr . and Meth . A 264 (1986) 294 . [2] A. Padmore, Rev. Sci. Instr. 60 (1989) 1608 . [3] R. Reininger, Nucl . Instr. and Meth . A 319 (1992) 110 . [4] C .T . Chen, Nucl . Instr. and Meth . A 265 (1987) 595 . [5] H. Hogrefe, M.R . Howells and E. Hoyer, SPIE 733
(1986) 274. [6] F. Sent', F. Eggenstein and W. Peatman, Rev. Sci. Instr. 63 (1992) 1326 . [7] H. Dietrich and C. Kunz, Rev. Sci. Instr. 43 (1972) 434 . [8] H. Petersen, Opt. Commun . 40 (1982) 402 . [9] R. Reininger and V. Saile, Nucl . Instr. and Meth A 288 (1990) 343 . [10] W. Jark, Rev. Sci. Instr . 63 (1992) 1241 .
[11] K.J . Randall, W. Eberhardt, J. Feldhaus, W. Erlebach, A.M . Bradshaw, Z. Xu, P.D . Johnson and Y. Ma, Nucl . Instr. and Meth. A 319 (1992) 101 . [12] A.R .B . de Castro and R. Reminger, Nucl. Instr. and Meth . A 307 (1991) 135 . [13] A.R .B . de Castro and R. Reininger, Rev. Sci. Instr. 63
(1992) 1317 . [14] J. Pflilger and P. Gürtler, Nucl . Instr. and Meth . A 287 (1990) 628 slightly changed parameters in the HASYLAB annual report 1992, p . 22. [15] M. Domke, T. Mandel, A. Puschmann, C . Xue, D.A .
Shirley, G. Kaindel, H. Petersen and P. Kuske, Rev. Sci. Instr . 63 (1992) 80 . [16] P . Gürtler, private communication . [17] C .T . Chen and F. Sette, Rev. Sci. Instr. 60 (1988) 1616 . [18] P . Heimann, F. Senf, W. McKinney, M. Howells, R.D . van Zee, L.D . Medhurst, T. Lauritzen, J. Chin, J. Meneghetti, W. Gath, H. Hogrefe and D.A . Shirley, Phys . Scripta T 31 (1990) 127.
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C U. S. Larsson et al /Nucl. Instr. and Meth. to Phys. Res. A 337 (1994) 603-608
[19] A. Yagishita, S . Masuj, T . Toyoshima, H . Maezawa and E . Shigemasa, Rev. Sci . Instr . 63 (1992) 1351 . [201 U. Gelius, S . Svensson, H. Siegbahn, E . Baisilier, A. Faxdly and K. Siegbahn, Chem. Phys . Lett . 28 (1974) 1 . [21] A .P . Hitchcock and C .E . Brion, J . Phys . B 13 (1980) 3269 . [221 F. Federmann, A . Beutler, C Larsson and T . M6ller, Chem . Phys . Lett ., to be published . [23] G .R . Wight and C.E . Brion, J . Electron . Spectrosc. Relat . Phenom . 4 (1974) 25 .
[24] D .Y . Kim, K . Lee, C.I . Ma, M . Mahalingam, D .M. Hanson and S .L. Hulbert, J . Chem . Phys . 97 (1992) . [25] F. Federmann, O. Bj6rneholm, F . F6ssing and T . M611er, to be published . [26] H . Petersen, C . Jung, C . Hellwig, W . Peatman and W . Gudat, BESSY Annual Report 1992, p . 484.
NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH Section A
First results from the high resolution XUV undulator beamline BW3 at HASYLAB C.U.S. Larsson b,1, A. Bevtler a, O. Björneholm b, F. Federmann b, U. Hahn b, A. Rieck b, S. Verbin a,2, T. Möller b, II Inst. für Experimentalphysik, Universität Hamburg, Luruper Chaussee 149, 22761 Hamburg, Germany
b Hamburger Synchrotronstrahlungslabor HASYLAB at Deutsches Elektronensynchrotron DESY (Hamburg), Notkestr 85, 22603 Hamburg, Germany
(Received 16 June 1993 ; revised form received 6 September 1993) Initial results from the XUV undulator beamline at the 4.5 GeV storage ring DORIS are reported . The beamline consists of three different undulators mounted on a revolver type device and a modified SX-700 plane grating monochromator with a spherical focusing mirror. It covers the spectral range from 50-2000 eV with excellent spectral resolution and high photon flux. In the energy range 400 eV to 1 keV a spectral resolution estimated to 4500-9000 is obtained which is slightly better than the best values previously reported . In the energy range 50-1700 eV the photon flux is of the order of 10 11 -1013 /s for a resolving power of a thousand normalized to a ring current of 100 mA . A study of the photofragmentation of H2O following K-shell excitation energetically close to the oxygen K-edge at - 540 eV demonstrates the excellent performance of the beamline, in particular for the investigation of secondary processes where a high flux is necessary. 1. Introduction In the past years several innovative designs for grating monochromators working in the soft X-ray range (XUV, 50-2000 eV) have been proposed and constructed . Several of the instruments were planned to achieve a spectral resolution better than 100 meV, i.e . smaller than the natural linewidth of C, N, O K-edges. The performance of some of these instruments however is severely limited by figure errors of the optical elements [1,2]. Bright undulator sources which emit radiation in a narrow cone allow one to make use of new optical concepts [2,3]. On the other hand, the collimated radiation of undulator sources may lead to heat load problems which may deteriorate the spectral resolution or reduce the photon flux . Therefore, the design of a monochromator and the undulator are strongly coupled. For the design of the monochromator it is of considerable importance that plane or spherical surfaces can be manufactured with Now with the Swedish Defence Research Establishment, Department of Information Technology, Link6ping, Sweden. 2 Home address: St . Petersburg University, Department of Physics, St . Petersburg, Russian Federation . * Corresponding author. 1
much better accuracy than aspherical ones . At present, two different concepts seem to be the most promising for achieving high flux and high resolving power simultaneously: (i) spherical grating monochromators (SGM) of the DRAGON type [4] or related designs [5,6], (ü) monochromators based on the early concepts of plane grating monochromators (PGM) [7] like the SX-700 [8-101 . In both cases, special emphasis has to be put on the construction of the pre-optics, e.g . efficient cooling systems. Preliminary results from the SGM at Brookhaven [11] indeed show the influence of heat load problems . In the following, we present initial results from the modified SX-700 monochromator with a spherical focusing mirror behind an undulator at HASYLAB. Making use of graphite coated with SiC and a triple-undulator being optimized for a total emitted power less than 100 W in the most frequently used energy range, the heat load problems were reduced to an acceptable level [12,13]. During the whole beamline commissioning period we had no indications of heat load problems. First tests show that the measured photon flux is close to the expected values . The spectral resolution obtained so far is even better than specified [9]. An estimated resolving power of approximately 7000-11000 at the N K-edge at 400 eV is ob-
0168-9002/94/$07 .00 © 1994 - Elsevier Science B.V. All rights reserved SSDI0168-9002(94)E0868-S
604
C.U. S. Larsson et al /Nucl. Instr. and Meth . in Phys. Res. A 337 (1994) 603-608
Side View
M3
full aperture of the focusing mirror . According to the SX-700 design the position of the exit slit is fixed by keeping c = cos /3/cos a = const, where a and /3 are the angle of incidence and diffraction at the plane
M2 ßm1 Fig. 1 Optical layout of the SX-700 plane grating monochromator S- undulator source ; MO and Ml . plar.- mirrors, G: grating, M2 : spherical focusing mirror; ES : exit slit ; M3 : toroidal refocusing mirror
S
grating, respectively, [8]. If r, and r2 are the distances
between the source and the focusing mirror and between the mirror and the exit slit, respectively, the
vertical spot size o-v* at the exit slit plane is given by [8,y] o-,* =o-,r,/r,c,
tained . At
the Ne K-edge at 870 eV the resolving
where o- is the vertical source size . In the standard
obtained by making use of a special high resolution mode of the SX-700 monochromator . Basically, the
design a value of c = 2.25 was adopted in view of grating efficiencies in the XUV regime . By choosing larger values for c the resolving power can be consider-
moved to larger distances from the monochromator.
limiting the resolution . The gain in resolving power is
parameter c in the SX-700 monochromator. The flux obtained so far should allow one to analyse secondary
result of the defocusing of the plane grating. This leads
power is approximately 4500 . These values could be
virtual source which limits the spectral resolution is
ably enhanced as long as the vertical source size is
This can be simply done by changing the fixed focus
due to an increase in the virtual source distance as a
processes, e.g . with time-of-flight spectroscopy, photoemission or XUV-fluorescence with a spectral resolu-
to a much larger demagnification of the source [9].
This feature is particularly useful for a monochromator
without an entrance slit where usually the limit for the
tion comparable to the natural linewidth .
resolution, given by the vertical source size, cannot be
2. Description of the beamline
refocusing mirror . The spot size at the sample is less than 1 mm 2.
overcome .
The last optical element in the beamline is a toroidal
The optical layout of the beamline is presented in Fig. 1. In order to cover a broad photon energy range a configuration of three 4 m long undulators (total scan-
3. Results
length of the different structures is 9, 12 and 18.6 cm, respectively. The total power delivered from the undu-
the exit slit for the three undulators and the different
ning range 15-2000 eV in the first harmonic) mounted on a revolver type device was adopted. The period
lators is kept below 100 W for the most frequently used energy range (200-2000 eV) [12-14]. However, if the
gap is closed down to 30 mm the first harmonic moves to 15, 90 and 150 eV, respectively, for the three structures while the totally emitted power increases up to
2000 W as a result of the high energy of DORIS III
Photon flux measurements were performed behind
mirror coatings . In the energy range 50-1000 eV a photon flux in the order of 10 12 up to 1 X 10 13 (photons/0 .1% bandwidth normalized
to
100 mA) was
measured in the first harmonic for the best undulator/
coating combinations . This is close to the theoretical
values using the calculated flux from the undulator [16], the grating efficiencies and the reflectivities of the
(4 .5 GeV) . The first optical elements (premirror MO in
mirrors. The effect of surface roughness is not included in the calculation .
for the horizontally deflecting mirror MO, the premirror M1 and the grating are made of graphite coated
the refocusing mirror M3 at the sample position. Fig. 2
shows a comparison between measured and calculated
to the working range of the different undulators . The
coating. The experimental values are somewhat lower than the calculated ones . Whether the losses in flux are
the ring plane mirror MI and grating G, see Fig. 1) are water cooled to withstand the heat load . The substrates
with SiC. Three different coatings (SiC, SiO, and Au) are available on the first mirror optimized with respect main modification in the new design with respect to the conventional SX-700 is the replacement of the refocusing ellipsoidal mirror which limits the resolving power in the original one [15] by a spherical one (0.19
arc sec slope error). Spherical aberrations which might limit the spectral resolution are reduced to an accept-
able value by using a focal length of 6000 mm . This allows one to obtain high spectral resolution with the
Further flux measurements were performed behind
photon flux for undulator I using the first mirror's gold
due to small misalignments or due to a reduced reflectivity as
a result of surface roughness is
an
open
question . The photon flux at 600 eV could be considerably increased up to approximately 3 X 10 12 photons/
0.1% (for 100 mA) with the undulator III using a
smaller gap and the SiC coating. It turned out that the useful working range of the beamline is 50-1800 eV . Below 50 eV the contribution of light from higher
C.U.S. Larsson et al. I Nucl. Instr. and Meth . in Phys. Res. A 337 (1994) 603-608
orders becomes troublesome . Above 1.8 keV the measured flux drastically decreases in accordance with the scanning range of undulator III (see Fig. 2, insert). It should be pointed out that the flux measured at 1 .7 keV using undulator III in the first harmonic is approximately four to six times larger than the flux obtained with the structures I or II in the third harmonic. The resolving power was tested directly by measuring the absorption of Ar, Ne and NZ gas. In addition, partial and total ion yields were recorded with a small time-of-flight mass spectrometer and an effusive molecular jet. Initial measurements, using the first order of diffraction of the monochromator, were performed in a small triple-cross chamber supplied by the Physics Department of University of Uppsala. The determination of the actual resolving power in the XUV regime is a severe problem since the natural linewidth of even the sharpest lines is comparable or larger than the bandpass energies of the best monochromators [11,17]. Generally numbers given in recent papers [15,17] are obtained from a fitting procedure of the measured absorption spectra using natural linewidths obtained from electron-energy-loss-spectroscopy or photoemission data . For the sake of com-
*lo lt 15 9 '0 .
10
by
5
ó rn
Undulator 1 150 mm gap measured 6
i 500
1000
1500
Photon Energy(eV) Fig. 2. Comparison between measured (bottom, GaAs photodiode, Hamamatsu) and calculated (top) photon flux (Undulator 1, gold coating of the first mirror) normalized to an electron current of 100 mA . The scanning range of the first harmonic of the different undulators is given in the insert . For small magnetic gaps with a totally emitted power greater than 100 W the curves are dashed.
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e) first order
b) second order
01 399
I
400
r
I
401
Energy [eV]
I-
402
.
I
403
Fig . 3. Vibrationally resolved K-shell absorption of N Z recorded by measuring the total ion yield using a high resolu tion mode in the first and second order of diffraction. Parameters: c = 6, 20 win exit slit . parison we give numbers relying on similar procedures although it is quite clear that the natural linewidths entering into the determination are not sufficiently precise . A standard test for high resolution is vibrational resolution in the NZ IS 'Tr * transition at - 401 eV . An estimated resolution of 150 meV. has been obtained in the standard SX-700 mode (c = 2.25) with an 80 ltm exit slit, which is close to the calculated value of 135 meV [9]. The resolving power of this instrument without an entrance slit is mainly limited by the source size (vertical height 0.8 mm (FWHM)). The design of the beamline (a spherically focusing mirror and a fixed exit slit which can be set to different positions for the different values for c) allows us to increase the resolution by using larger values for c. With c = 6 and a 20 win exit slit excellent spectral resolution is obtained (see Fig. 3) . To obtain the ultimate resolution it was necessary to take only the central part of the undulator cone (corresponding - 50% of the optical active surface) . This reduces the spherical aberration by a factor 4 [9]. The valley to peak ratio between the first valley at 400.95 eV and the third peak at 401 .31 eV, respectively is a sensitive measure of the spectral resolution [17] . We obtained a value 0.75 (first order) and 0.70 (second order), respectively . The value obtained in the second order of diffraction is even better than the best value of 0.73 reported so far for the Brookhaven XIB beamline [11] while for other high resolution monochromators the value ranges be-
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tween 0.85 and 1 [15,17,18] . The quoted spectral resolution of these instruments ranging between 40 and 74 meV shows the uncertainties in the determination of the resolution . We estimate that our spectral resolution measured in the first order is similar to that at Brookhaven X1B which is suggested to be below 50 meV [11] . Therefore, a conservative estimate gives a resolving power (first order) of 5000-8000 at 400 eV . Using the second order the resolution could be increased to approximately 7000-11000. These values are based on the results obtained in the first order and the ratio between the dispersion in the first and second orders . According to the dispersion of the instrument [8,15] the resolving power in the second order should be V~2 times higher than in the first order. Since the natural linewidth of the N is - "rr * transition (130 meV [15]) is much smaller than the bandpass of the monochromator the shape of the lines measured in the first and second order differs only slightly (see Figs . 3a and 3b). Measurements at the Ne K-shell at 867 eV are a more critical test because the resolving power generally decreases with increasing energy. For the Ne is-3p transition (see Fig. 4) a width of 350 meV (FWHM) is obtained which is better than the best values of 400 meV [15] and 450 meV [19], respectively, reported so far. By using the second order (see Fig. 4b) the width reduces to 290 meV. Using the relation between the resolving power in the first and second orders the natural linewidth and the spectral resolution can be
m . G 5
Fig. 4. Total ion yield of neon gas following K-shell excitation . The lines are due to transitions mto 3p, 4p and 5p states. Parameters : c = 6, 20 Wm exit slit .
H'
c
C:2 0
U
532
542 537 Energy [eV] Fig. 5. Partial ion yield curves of H2O measured with a time-of-flight mass spectrometer, spectral resolution 300 meV. estimated from the measured widths of the transition is - 3p . For the natural linewidth we obtained a value of 215 meV. This value is slightly lower than the values of 230 meV obtained from photoemission data [20] and 310 meV from EELS measurements [21] . For the bandpass of the monochromator we obtained 270 meV (first order) and 195 meV (second order), respectively . This corresponds to a spectral resolution of 3200 (first order) to 4500 (second order) at 870 eV . These values seem to be somewhat low in view of the quoted resolving power, estimated to be 4000 for 450 meV [19] measured with the 10 m monochromator at the Photon Factory and 3300 for 400 meV [15] width obtained with the SX700/11 at Bessy. Presumably, the natural linewidth of 310 meV assumed for the determination of the resolution is too large while our value of 215 meV is probably slightly too small . Another advantage of the monochromator is that photons of the second diffraction order can be sufficiently suppressed by using a small value for c, e.g . c = 1 .6. We note in passing that high spectral resolution can also be obtained in this higher order suppression mode when the position of the exit slit is adjusted . (For a constant value of c the position of the exit slit is fixed for all energies, however, the distance from the focusing mirror is different for different values of c.) First scientific results are presented in Fig. 5 as an example of the performance of the beamline . An im-
CU. S . Larsson et al. I Nucl. Instr. and Meth . in Phys. Res. A 337 (1994) 603-608
portant feature of the new beamline is that the gap of the undulator can be changed simultaneously while scanning the monochromator, and Fig. 5 shows partial ion yield curves of H2O gas excited close to the oxygen K-edge are recorded while simultaneously changing the undulator gap in steps of 0.05 mm, corresponding to 0.5 eV in the first harmonic . The time structure of the synchrotron radiation in the 4 bunch mode (240 ns separation) is directly used to start and stop a short time-of-flight mass spectrometer (10 mm total length of the flight tube allowing detection of particles up to 1600 atomic mass units in single bunch mode). Partial ion yield counting rates up to 5 x 10 4 cps are obtained at a spectral resolution of 300 meV which is smaller than the intrinsic linewidth of the absorption bands of H 2O. The partial ion yield curves exhibit some structure [22] previously not seen in absorption [23] or H+ yield curves [24] . Furthermore, the measurements show that the fragmentation pattern depends on the primary excited state. Very recently, these measurements have been extended to small (H 20) and Arn clusters [25] . Further improvements to the beamline are in progress. Ray-tracing calculations for minimizing the spot size at the sample position have been performed for two different optical concepts . A small spot size is highly desirable in view of the scientific program at the beamline, e.g . XUV fluorescence, time-of-flight spectroscopy on atoms, molecules and clusters. At present, the spot size is limited by aberrations of the toroidal refocusing mirror due to the fact, that this mirror is the only horizontally focusing element in the beamline . A vertical spot size of 100 ltm or less should be possible by using a combination of two toroidal mirrors to compensate for aberrations. Alternatively, a special ellipsoidal mirror with three different axis lengths should also give a rather small spot size while the reflectivity losses would be smaller. After this work was completed we became aware of recent results obtained with a plane grating monochromator at Bessy. Similar to our optical layout this monochromator makes use of a spherical focusing mirror [26] . As a result excellent spectral resolution with values only slightly lower than those reported here is obtained. This underlines again the usefulness of this concept. 4. Conclusion The first successful tests show the excellent performance of the XUV beamline at the HASYLAB wiggler laboratory . The ultimate resolving power obtained at the nitrogen and neon K-edge is superior to that obtained with other XUV-monochromators . In particular, if the time structure of the DORIS III storage ring at HASYLAB (1-5 MHz repetition rate) is considered this beamline will have advantages over similar beam-
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lines at third generation machines in the domain of time-resolved measurements . We also draw attention to the excellent performance of the beamline in the high energy range 1-1.8 keV, an energy range which can only be covered with great difficulties by undulator beamlines at the new machines like the ALS, ELETTRA and BESSY II . Acknowledgement A fruitful cooperation with R. Reininger (SRC, Wisconsin) is kindly acknowledged . We are also grateful to P. Giirtler, who performed numerous calculations for the flux of the undulator and Th . Kracht who wrote the software for running the experiment . Special thanks also to D. Mancini and N. Wassdahl (University of Uppsala) for support in the first beamtime period . Furthermore, we are indebted to the Fritz-Haber-Institut (U . Becker), the Kernforschungsanlage Jiilich (W . Eberhardt) and the University of Uppsala (J . Nordgren) who have also taken part in funding the beamline . References [1] G.P . Williams, Nucl. Instr . and Meth . A 264 (1986) 294 . [2] A. Padmore, Rev. Sci. Instr. 60 (1989) 1608 . [3] R. Reininger, Nucl . Instr. and Meth . A 319 (1992) 110 . [4] C .T . Chen, Nucl . Instr. and Meth . A 265 (1987) 595 . [5] H. Hogrefe, M.R . Howells and E. Hoyer, SPIE 733
(1986) 274. [6] F. Sent', F. Eggenstein and W. Peatman, Rev. Sci. Instr. 63 (1992) 1326 . [7] H. Dietrich and C. Kunz, Rev. Sci. Instr. 43 (1972) 434 . [8] H. Petersen, Opt. Commun . 40 (1982) 402 . [9] R. Reininger and V. Saile, Nucl . Instr. and Meth A 288 (1990) 343 . [10] W. Jark, Rev. Sci. Instr . 63 (1992) 1241 .
[11] K.J . Randall, W. Eberhardt, J. Feldhaus, W. Erlebach, A.M . Bradshaw, Z. Xu, P.D . Johnson and Y. Ma, Nucl . Instr. and Meth. A 319 (1992) 101 . [12] A.R .B . de Castro and R. Reminger, Nucl. Instr. and Meth . A 307 (1991) 135 . [13] A.R .B . de Castro and R. Reininger, Rev. Sci. Instr. 63
(1992) 1317 . [14] J. Pflilger and P. Gürtler, Nucl . Instr. and Meth . A 287 (1990) 628 slightly changed parameters in the HASYLAB annual report 1992, p . 22. [15] M. Domke, T. Mandel, A. Puschmann, C . Xue, D.A .
Shirley, G. Kaindel, H. Petersen and P. Kuske, Rev. Sci. Instr . 63 (1992) 80 . [16] P . Gürtler, private communication . [17] C .T . Chen and F. Sette, Rev. Sci. Instr. 60 (1988) 1616 . [18] P . Heimann, F. Senf, W. McKinney, M. Howells, R.D . van Zee, L.D . Medhurst, T. Lauritzen, J. Chin, J. Meneghetti, W. Gath, H. Hogrefe and D.A . Shirley, Phys . Scripta T 31 (1990) 127.
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[19] A. Yagishita, S . Masuj, T . Toyoshima, H . Maezawa and E . Shigemasa, Rev. Sci . Instr . 63 (1992) 1351 . [201 U. Gelius, S . Svensson, H. Siegbahn, E . Baisilier, A. Faxdly and K. Siegbahn, Chem. Phys . Lett . 28 (1974) 1 . [21] A .P . Hitchcock and C .E . Brion, J . Phys . B 13 (1980) 3269 . [221 F. Federmann, A . Beutler, C Larsson and T . M6ller, Chem . Phys . Lett ., to be published . [23] G .R . Wight and C.E . Brion, J . Electron . Spectrosc. Relat . Phenom . 4 (1974) 25 .
[24] D .Y . Kim, K . Lee, C.I . Ma, M . Mahalingam, D .M. Hanson and S .L. Hulbert, J . Chem . Phys . 97 (1992) . [25] F. Federmann, O. Bj6rneholm, F . F6ssing and T . M611er, to be published . [26] H . Petersen, C . Jung, C . Hellwig, W . Peatman and W . Gudat, BESSY Annual Report 1992, p . 484.