The LURE grazing incidence spectromonochromator: Two years operating experience

NUCLEAR

INSTRUMENTS

AND

METHO[)S

152

(1978)

85-93

,

©

NORTtI-IIOLLAND

PUBLISHING

CO

T H E LURE GRAZING INCIDENCE S P E C T R O M O N O C H R O M A T O R : TWO YEARS OPERATING EXPERIENCE* P I E R R E D H E Z . P I E R R E JAEGL[~. F R A N C O I S J W U I L L E U M I E R , E L I S A B E T H K A L L N E * , V O L K E R S C H M I D T ' , M A U R I C E B E R L A N I ) ~ and A N T O I N E C A R I L L O N ~

Eqmpe de Recherche "Spectroscopw Atomtque et Iomclue du C:\RS"

and LI_,RI~. Lmver.stte Parts Sud. 91405 O~sa). trance

T h e g r a z m g m o d e n c c s p c c t r o m o n o c h r o m a t o r spectally d e s i g n e d to u s e the s y n c h r o t r o n radJanon e m i t t e d by the A c e storage ring m the soft X-ray reglon ~'30-200 eV) has been m opcratton for m o r e t h a n two years on the four degree b e a m h n e o f L U R E Th~s ~s o n e o f t h e m o s t versanle a p p a r a t u s , s m c c tt can work as a m o n o c h r o m a t o r or as a s p e c t r o m e t e r , wqth one or t ~ o g r a t i n g s , w~th a mobtle or a fixed exit sht l h e u m q u e features o f th~s i n s t r u m e n t ~tll be d t s c u s s e d , i n c l u d i n g the poss~bdlt2, o1 varying c o n t i n u o u s l y , m an e x t e n d e d a n g u l a r range, the incidence angle on the gratings, as well as the angle betv.een t h e e n t r a n c e sht and t h e cx~t sht Actual p e r l o r m a n c e figures ~,iIl be g~ven, including absolute o u t p u t p h o t o n flux, order sorting a n d rate of scattered h g h t A large vaneD' of c x p c r m a e n t s have already, m a d e u s e of th~s m o n o c h r o m a t o r tn vartous fields of atomic ph}s~cs s o m e e x a m p l e s o f n c ~ results recently obtained v, dl bc g~vcn

1. Introduction The use of grating systems under grazing incidence ts presently the most favorable opttcal solunon to obtain monochromanc radiatton m the 20-300 A wavelength range. Recent developments of transmtss~on granngs ~-2) seem to be prom~smg for the future, but several problems raised by the destgn and construcnon of an apparatus using such gratings as well as production of statable gratings have to be solved before ~t can be effecnvely used as a monochromator routinely operated wtth synchrotron radtatton 3) For cxpenmantahsts the most ~mportant requtrements are high flux, htgh resolunon and radmnon free of order overlapping More precisely, some expertments need a high-flux of photons m a given bandpass w~thout strict reqmrements about resolunon, order overlapping or scattered light, while some others requtre chiefly h~gh resolunon and monochromattztty For some experiments another ~mportant point exists, the knowledge about the polanzatton state of the monochromanzed radtanon Of interest for all experiments ~s the wavelength range where these requwements must be sattsfied Usually the design of a monochromator tries to fulfil mainly one of these reqmrements and to compromtse w~th the other

The use of a toro~dal gratmg w~th s~mple rotatton ~-6) or the combmanon of a plane grating with a toroldal mirror 7) have recently been developed to solve mainly the asngmansm problem encountered wtth spherical gratings In the grazing incidence spectromonochromator firstly designed for LURE, wc have tried to sansfy alternanvely the different needs with a versatile system based on a classical Rowland circle m o u n n n g The monochromator can be used w~th one or two gratings, m the latter case, ~t can provide flux of photons completely frcc of higher mterfc."encc orders tn w~de energy ranges The incidence angle on each grating can bc vaned continuously and the gratings can be easily changed and readjusted properly w~th v~s~ble hght The extt sht can be kept fixed m posmon whde scanmng m energy, makmg a fixed d~rectton of the exit beam possible Thc instrument can also work as a spectrograph w~th a photographic chamber or a photoelectric detector array Th~s instrument can be used for synchrotron radmnon or w~th plasma laser sources A detatlcd descnpnon has been given elsewhere 8) In this paper we wdl descrtbe how the monochromator was adapted to the synchrotron radmnon emitted by the A c e storage rmg on the soft X-ray (4 =) beam hne of LURE and to present the performances actually achteved m various experiments d u n n g these last two years

ones • Research s u p p o r t e d by the C e n t r e N a n o n a l de la R e c h e r c h e Sc~ennfiquc a n d the Untverstt6 P a n s Sud * Present a d d r e s s UntvcrsJty of BntJsch C o l u m b i a , V a n c o u ver B C , C a n a d a V 6 T 1W t Fakultat fur P h y s l k dcr U n t v e r s l t a t Frclburg, D-7800-Frctburg t Brelsgau, G e r m a n y E R Spectroscopic A t o m l q u e et l o m q u e du C N R S , U m verslt6 P a n s Sud, Orsay

2. Design considerations Ftg 1 mdtcates the system of reflecting opttcs used m the LURE 4° beam hne and in the monochrornator The hght coming from the storage 11

VUV

MONOCHR()MATORS

AND

SPECIROMI..TERS

86

e

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Iqg 1 Optical layout of the synchrotron radiation beam hnc and monochromator M - c>l,ndncal m~rror, P.k.l-spherical prefocusmg mtrror. S 1 and S2 = e n t r a n c e and exit shts, G grating

ring ~s reflected on a mirror M, u n d e r a grazing incidence angle of 4 ~ onto the m o n o c h r o m a t o r A spherical prefocusmg m~rror PM focuses th~s light on the entrance sht S~ After dtffractton on the grating G, the m o n o c h r o m a t i c light passes the exit sht S~, whzch can be a rnobde exit slit or a fixed exit sht, m this latter use, S~ is a mirror-knifeedge c o m b m a t m n o f the C o d h n g type'S), as mdxcated m the figure The mirror M was originally a plane mlrror~°), whtch was later replaced by a pohshed glass cylindrical mirror overcoated w~th platinum It locus the s y n c h r o t r o n radmtmn m the vertical plane T h e radius of c u r v a t u r e of this marror ~s 5 0 c m as calculated, m order to place the focusing point on the entrance sht S~, according to the equation 1 + 1 - 2 sm fl

p

q

R

where p (6 8 m) is the d~stance between M and the light source (electron b u n c h m the storage ring), q (7 6 m) is the d~stance between M and S~ and /; ~s the grazing incidence angle M ~s kept fixed and the incidence angle on it can be adjusted u n d e r v a c u u m from the outside w n h three m i c r o m e t e r s screws This m~rror M ~s 30 cm long and 20 m m high and thus intercepts the full horizontal and vertical d~vergences accepted by the beam hne ( 3 x 3 mrad 2) T h e s~ze of the electron beam m the ring Is neghg~ble (~t is typically 1 ram, but can be eas,ly adjusted from 0.3 to 2 m m m d m m e t e r ) ; ,n add~tton this beam ~s very steady (within 2 0 / , m ) u n d e r usual operating condtttons of the ring T h e s~ze o f the p h o t o n beam reflected by M has been m e a s u r e d to be 2 m m high and 40 m m w~de at the place of the prefocusing m,rror PM. Th~s mtrror PM is a 30 x 30 m m 2 spherical m~rror with a radms o f curvature o f 1200 ram, over-

ct al

coated with platinum It focuss part o f the mooremg p h o t o n beam on the entrance slit Sl and allows to c o n l m u o u s l y d l u m m a t e the grating w~th divergent hght where the central ray (from the mtddle of PM to the mtddle of G) can have angles (0 of grazmg incidence ranging from 5 ° to 20': Changes tn ~p follow a change of the angle :~ o f grazing incidence on the mirror PM [~-- ~ 0 - ( ~ - 4 : ) / 2 ] T h e center of PM, S~ and G must lay on a straight hne which is achmved by a ng~d frame The different angles ~ (and :z) are provided and th~s relation is fulfilled by moving the mirror PM and the gratmg G on that straight line whereby PM and S1 are connected by two arms (300 m m long) whose center describes a circle around the fixed entrance sht S~ To cover the ~ a v e l e n g t h range o f interest, the mtrror PM is m o v e d on its constrained path by the displacement of the grating G along the Rowland circle, which is achieved with a stepping m o t o r Stmple geometrical constderattons show that, for a varmtlon zl~0 of the grazing mczdence angle on G, one has a variation ,4¢/2 o f the grazing mctdence angle ~. on PM and a rotation z,l~0/4 of the mtrror at tile exit slit S2 [to keep fixed the direction of the exit beam for a fixed p o s m o n of S, ~)] Covering o f the wavelength range reqmres also m o v i n g the mirror PM m the photon beam reflected from M T h o u g h this photon beam is 40 m m wide, the mirror PM accepts only a small percentage o f the horizontally divergent beam (a few t e n t h s o f a m~lhradmn), due to the small values of :z. A toro~dal m~rror will soon replace the cyhndncal mirror M In this new configuration the whole 3 mrad horizontally d w e r g e n t beam wdl be focussed on PM, which should increase the pho-

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N,< O Fig 2 Basic principle of "the lwo-gralmg system

LURE

GRAZING

INCII)ENCE

SPEC'IROMONOCIIROMAqOR

ton llux available by nearly one order of magmtude However, continuous scanning of the wavelength range will not be possible m the one grating mode wtthout readjusting the m~rror M to keep the reflected beam mc~dcnt on the prefocusmg mtrror PM The monochromator can work also in the twogratmg mode In th~s configuratton, h~gher orders are suppressed, the scattered hght ts decreased and the resolution ts improved, at the cost of a reduced output hght flux In th~s mode, the prefocusmg mwror ~s kept fixed and the scanning of the wavelength range ~s obtained by d~splacmg the second grating F~g 2 shows, m more details, the pnnc~ple of the apparatus ~) The two gratings G~ and G 2 are mountcd on the same Rowland c~rcle of radtus R. A mtrror m wtth a radms of curvature =2R ~s mounted on the Rowland ctrclc betwecn the two gratings m such a way that they arc set symmetrically Th~s mtermedmte m~rror m selects from the spectrum diffracted by G~ a small spectral band, the width of which ~s determined by the w~dth x of the m~rror The m~rror acts to make the various rays convergent onto G~ The midpoint of G, ~s the opncal ~mage, through the mtrfor, of the midpoint of G~ T hen G~ diffracts the beam reflected by the m~rror m and focuses monochromatic rays all along the Rowland c~rcle The

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position of the mtdpomt of the mirror determines the central wavelength of the band reflected and later diffracted by G2 Posmve and negative ord~ers must be used alternauvely in thts mounting m v~ew to obtain d~spers~on Usually m ~s set m the first negauve order of G~ and G2 ~s used m the first posn~ve order This gives the band ( - 1, + 1) Since G2 recewes only a narrow spectral band, after the second grating the bands ( - n , ~-1) and ( - 1, +n), w~th n > 1, can be completely separated and the band ( n, - n ) c a n be suppressed considerably by using entrance angles on the opucal elements PM, G~, m and G2 that g~ve low reflecnv~y lbr higher order radmUon F~g 3 gtves a schematic v~ew of the system Each opttcal element (sht, grating, m~rror) is connected independently on a horizontal wheel and the versatthty of the instrument comes from the poss~bthty to easily build up the working configuration that Js wished one or two gratings, mobile or fixed exit sht AdJustment of the components on the Rowland c~rcle ~s achieved with the help of a telescope placed at the ccntcr of the apparatus Th~s telescope can be oriented towards the d~fl'crent opucal elements. An attached optical encoder wtth a resolunon of 0 001:: allows one to precisely measure the angular d~stance between the elements The optical components arc connected together wtth specml welded bellows that ensures to move all optical components under vacuum In th~s way, h~gh vacuum has to be maintained only w~thm these bellows and adjustments can be made w~thout braking the vacuum. Another pecuhanty of the instrument ~s the option to freely choose the angle between the entrance sht S~ and the extt sht $2, according to the type of experiment and the wavelength range of interest It ~s also possible to continuously vary thc width of the shts under vacuum, from a few m~crometers to one mflhmeter, thts characteristic has been found to be very advantageous to adapt the operating conditions to the actual photon flux F~g 4 shows a photographic overview of the whole apparatus, mounted on a gramt slab through three m~crometer jacks allowing adjustment of the hetght of the focusing circle This slab ~s supphed w~th three shppcrs set w~th a~r cushions offering an easy adj u s t m e n t of the m onochrom at or m the synchrotron radtauon beam Pressure ms,de the bellows ~s kept m the 10 6 tort region However the instrument can been used to dehver monochromattc flux of pho-

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Ftg 3 S c h e m a t t c d r a ~ , m g o f t h e m o n o c h r o r n a t o r 1 - horizontal w h e e l s , 2 . o p t t c a l b l o c k , 3 - b l o c k support, 4--support ~ , h e e l . 5 = g r a m t e s l a b , 6 - m i c r o m e t e r j a c k , 7 = s h p p e r set v, n h atr c u s h t o n s , 8 = g r a m t e lbundanon base. 9 stepping motor, 10= central telescope, I I = desk. 12- optical encoder

11

VUV

MONOCIIROMATORS

AND

SPECTROMETERS

88

P

et al

DHEZ

tons lbr experiments with gaseous targets without window, because of the safety device ~'~) specially designed for the VUV beam hnes of LURE In this case, pressure in the monochromator can raise up to 10 ~ tort without effectmg neither the beam hne vacuum (10 : to 10 -~ tort), nor the ultrahigh vacuum lnsLde the nng chamber (10 retort) llowever, conccrmng the reflectzvtty of the optical elements, a better vacuum would be advantageous. This point will be discussed later 3. I)erformances and operating conditions The monochromator has been operated for two years w~th Bausch and Lomb platinum coated 1 m concave rcphca gratings (600. 1200 and 2400 h n e s / m m ) and w~th Jobm and Yvon holographic gratings (1200 h n e s / m m ) The relative intensity observed with these gratings was measured, as a function of photon energy

w~th a photomultipher sensitized wxth sodium sal~cylate, or by counting the number of photons at the exit sht of the monochromator wnh a photoelectric detector Fig. 5 gwes an example of such spectra for several values of the grazing incidence angle, taken wroth the mobde exit sht and a 1200 h n e s / m m grating Fig. 6 shows another case' it displays the relative efficiency of a 2400 h n e s / m m gratmg where the curves have been normalized to the ACO spectrum, the black dots indicate, for each curve, the blaze wavelength calculated by usmg the blaze angle indicated on the gratmg. Figs 5 and 6 demonstrate clearly the effect of the cut-off angle on the diffracted intensity They show also that the calculated blaze wavelength does not comcnde with the maximum of the curve and, consequently, how advantageous it is to be able to choose freely the incidence angle Tests of other gratings have not always shown such large differences between the relauve positions of the calculated blaze wavelengths and the observed maxima, which seem to indicate that the blaze angles indicated o n the gratings are not always rehable These results demonstrate the dfff'tculttes to design an efficient system, without prehmmary

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Ftg 5 Light output as a functnon of the pholon wavelength at the exit shl recorded v,~lh a photomultnphcr m the one-grating setting ~tth the rnobde exnt shl ACO was operated at 350 MeV The dotted curve g~,es, on an arb,trary scale, the shape of the spectral distribution ernxtted by ACO

LURE G R A Z I N G

INCIDENCE

SPEC1ROMONOCHROMATOR

89

TABLE 1 Output photon flux at the ex,t slat of the monocbromator m the one grating (576 hnes/mm)-fixed cx2t sht mode-with ACO operated at 536 McV and 100 mA The third column gives the number of photons emitted m a l'/,, band v,Jdth (column 4) and m the horizontal acceptance of the prefocusmg m~rror ~column 5) The verucal acceptance of the system ~s 3 mrad :z ts the grazing m o d e n c e angle on the prefocusmg m=rror (column 6), • and qb' are respccuvely the grazing m o d e n c e and exit angles (columns 7 and 8) Column 9 g~ves the estimated number n of photons at the ex.t sht, rcflcctp.'mes of the optical surfaces a r e taken from ref 13 Numbers have been calculated for two dlflerent values of the angle 0 between the entrance sht and the ex=t sht ,:.

E

(A)

(eV)

N (photons/see)

dk. (eV)

1lonzontal acceptance (mrad)

~ (degree)

¢ (degree)

~' (degree)

n (photons/see)

8 280 8 133 7950 7767 7400 7033

12 560 12 266 11900 11 533 10799 10065

13 440 13 734 14101 14468 15201 15936

1 9 x l0 Ifj 4 8 × 10 I° 4 3 x 1 0 l° 39×101° 31×101° 26x101°

5 645 5407 5111 4814 4 221

7 289 6814 6221 5628 4 441

8 712 9 186 9779 10372 11 559

1 4× lOl° 32x10 m 28×101° 2 4 × 1 0 l° 1 8 × 10 l°

0 = 52 ~ 60 100 150 200 300 400

206 7 123 9 8265 61 98 4133 3100

8 70× 10 ll 1 51 × 1012 202×1012 2 4 8 × 1012 2 9 6 x 1 0 ]2 327×1012

2 07 1 2,.1 083 062 041 031

0 30 0 30 029 029 027 026 O= 32:

60 100 150 200 300

206 7 1239 8265 6198 41 33

6 09 × 10 ]l 1 01×1012 132×1012 154×1012 1 75 × 10 ]2

2 07 1 24 083 062 0 41

0 21 020 019 018 0 16

tests of the gratings, when the angle of mctdencc ts fixed The absolute output flux from the monochromator expected with the cylindrical m~rror M has been calculated m the one grating-fixed exit sht mode of operation (four reflecttons) Table 1 lists the relevant parameters at six d-fferent wavelengths for two dtfferent values of the angle 69 between the entrance sht and the exit sht Numbers have been calculated for a constant spectral band pass of 1% Reflecttvity of optical surfaces have been taken from the results of Luktrskt 13) for gold, due to the lack of data for platinum. An overall gratmg efficiency of 5% has been assumed, whmh seems reasonable, taking mid account the value measured by Speer at 44,/k 14) Calculations have been made for the normal operating condmons of the storage rmg ACO (536 MeV, 100 mA) The actual photon flux observed m recent photoemxssmn experiments on gaseous species [He is), Xe ~)] have been found to be in agreement with the esumated values (last column of table 1) between 100 and 300 .A ~7).

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F=g 6 Efficmncy of a 2400 h n c s / m m platinum-coated grat,ng as a function of the wavelength The actual shape of the ACO spectrum has been used to normahzc the maxima of the curves to the same value (Bcrland et al, ref 27)

II

VUV M O N O C H R O M A T O R S

AND S P E C T R O M E T E R S

90

I'

DHEZ

F~g 7 shows the photoelectron spectrum of He ~omzed with 93 eV photons ~s) This spectrum was obtained with all ophcal surfaces freshly coated w~th platinum The integrated flux of photons esttmated from this spectrum is about 4 × 10 ~° photons/see eV, which means a flux of photons even higher with 130 mA Jn the ring at the beginning of the expertment It should be noted however that the deposmon of hydrocarbon layers on the opttcal surfaces drasucally decreases the intensity available at the exit slit of the monochromator After three to four hours, the photon flux reflected by the prefocusmg mirror is lowered by a factor 2 to 3. The same reducuon m intensity ~s observed for the grating after a few tens of hours The effect o f hydrocarbon layers cracked on the opucal surfaces by the UV radiation is also to shift the position of the maximum of the efficiency curve However the easy change and adjustment of the optical surfaces (the whole operatton, including BINDING ENERGY (eV) 68 66 6L 62 28 26 2t. 22 I

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26 28 30 66 68 70 72 KINETIC ENERGY (eV) Fig 7 Photoelectron s p e c t r u m lollowmg ~omzatton of He w~th 93 c\. p h o t o n s "I hc m o n o c h r u r n a t o r was operated in the onegratmg-l'ixed extt sht mode v,Jth a 576 h n e s / r n m platinum coated grating The grazing incidence angle was 10 593: The restdual p o s m v c ion can be left m the g r o u n d state ( H e - I s) or m the n = 2 excited state (tle~ 2 s or 2 p ) ( W ' u d l e u r m e r et al , rcf 15)

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braking of the vacuum and repumpmg of the system takes half an hour for the prefocusmg mirror and two hours for the grating) allow to keep the apparatus working most of the time m the optim u m condmons The presence of higher orders diffracted by the grating at the exit slit of the monochromator is one of the major problems raised by the use of synchrotron radiation Higher orders are troublesome not only for photo-absorpuon experiments, m which case they can introduce large errors m absolute cross-sect~on measurements, but also for photoemlsslon studies when photoelectron lines produced by iomzat~on of tuner shells wtth the second order radlauon can be several times more intense than photoelectron hnes produced by ionizauon of outer shells with first order radlauon, like m the case of xenon iT) F~g 8 shows how the poss~blhty to vary the grazing incidence angle helps to suppress higher orders in the left part, the photoelectron spectrum produced by ~omzauon of hehum in the ls shell with photons of nominally 50 5 eV energy is displayed, m addition a hne due to lonlzauon of He w~th the second order radmhon of 101 eV energy ]s observed with a high intensity Taking into account all instrumental corrections the relattve amount of second order radiation, compared to the first order radiation, is about 80%J-) When the grazing incidence angle Js increased (right part of the figure), th~s proporuon drops to about 5% at a photon energy of 39 3 eV where the second order radlaUon (78 6 eV) was expected to be even more intense than in the preceding case tlowever a few percent of second order contnbuUon is still too high m some cases and figs 5 and 6 confirm that it ts difficult to suppress completely the radmuon diffracted in the second order when the wavelength becomes larger than about 200 A. Another difficulty comes also from the h~gh level of scattered light, since the grating receives a large spectral band wtth high intensity in the short wavelength range T hen the use of the two grating system overcomes these two kinds of problems Fxg 9 shows an example of spectrum obtained m the two gratmg-mobde exit sht operating mode w~th a photoelecmc detector From the left to the right of the figure, one founds first the ( - 1 , 0 ) band that corresponds to the spectral band reflected by G, in the first negative order and by G2 m the zero order. The zero order on G2 ~s not sharp, hke m the one gratmg case, because all wave-

LURE G R A Z I N G

He 1~

INCIDENCE

SPEC]ROMONOCHROMATOR

91

[ACO 536MeV-B0rnA] h~Vl=50 5 eV Bond pass=0395 eV

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Fig 8 Photoelectron spectra fol)ov, mg ionization of lid. with first order .50 5 eV photons and second order lt)] cV photons respcctlvcly (left part) and v, lth first order 39 3 eV photon~, and second order 78 6 e\" photons (right part) Only the mare peak ( H e n = 1) Is shown ~p and (#' respecuvely arc the grazmng incidence and exit angles measured with reference to the grat|ng surface The grating was a 576 hnes m m grating overcoated with platinum (Adam e t a l . ref 16)

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Fig 9 b g h t output as a lunct~on or the wavelength at the exit sht of the monochromator m the two-grating setting with the mobile cx,t sht S,gmficat~on of the ( - n . - n ' ) bands ~s g~vcn m the text 0 ~s the angle between the entrance and the d×lt sht

ments and much lower tn mtenstty Comparison of the ( - l , 0) and ( - 1 . + l) bands shows clearly the additional dispersion introduced by d~ffractton from G2 The last band to the right ts the weak ( - 1, +2) band This figure demonstrates how the higher order bands are spatially resolved from the

lengths m c o m m g on G 2 c o m e s from different po~nts of the intermediate mirror m set along the

Rowland circle For this zero order, G2 does not bring additional dispersion. The next band is the ( - 1 , - 4 - 2 ) band It ts completely separated from the ( - 1 , + 1) band that ]s used for the measure11

VUV M O N O C H R O M A T O R S

AND SPECTROMETERS

92

P

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et al

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50

!

I

!

1

100

!

I

0

0

150 E(eV)

Fig 10 VarlaHon o f the ~.bsorptlon co~rllClents of t h o r i u m and u r a m u m

mare ( - 1 , + l) band The only contnbuuon of higher orders could come from the ( - 2 , + 2) band that would coincide exactly with the ( - 1 , +1) band However, from the low intensity of the (-2, +1)and (-1,-2) bands compared to the ( - 1, 0) and ( - 1, + 1) bands, it .s possible to conclude that the contnbuuon of the ( - 2 , +2) band ts neghg[ble. In addmon the rate of scattered hght has been reduced to 1 part over 10~ Accurate determinations of the relative mtensmes sent by G 2 m the 0, + 1 and + 2 orders can be easily obtained from such spectra Spectra hke the one shown tn fig 9 could also be obtained m the fixed exit sht mode Th~s mode has actually been used for the measurement of the double photoiomzauon cross secuon m rare gases at low photon energies ~8) However the photon flux available m the two grating system ,s about 30 umes lower than m the one grating mode, which explmns that th.s operating mode has been used mostly to measure photoabsorpuon cross secuons. Fig 10 shows, for example, the variation of such cross secuons, as a funcuon of photon energy, m the case of thorium and uramum ~9) W.th the two grating system, the values of the maxima have been found to be 25% higher than m some preceding measurements earned out w~th the one grating system. The ultimate resolution of the monochromator should be ~ 0 0 2 , ~ at 100A, but has not yet been determined. However, m a stmflar apparatus built for plasma laser experiments, emission hnes m a

5O

I

10o

I

15o

E.(= v)

as a l u n c H o n o r p h o t o n e n e r g y (Cuk=er et a l , ref

19)

Mg plasma separated by 0 04 ~ have been eas.ly resolved at 170 ,/~ 20) This monochromator has been extenswely used for photoabsorption~" 2~.22), photolon~), photoelectron '5'16'232a) and Auger 25) spectroscopy experiments, producing new s~gmficant results m the study of single and muuple photo~omzahon and Auger processes, hke the first observation of the post colhslon effect by photon ~mpact 25) or the study of correlauon satelhtes m photoelectron spectra at low photon energy'-~.'4). More recently ~t has been used for the first experiment of photoelectron spectrometry w.th metalhc vapour in the grazing incidence region (30-105 eV photon energy)26) As an example, fig 11 shows a typical spec-

_

1200

_

_

m

1

hr.,(28 e7

'

15

r

fve,, I''-

20

25

20 [ev~ ~5

l"Jg I] Photoelectron and Auger spectra of momJc lead following Jomzatlon by 42 8 c V photons The m o n o c h r o m a t o r was operated with a 0 2 eV bandpuss and a 5"76 h n e s / m m grating (gandner et a l , ref 26)

LURE G R A Z I N G

INCIDENCE

93

SPEC'IROMONOCttROMATOR

"~) D Lepere, Nouv Revue Opaque 3 (1975) 173 -~) Y Petrol'r, P Thlry, R Pmchaux and D Lepere, 5th, Int C o n f on I'(l(ultt?! Lt/II(IVIOIPt radiation p/t.i.st(s, Montpelher, France, 1977, Extended Abstracts, Vol 111, p 70 6) C Depautex, P Th~ry, R Pmchaux, Y Petroff, D Lepcre, G Passereau and J Flamand. th~s issue, p 101 ~) W Eberhardt, G Kalkoffen and C Kunz, th~slssue, p 81 ~) P Jacglc, P Dhez and F Wuflleumler, Rev ScJ lnstr 48 (1977) 978 '~) K Codling and P Mitchell, J Phys E3 (1970) 685 10) p Jaegl6, P Dhez and F Wudleummr, lacuum uhtavtolet Jadumonpto,~cs(eds E E Koch, R Ilaensel and C Kunz, Pergamon-Vteweg. Braunschwe~g, Ilamburg, 1974) p 788 ]l) p Jaeglc:, J Phys (Pans)24 (1963)179, C R Acad Sci (Paris) 259 (1964) 533, ibtd 259 (1964) 4556 I,~) p M Guyon, C Depautex and G Morel. Rev Sc~ Instr 47 (1976) 1347 i3) A P LukJrskL E P Savmovand Y F Shepelev, Opt Spectr. 15 (1963) 290 I.~) R J Speer. J Spectr Soc 23 (1974)53 If,) F Wudleum~er, M Y Adam. N Sandner, V Schm~dt and W Mehlhorn, to be pubhshed, see also LURE Annual Report, Orsay (1977) 16) M Y Adam, F Wuflleumler, P Dhez, iX Sandner, V Schmldt, W Mehlhorn and J P Dcsclaux, to be pubhshed, see also LURE Annual Report, Orsay (1977) ]~) M Y Adam, F Wmlleummr, N Sandner and V Schmldt, to bc pubhshed, see also LURE Annual Report. Orsay (1977) ]~) V Schmldt, N Sandner. H Kuntzemuller. P Dhez. F Wudleumler and E Kallne, Phys Rev A13 (1976) 1748 ]9) M Cukter. P Dhez. B Gaulhc. P Jaegle, C Wehenkel and F Comber Farnoux, 5th int Conf on I a c u . m ultraviolet radiation phystcs, Montpelher, France. 1977, Vol 1. no 9 20) A Carillon et al, unpubhshcd 21) M Cukler, P Dhez and P Jacgl6, Abstract book 2nd Int Conf on Inner ~hell tomzatmn phenomena, Fretburg, Germany, 1976, p 89 22) M Cukler, P Dhez and P Jaegle, Int Conf on Phys.s ol X-~a.v spe~oa. Extended Abstracts, GaJthesburg. Maryland. USA, 1976, p 321 23) F Wufllcumter, M Y Adam. P Dhez, N Sandner, V Schmldt and W Mehlhorn, Plays Rev AI6 (1977) 646 24) M Y Adam, F WuflleumJer, N Sandner, V Schmldt and G Wendm, J Phys (Pans) 39 (1978) 129 25) V Schmldt. N Sandncr, W Mclhorn, M Y Adam and F Wudleummr, Phys Rev Lett 38 (1977) 63 26) N Sandner, S Krummacher, V Schmtdt, W Mehlhorn, F Wmlleummr and M Y Adam, 5th Int Conf on Va~tntm ultraviolet ladtatton physics. Montpclher, France. 1977. Extended Abstracts, 1-105 27) M Bcrland, P Dhez, P Jaegle and J Flamand, 5th Int Conf on Va(uum ttlttavtolet tadtanotl physt(s, Montpelher, France, 1977, Extended Abstracts, Vol I11, no 15, p 26-28

trum It Is produced by photoionizatton of atomic lead by 42 8 e V photons with a monochromator bandpass of about 0.2 eV 26) in one grating (576

h n e s / m m ) mode 4. Conclusion We have described the design cons]derauons and actual performances of the grazing inctdence spectrochromator installed on the 4 ° beam line of LURE at the ACO storage ring The instrument has been in operation for more than two years and provides photon beams with the expected characteristics The adjustment is relatively simple Its use has made possible a large number of new experiments in atomic physics The major problem still to be solved ~s the contmuous deposition of hydrocarbon layer on the optical surfaces, causing a drastic reduction of the photon flux available, partly compensated by the easy change of the optical components. Further modifications should mclude a new pumping system to improve the vacuum in the optical component blocks and the possibility to evaporate inside clean layers of fresh gold on the prefocuslng mirror to restore permanently its efficiency The authors express their thanks to the members of the Laborato~re de rAccelerateur Lln6alre and m pamcular to P Marm, for their constant help in operating the storage ring They would hke also to thank M. Y. Adam, M Cukter and N Sandner whose active participation m the various experiments mentioned in th~s paper have greatly contributed to a better knowledge of the possibilities of the instrument and of the best operating conditions

References I) j H l)tjkstra and L J Lantwaard, Opt Commun 15 (1975) 300 2) H W Schnopper, L P Van Speybrocck, J P Delvaflle, A Epstein, E Kallne, R Z Bachrach. J H Dukstra and L J Lantwaard, Appl Opt 16 (1977)1088 3) E Kallne, II W Schnopper, J P Delvatlle, L P Van Speybroeck and R Z Bachraeh, this issue, p 103

11

VUV

MONOCHROMATORS

AND

SPECTROME]ERS