SPECTROSCOPY IN THE GRAZING-INCIDENCE SPECTRAL REGION

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SPECTROSCOPY IN THE GRAZING-INCIDENCE SPECTRAL REGION

G. Sørensen

To cite this version:

G. Sørensen. SPECTROSCOPY IN THE GRAZING-INCIDENCE SPECTRAL REGION. Journal

de Physique Colloques, 1979, 40 (C1), pp.C1-157-C1-160. �10.1051/jphyscol:1979129�. �jpa-00218409�

JOURNAL

DE PHYSIQUE Colloque Cl , supplt?ment au n " 2, Tome 40, fgvrier 1979, page C1-157

SPECTROSCOPY IN THE GRAZING-INCIDENCE SPECTRAL REGION

Institute of Physics, University of Aarhus, DK-8000 Aarhus C, Denmark R6sum6. Dans la r6gion spectrale 'grazing incidence', la lumigre Provenant de la zone de la collision a 6t6 focalis4e sur la fente d'un spectrometre

McPherson ~ o d k l e 247 par un mirroir ellipsoIda1. Une r6solution spatiale d'en- viron 100 a 6t6 obtenue. Le spectromgtre est control6 entigrement par un calculateurqui enregistre les spectres. Des exemples sur les collisions ion-

gaz et ion-feuille sont donn6s.

Abstract. Mirrorfocuss'ingof the light emitted in the grazing-incidence spec- tral region during ion-atom collisions has proven to be an improvement compared with earlier measurements, where the collision region was placed as close as possible to the entrance slit of the spectrometer. The present contribution describes the use of a McPherson Model-247 grazing-incidence spectrometer for the study of ion-atom collisions at the 600-keV heavy-ion accelerator at Aarhus University. An ellipsoidal, gold-coated mirror projects the light from the col- lision zone onto the spectrometer entrance slit, enabling a spatial resolution of $100 pm in the light source. A simple collision chamber for ion-gas collisions is described. The operation of the spectrometer is steered by a computer; hence data can be accumulated in both scanning directions. The principle of this auto- matic recording system, which is especially suited for the study of faint light sources, is discussed. Examples of ion-gas and beam-foil excitations will be shown, and general problems of spectroscopy in this spectral region will be dis- cussed.

INTRODUCTION

Beam-foil as well as beam-gasexcitation in the vacuum ultraviolet spectralregion has been studied extensively. Among the problems encountered here is refocussing of the collision region onto the entrance slit of the monochromator, and various sol- utions have been suggested in the literat- ure. Thus ~arettel) has used a system with an auxiliary slit, and Buchet et a1. 2, have applied a toroidal mirror in the 300-1700 A

r l O N BEAM rMIRROR

L ~ ~ ~ ~ l ~ l O N ZONE -

ENTRANCE

SIT-\

spectral region with a normal incidence

Fig. 1. The experimental arrangement shown spectrometer. The present contribution de- schematically.

scribes beam-gas and beam-foil experiments performed at the 600-keV heavy-ion acceler- ator at the University of Aarhus, where a McPherson Model-247, 2.2-m grazing-incidence monochromator was used in the 75-1 500 A spectral region.

EXPERIMENTAL ARRANGEMENT

The experimental arrangement is shown schematically in Fig. 1 . A gold-coated, el- lipsoidal mirror is placed on a mount in a vacuum chamber so translation (x,y,z) and

rotation (+3O) around (x,y, z) is possible during the alignment procedure. The colli- sion zone and the spectrometer entrance slit are both in the focal points of an el- lipse, to which the focussing mirror is mounted tangentally. The distance of the mirror to the two focal points is 0.5 and

1.0 m, respectively, and in this arrange- ment, a demagnification of (2: 1 ) is achieved. The alignment of the experimen- tal setup is performed by means of a lamp

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979129

(21-158

DE

at the exit slit of the monochromator in the zero-order position. In this way, the position of the collision zone is de- termined and the target arrangement prop- erly installed. Thus, a good alignment will ensure a spatial resolution of approximately 100 iJm,which is of ,importance, especkally whenshort lifetimes have to be measured.

For ion-gas excitations in the single collision region, the weak light source requires an automatic 'scanning 'system for several scans to be accumulated. This was accomplished by installing a shaft encoder

(LOGIK, Denmark) on the drive shaft, connect- ing the gear box to the grating assembly.

The encoder, which yields 50 pulses per shaft revolution, makes it possible to per- form the scanning by means of a computer

(NORD) and also enables an accumulation of data for several scans when scanning in both directions. To overcome the clearance when the scanning direction is reversed, the scanning range is slightly larger than the spectral range in which the computer accumulates data. All relevant parameters are included in the computer programme; on the display, built-in markers facilitate wavelength registration and area determin- ation as well as other computations.

LIGHT-EMISSION SPECTRA

Although the automatic scanning system described above is particularly well suited for studies of single ion-gas collisions, it may also be useful in beam-foil studies with extremely small slits. The collision cell used in the ion-gas studies is shown schematically in Fig. 2. While the position of the observation window is fixed, it is - ..

PRINCIPLE OF GAS CELL FOR OPTICAL SPECTROSCOPY ACUUM FLANGE

G4S INLET v' ,MOVABLE GAS CELL ,DIAFRAGM

1 'OBSERVATION REGION ION &AM

Fig. 2. A simple collision chamber for ion- gas excitations.

possible to change the position of the inner tube and, consequently, the entrance hole of the gaseous cell. This is of im- portance for collisions outside the sinqle- collision region, and in this case, a cryo- genic pump surrounding the cell facilitates the differential pumping of the gas leak- ing out via the cell-entrance hole and the observation window. An example of the re- solution with fairly small slits is shown

CHPNNEL NR

Fig. 3..A partialemission spectrum showing 400-keV ~ e + + ~ r , where the transitions from Is2,1s,, in Ne I at 435.8 A and 743.7 A have been resolved from blending Ar I1 lines.

Ar I1 excited in collisions of neon ions with argon are 1.2 A apart and well re- solved. In another experiment, a 400-keV lithium-ion beam was beam-foil excited, and the spectrometer slits varied between 30 and 480 um.The spectra recorded with varying slit sizes are shown in Fig. 4, where the importance of being able to re- cord spectra with small slits is clearly demonstrated; hence the automatic scanning system seems to be fabourable.

EXPERIMENTAL RESULTS

An example of single-collision stud- ies has been reported elsewhere in these procedings3). As examples of beam-foil studies, lifetimes have been measured in two isoelectronic sequences. The radiative lifetime for the 2s22$

- ^{2s22p3s 'P}

in the carbon sequence has been measured

with the present experimental arrangement

for N I1 and 0 111. In N 11, Livingston et

a1. 4, have measured the 747.0 11 transition

Slits *a

480 p

240 p g

I ; \ ^!

u A - A - A .

S: 1 6 C 3 2 0 (I80 6 a i 8 L O 9 6 0 312 128 CHIINhCL V R

L ., .'. ^{,... ,,}

F i g . 4. The s p e c t r a l r e s o l u t i o n shown as a f u n c t i o n of s l i t s i z e when a 400-keV ~ i + beam was f o i l - e x c i t e d . Upper c u r v e 480 p s l i t s ; l o w e r c u r v e 30 p s l i t s .

w h i c h t h e y s o c c e e d e d i n r e s o l v i n g from t h e n e i g h b o u r i n g l i n e s o f 745.8 A a n d 748.4 A.

F o r t h e 747.0

t r a n s i t i o n , t h e y r e p o r t e d a l i f e t i m e o f 0.22k0.03 n s . I n t h e p r e s e n t s t u d y , a somewhat l o n g e r l i f e t i m e of

0.35*0.10 n s was m e a s u r e d , a n d by u s i n g t h e t h e o r e t i c a l b r a n c h i n g r a t i o o f Warner a n d

~ i r k ~ a t r i c k ~ ! a n f v a l u e o f 0.14 i s e v a l - u a t e d . I n t h e p r e s e n t s t u d y , t h e l i f e t i m e o f t h e 394 A t r a n s i t i o n i n 0 111 i n t h e s a m e s e q u e n c e was m e a s u r e d t o b e 0 . 3 2 t 0 . 0 8 n s , i n good a g r e e m e n t w i t h t h e r e s u l t o f P i n n i n g t o n e t a1. 6 , . I n f i g . 5 i s shown .< .. f

I 1

v a l u e s c o r r e c t e d f o r b r a n c h i n g t o g e t h e r w i t h r e f e r e n c e s from t h e c o m p i l a t i o n of Smith a n d wiese7), and a s shown, some d i s - c r e p a n c y i s p r e s e n t . The 2 s 2 2 p 'P -

2s2p2 2 P t r a n s i t i o n a t 904 A i n C I1 be- l o n g i n g t o t h e B 1 s e q u e n c e a l s o e x h i b i t s some d i s c r e p a n c y from t h e s y s t e m a t i c t r e n d af R e f . 7. I n t h e p r e s e n t s t u d y , t h e l i f e - t i m e was measured t o b e 0.25+0.03 n s , a n d t h i s v a l u e , t o g e t h e r w i t h t h e d a t a shown i n F i g . 6 , i n d i c a t e s t h a t f v a l u e s f o r t h e l o w e r members o f t h e i s o e l e c t r o n i c s e q u e n c e h a v e b e e n u n d e r e s t i m a t e d .

0.8 ..

- B-sequence

0.6 - 2 ~ 2 2 ~ 2 ~ - 2s 2 p 2 2 ~

-

trend

- (Sm~th and Wiese)

F i g . 6. S y s t e m a t i c t r e n d o f f v a l u e s f o r - 2 s 2 2 p

- 2 s 2 p 2 'P t r a n s i t i o n i n t h e B I s e q u e n c e from S m i t h and w i e s e 7 ) a n d r e f e r - e n c e s t h e r e i n . The p r e s e n t r e s u l t f o r C I1 h a s b e e n i n c l u d e d +.

CONCLUSION

With t h e p r e s e n t e x p e r i m e n t a l a r r a n g e - ment, i t i s p o s s i b l e t o s t u d y t h e e m i s s i o n s p e c t r a o f ion-atom c o l l i s i o n s i n t h e f a r - u l t r a v i o l e t s p e c t r a l r e g i o n s f o r i o n - g a s , b e a m - f o i l , a n d i o n - s o l i d i n t e r a c t i o n s . C o n c e r n i n g b e a m - f o i l l i f e t i m e s , t h e s p a t i a l r e s o l u t i o n o b t a i n e d by t h e p r e s e n t r e f o c u s - s i n g s y s t e m may b e u s e f u l i n more p r e c i s e l i f e t i m e s t u d i e s i n t h e f a r - u l t r a v i o l e t s p e c t r a l r e g i o n .

t ^C 2s' 2p2 ' 0 - 2s22p 3s'p0 ^{- sequence}

--- systematic trend 0,20 t ^{(Smith f} present results ^{and Wiese)}

ACKNOWLEDGEMENT

The c o o p e r a t i o n o f J . R . R o b e r t s a t t h e i n i t i a l s t a g e s o f t h i s work i s g r a t e - f u l l y acknowledged.

F i q . 5. S y s t e m a t i c t r e n d o f f v a l u e s f o r 2 s 2p2 'D - 2 s 2 2 p 3 s 1 p t r a n s i t i o n i n t h e C I s e q u e n c e f r o m R e f . 7 a n d r e f e r e n c e s t h e r e i n I n c l u d e d a r e p r e s e n t r e s u l t s +.

REFERENCES

( 1 ) L. B a r e t t e , D r - S c . T h e s i s , D e p a r t e m e n t d e P h y s i q u e , U n i v e r s i t g L a v a 1 (1975) ( 2 ) J . P . B u c h e t , M.C. B u c h e t - P o u l i z a c , a n d

M. D r u e t t a , J . O p t . S o c . A r n . ~ , 842 ( 1 9 7 6 )

c1-160

( 3 ) E.G. Boving, E. Edlund, and G. SBren- ( 5 ) B. Warner and R.C. K i r k p a t r i c k . Month.

s e n , t h e s e P r o c e e d i n g s Not.Roy . A s t r . S o c . 142, 265 (1969) (4) A.E. L i v i n g s t o n , P.D. Dumont, Y Baudi- ( 6 ) E.H. P i n n i n g t o n , D . J . G . I r w i n , A.E.

n e t - R o b i n e t , a n d H.P. G a r n i e r , P r o c - I V L i v i n g s t o n , a n d J . A . Kernahan, Can.J.

Conf.on Beam-Foil S p e c t r o s c o p y , Vol. I , Phys. 2 , 1961 (1974)

(Plenum P r e s s , N e w York, 1976) p . 339 ( 7 ) M.W. S m i t h and W.L. Wiese, A s t r o p h y s .

J . S u p p l . S e r . 2 , 1 0 3 (1971)