Anisotropic Excitation by Foils and Surfaces

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Anisotropic Excitation by Foils and Surfaces

D. Church

To cite this version:

D. Church. Anisotropic Excitation by Foils and Surfaces. Journal de Physique Colloques, 1979, 40

(C1), pp.C1-304-C1-306. �10.1051/jphyscol:1979164�. �jpa-00218444�

JOURNAL DE PHYSIQUE Colloque C1, suppl6ment au

n o

2 , Tome 40, ficvrier 1979, page C1-304

A n i s o t r o p i c E x c i t a t i o n by F o i l s and Surfaces

D. A . Church

Physics Department, Texas A&M U n i v e r s i t y , College S t a t i o n , Texas, USA

Resume. Certaines t h e o r i e s recentes s u r l a p r o d u c t i o n d ' o r i e n t a t i o n e t d'alignement dans des 6 t a t s

~ZGFS d ' i o n s r a p i d e s p a r c o l l i s i o n avec des f e u i l l e s ou des surfaces i n s i s t e n t s u r l a d e s c r i p - t i o n de l l ~ v o l u t i o n de l l e t a t e x c i t e i p a r t i r d ' u n 6 t a t i n i t i a l i 1 1 i n t 6 r i e u r de l a m a t i e r e p a r une m a t r i c e d e n s i t 6 ayant des p r o p r i e t e s de symetrie p a r t i c u l i 6 r e s . A cause de l a longue,urdu t r a - j e t parcouru dans l a m a t i e r e , e t du moindre dear6 de d e f l e c t i o n des ions rapides, l e s & t a t s i n i - t i a u x d e f i n i s dans une c o l l i s i o n i o n s - f e u i l l e n ' a u r o n t pas necessairement l e meme c a r a c t e r e que dans une c o l l i s i o n avec un sol i d e . Des mesures de coherence,dlorientation e t d ' a l ianement des niveaux e x c i t e s d'atomes e t d ' i o n s He, B e t N p r o d u i t s p a r c o l l i s i o n avec des f e u i l l e s de carbone e t des s o l i d e s r e c o u v e r t s - d e c a r b o n e m o n t r e n t . d e t e l l e s d i f f e r e n c e s d a n s l e s e t a t s f i n a u x q u i p e u v e n t e t r e c o r r e l e s a l a d i s t r i b u t i o n i n i t i a l e d e c h a r g e e t d e moment.

A b s t r a c t . Recent t h e o r i e s f o r t h e p r o d u c t i o n o f o r i e n t a t i o n and alignment i n f a s t i o n ex- c i t e d l e v e l s b y c o l l i s i o n w i t h f o i l s o r surfaces have emphasized t h e development o f t h e ex- c i t e d s t a t e from an i n i t i a l s t a t e i n s i d e t h e m a t e r i a l d e f i n e d by a d e n s i t y m a t r i x w i t h p a r - t i c u l a r symmetry p r o p e r t i e s . Due t o t h e path l e n g t h i n s i d e t h e m a t e r i a l , and t o t h e l e s s e r degree o f d e f l e c t i o n of t h e f a s t ions, t h e i n i t i a l s t a t e s d e f i n e d by a f o i l c o l l i s i o n w i l l n o t n e c e s s a r i l y have t h e same c h a r a c t e r as those d e f i n e d by a c o l l i s i o n w i t h a s o l i d .

Coherence measurements of o r i e n t a t i o n and alignment o f e x c i t e d

0

l e v e l s of He, 6, and N atoms and ions produced by c o l l i s i o n s w i t h carbon f o i l s and carbon-coated s o l i d s a r e c o n t r a s t e d t o d i s t i n g u i s h such differences i n f i n a l s t a t e s which may be c o r r e l a t e d w i t h t h e i n i t i a l d i s - t r i b u t i o n o f charge and momentum.

I n t r o d u c t i o n

Fast ions e x c i t e d and deflected through small angles b y s u r f a c e c o l l is i o n s exhi b e t a h i g h degree of o r i e n t a t i o n o f t h e e x c i t e d l e v e l s , b u t r e l a t i v e - l y l e s s a1 ignment [I]. The momentum t r a n s f e r i n t h e c c , l l i s i o n appears c o n s i s t e n t w i t h b i n a r y c o l l i - s i o n s 3 t o r near t h e surface, w i t h l i t t l e penetra- t i o n o f t a r g e t m a t e r i a l [Z]. These r e s u l t s may be c o n t r a s t e d w i t h t h e a n i s o t r o p y produced i n e x c i t e d l e v e l s due t o t h e passage of ions through a t h i n f o i l w i t h normal i n c l i n e d t o t h e beam d i r e c t i o n , [ 3 , 4 ] where s i g n i f i c a n t alignment i s a l s o sometimes observed [5]. Even f o r t h i n f o i l s , t h e increased t r a v e r s a l d i s t a n c e due t o l a r g e f o i l angle i . n c l i - n a t i o n produces considerable slowing and s c a t t e r i n g of t h e i n c i d e n t beam. The b u l k s c a t t e r e d i o n s then emerge from t h e s u r f a c e w i t h v a r i o u s d i r e c t i o n s ,but 1 i t t l e i n f o r m a t i o n about t h e momentum t r a n s f e r i n t h e f i n a l e x c i t i n g c o l l i s i o n i s a v a i l a b l e . I t has been shown, however, t h a t o n l y t h e d i r e c t i o n o f t h e i o n s r e l a t i v e t o t h e s u r f a c e m a t e r i a l normal i s t h e s i g n i f i c a n t parameter i n s u r f a c e c o l l i s i o n s [2] ,and s i m i l a r l y ,

t h e s i g n i f i c a n t parameter i n f o i l c o l l i s i o n s i s t h e v e l o c i t y r e l a t i v e t o t h e e x i t surface normal, i n - dependent o f t h e i n c l i n a t i o n o f t h e f o i l 16).

Theories by Herman, [7] Band, [8] and Lewis and S i l v e r [9] assume an a x i a l ly-symmetric i n i t i a l d e n s i t y m a t r i x f o r f o i l - e x c i t e d ions, which i s a l s o symmetric under r e f l e c t i o n s i n any plane c o n t a i n i n g t h i s symmetry a x i s . This m a t r i x i s m o d i f i e d by capture, e x c i t a t i o n , o r precession when t h e i o n emerges from t h e f o i 1 surface, t o produce t h e a l i g n - ment and o r i e n t a t i o n . For small-angle i o n - s u r f a c e c o l l i s i o n s , ~ c h r g d e r

[ l o ]

assumes t h a t s i n g l e o r m u l t i p l e charge capture i s t h e p r i n c i p l e mechanism f o r o r i e n t i n g t h e e x c i t e d l e v e l s , a1 though core po- l a r i z a t i o n may a l s o p l a y a r o l e .

A t comparable i n c l i n a t i o n angles, one may hope t o d i s t i n g u i s h d i f f e r e n c e s o r s i m i l a r i t i e s i n r e l a - t i o n t o t h e f o i l o r surface e x c i t a t i o n processes by examining t h e r e 1 a t i ve magnitudes o f t h e o r i e n t a t i o n parameter [ I 1 1 0;" and o f t h e t h r e e alignment parameters A:'',

:' A: ,

and ':A: produced by c o l - l i s i o n s of each type. Considered here a r e D l e v e l s 1

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979164

of three configurations in two l i g h t ions and one atom. 'D l e v e l s a r e the lowest

L

level f o r which higher order (unobserved) anisotropic tensor mo- ments can be coupled i n t o moments observable by l i g h t polarization, and a l s o a r e i n t e r e s t i n g since core polarization may a f f e c t the anisotropy in mul- tip1 e capture processes [lo].

Experimental Method

To obtain similar data, l i g h t from a small range.of s c a t t e r i n g angles was collected i n each case. In a typical beam f o i l coherence measurement, l i g h t emitted from ions scattered into a range of angles up t o 58' could be collected. Since the em- ergence angle of the f a s t p r o j e c t i l e s i s t h e c r i t i - cal parameter, t h e observation region f o r ion-solid s c a t t e r i n g was positioned a t the end of the t a r g e t surface. This geometry was chosen t o be closer t o the f o i l s c a t t e r i n g average, although the mean o r i - entation measured in t h i s configuration i s approxi- mately 10% below the maximum obtained when l i g h t i s collected from a minimal angular range well down- stream [21.

The experimental confi'gurations a r e shown sche- mati cal l y i n Figure 1 (a) and Cbj .

^A

coherence measurement technique was used in each case, [5>ith polarization analyzers fixed in angle f o r a given measurement. The emitted polarized 1 ig h t was col- lected from a narrow s p a t i a l region as a function of a l i n e a r l y swept magnetic f i e l d strength. In the case of f o i l e x c i t a t i o n , t h i s produced aquantumbeat s i g n a l , while f o r a s o l i d t a r g e t a finite-time Hanle-effect signal [1,13J was observed. In each case, the r e l a t i v e modulation of the analyzed 1 ig h t i n t e n s i t y i s related t o two o r more of the four orientation and alignment parameters. In quantum- beat s i g n a l s , these modulations often occur a t d i f - f e r e n t mu1 t i p l e s of the Larmor frequency. Four sepa- r a t e r e l a t i v e polarization measurements a r e s u f f i - c i e n t t o obtain the magnitude of t h e four multipole parameters [5].

The t r a n s i t i o n s and level s investigated were t h e 49228 t r a n s i t i o n from the ls4d 1

D

level of He I , the 34518 t r a n s i t i o n .from the 2p2 'D level of B 11, and the 39968 t r a n s i t i o n from t h e 2 p 3 p 1 ~ level of

N 11.

Cb)

Figure 1 : ( a ) f o i l excitation configuration Cb) surface exci'tation configuration

Table 1: 0;:' vs

6

Energy (keV

60 80

9 5

75 .058 .053 .050

f o i l 80 .052 .050 .050

a n g 1 e 8 3 .040 -

[degree) -

sur-

85

face .033 .030 .030

angle

Yagnitude of 0:: ' f o r the He I 4d 1 D s t a t e excited by carbon.

. I . .

75 80 1f l a

&-

Energy (a 1

(b)

Figure 2: (a) Anisotropy parameters f o r f o i l

excitation and (b) surface excitation

C1-306 JOURNAL DE PHYSIQUE

Table I1 tensor moments f o r both ion levels were l e s s than 2%.

Ion level Energy (keV) 0'0' angle Both ions have a nuclear s p i n , so the observed or- 1

-

NI I 120 .050+.002 86' ientation are l i m i t s : some of the electronic orien-

BII 120 .050f .002 88' tion could be coupled into the nucleus during the

He1 125 .032+.004 86' f r e e precession i n t e r v a l . Since the external mag- He I 60 .030f .006 85' n e t i c f i e l d was present the amount of t h i s reduction

Magnitude of 0;:' f o r several levels i s uncertain. For maximum coupling, the t r u e elec- Results and Oi scussion t r o n i c orientation would be increased over the ob-

The principal r e s u l t s f o r the 4d 1 D level of served value by a f a c t o r of 1.55 f o r "B and 1.26 He I a r e shown i n Table I and Fig. 2 . The i n c l i - f o r 14d. In any event, the orientation of the ions nation angles f o r each measurement type a r e s l i g h t - in both configurations i s s i g n i f i c a n t l y higher than l y d i f f e r e n t , but the variations of the parameters the atom o r i e n t a t i o n .

over a few degrees i n t h i s range a r e not discon- Conclusions

tinuous. The energy tabulated f o r the surface col- The anisotropy in the 1 D levels of ions pro- l i s i o n i s t h e incident ion energy, while the energy

duced

by.

carbon surface toll isions i s primarily ori-

1

tabulated f o r f o i l excitation i s the e x i t ionenergy

entation,

w h i l e

the

4d level

of neutral h e l i u m si -

based

On energy losses.

One see that

i n milarly excited shows a considerably smaller or.ien- both cases non-negl i gi bl e a1 i gnment 1 arger then the

t a t i

o n ,

and

c o r r e s p o n ~ ~ n g ~ y

1

arger

a l i g n m e n t . ~~i 1 orientation i s present: an unusual s i t u a t i o n f o r

excitation of the same He

level

a t emer-

ion-solid s c a t t e r i n g , where alignment i s often neg-

gence

a n g l e s a l s o

favors

a l i g n m e n t . ~h~ r e s u l t s in- lected compared t o t h e orientation in theoretical

dicate that the two types of

c o l l i s i o n

are certain-

treatments. In p a r t i c u l a r , the ion-surface c o l l i -

l y related for the formation of

t h i s

l e v e l ,

but

they

sion emphasizes the alignment parameter 4k01 and

a r e not completely equivalent. This favoring of ccr-

A::',

whi 1 e i s 1 argest f o r the ion-foi 1 toll i - t a j n ignment

components may

be

to

different

sion. The foil-generated orientation i s l a r g e r than

momentum transfer

i n

the f i n a l

c o l l i s i o n ,

or to

d i f - t h a t generated by the surface, but 7s converging to- ferences i n

capture i n

t h e formation ward t h e surface value (Table I ) . The largestvalue

the

f i n a l

state.

of 0;:' i s about 25% of the theoretical maximum. Acknowledgment In Table I1 i s a comparison of the orientations

measured f o r t h e D 1 levels of t h e ions and the He Mr. C.S. Lee a s s i s t e d

i n

some of these measure- atom, a l l f o r surface s c a t t e r i n g from a carbon t a r - ments. This research was i n i t i a t e d and continued g e t . Data a r e shown both f o r comparable energies, with support from the Research Corporation. Profes- and f o r comparable v e l o c i t i e s . The coherent a1 ignment s o r

W.A.

Porter and Professor

E .

Schweikert a r e

thanked f o r the use of t h e i r accelerator faci 1 i t i e s . BIBLIOGRAPHY

[I]. H . H . Andra, R . Frohling, H.J. Plohn and J.P.

S i l v e r , Phys. Rev. Letters

37,

p . 1212 (1976);

H.J. Andra, Phys. Letters 54J, p.315 (1975).

[2]. H.G. Berry, G . Gabrielse, and

A.E.

Livingston, Phys. Rev. AlJi, p.1915.(1977).

[3]. H . G . Berry i n "Beam Foil Spectroscopy: Heavy Ion Atomic Physics", I.A. S e l l i n and D.J.

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,

(Plenum, 1976).

[4].

D.A..

Church, 'lInvited l e c t u r e s of the

ICPEAC

,"

J . Risley and R . Geballe, Eds, (Univ.

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,

and W . Kolbe, Phys.

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El.

D.A. Church, and C.S. Lee (unpublished) [7]. R.M. Herman, Phys. Rev. Letters

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