FAST ION DOUBLE COLLISION SPECTROSCOPY

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FAST ION DOUBLE COLLISION SPECTROSCOPY

D. Church, C. Lee

To cite this version:

D. Church, C. Lee. FAST ION DOUBLE COLLISION SPECTROSCOPY. Journal de Physique

Colloques, 1979, 40 (C1), pp.C1-324-C1-326. �10.1051/jphyscol:1979169�. �jpa-00218449�

JOURNAL DE PHYSIQUE Colloque C1, suppl6ment au n o _2, Tome 40, fgvrier 1979, page cl-324

FAST ION DOUBLE COLLISION SPECTROSCOPY D.A. Church and C.S. Lee Physics Department, Texas A&M University R-

. Les collisionr: d'ions rapides avec des photons, mo~dcules, f e h l e s ou surfaces peuvent

&re u t i l i s k e s pour prgparer l e s ions dans des Q t a t s de charge sp6cifique, des niveaux excites ou des sous-niveaux. Une rleuxisme collision cons6cutive, qui peut &re diffgrente de la,premic!re, u t i l i s e ces p r o j e c t i l e s produits comme d & c r i t ci -,dessus dans 116tude des collisions specifiques.

L'analyse de l a deuxieme c o l l i s i o ? peut se f a i r e a p a r t i r de l'observation de l J i n t e n s i t e ou de l a polarization des photon kmis, ou a ~ a r t i r de l a i s t r i b u t i o n des particules rapides diffus6es.

Nous d6crivonsci-dessous14application de c e t t e m thode g6ne'rale

h

lt6tude de c o l l i s i o n s ion- surface qui produlsent une orientation

a

Abstract. Collisions of f a s t ions with photons, molecules, f o i l s or surfaces can be used to prepare the ions in s p e c i f i c charge s t a t e s , excited s t a t e s , o r substates. A subsequent second c o l l i s i o n , perhaps d i f f e r e n t from the f i r s t , employs t h e s e previously prepared p r o j e c t i l e s t o study s p e c i f i c c o l l i s i o n s . The analysis of the second collision may proceed by observing the i n t e n s i t y or polarization of emitted photons, or the d i s t r i b u t i o n of scattered f a s t p a r t i c l e s . Application of t h i s general method t o the study of ion-surface orienting c o l l isions i s described.

rNTRODUCTION

Collisions of low energy ions with atoms have occasionally been used t o prepare the ions i n spe- c i f i c excited or ground s t a t e s o r substates [ l ] . A second c o l l i s i o n i s used t o analyze the prepared s t a t e . Fast ion c o l l i s i o n s have more e x c i t a t i o n channels, b u t s u f f i c i e n t l y short relaxation times permit the formation of r e l a t i v e l y few f i n a l s t a t e s of charge and e x c i t a t i o n . Subsequent collisions with molecules, photons, e l e c t r o n s , or surfaces may be used t o study these r e l a t i v e l y more s p e c i f i c col- l i s i o n s . The analysis of the second c o l l i s i o n can proceed by observing the i n t e n s i t y or polarization of emitted photons [2]. As an example of t h i s gen- e r a l method, an equilibrium charge d i s t r i b u t i o n of f a s t p a r t i c l e s i s created by passing a beam through a f o i l . The orientation produced by a subsequent inclined surface c o l l i s i o n i s compared with the orientation produced i n a similar surface coll i sion by incident ions, t o seek f o r any e f f e c t of t h e mean incident charge d i s t r i b u t i o n on the average orientation of an ion level. Such an e f f e c t might occur i f the level were produced primarily by one process r a t h e r than by another: e.g. capture as opposed t o e x c i t a t i o n . The penetration of a grazing beam i n t o the s o l i d i s i n s u f f i c i e n t t o produce the charge-state equilibrium observed with f o i l e x c i t a - t i o n .

Experfmental Method

The basic coll i s i o n in fhese measurements was t h a t of beam ions with a s o l i d surface 131 inclined with surface normal a t an angle B t o the incident beam d i r e c t i o n , as shown i n Fig. 1 . The background gas;pressure was i n the usual beam-foil spectroscopy range: 2 x t o r r o r below. The l i g h t emitted perpendicularly t o the beam direction b y ions having

experienced t h i s colli'si'on was collected from a ver- t i c a l s p a t i a l region 200 mfcrons wfde and 5 mm high located near the surface as shown. This l i g h t was polarization and s p e c t r a l l y analyzed, and detected using photon counting techniques as a function of the strength of a swept uniform magnetic f i e l d ap- plied parallel t o the incident beam d i r e c t i o n . For undeflected ion t r a j e c t o r i e s , the resulting signal in c i r c u l a r l y polarized l i g h t i s described by the expression [4]

I(H) = (A + B cos (2ngu8Ht/h))exp (-t/T)dt where ^OJ = 2 ~ H/h i s the Larmor precessional f r e - 9 ~ ~ quency of the i p p e r level of the t r a n s f t i o n , T i s the mean l i f e of t h i s level; and tl and t2 are the extreme times of creation of the level i n d i f f e r e n t ions r e l a t i v e t o the time of observation of the emitted l i g h t . The r e s u l t i s a finite-tlme Hanle e f f e c t s i g n a l . The r e l a t i v e amplitude B/A of t h i s signal can be used as a measure of the orientation parameter 0::' of the excited level produced by the surface c o l l i s i o n [3], a1 though t h e small angular differences of the p a r t i c l e t r a j e c t o r i e s produced by the surface c o l l ision cause deviations.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979169

Fig. 1 .

SOLID

FOIL

The l i g h t collection region i s indicated by t h e rectangle below the s o l i d t a r g e t in t h i s schematic apparatus configuration.

W i t h i n t h e t o t a l experimental configuration, t h e i n i t i a l charge s t a t e of the incident ions can be changed by a prior c o l l i s i o n with a f o i l , which may be inclined a t an angle a . A t the incident ion energies used here (5 ^{140 keV) -3} predominant frac- tion of a boron beam i s neutralized by t h i s f o i l c o l l i s i o n , and the d i s t r i b u t i o n incident on the s u r - face i s characteristl'c of the charge-state equilibiun previously measured by Hvel plund e t a1 [5]. 9ue t o the s c a l e of t h e apparatus, many of the excited s t a t e s produced i n t h i s foi'l pre-collision did not have the opportunity t o relax t o the ground s t a t e before the subsequent surface c o l l i s i o n .

Results

Consider the 3451

fi

t r a n s i t i o n of B 11. When a 'B+ beam i s incident on the(ll1)face of a polished p-doped Si c r y s t a l inclined with surface normal a t an angle ~ ~ 8 4 ' t o the beam d i r e c t i o n , a Hanle-type signal shown i n Fig. 2 i s observed i n c i r c u l a r l y po- l a r i z e d l i g h t . A r e l a t i v e modulation amp1 itude of 23% i s found, well below both the minimum value of 65% i f f u l l hyperfine coupling occurs, and a tan a extrapolation 161 t o t h i s angle of the orientation signal produced i n the level by a f o i l c o l l i s i o n a t a=55O.

F i g . 2. Ca] The finite-time Hanle-effect surface s c a t t e r i n g signal i n c i r c u l a r l y polarized l i g h t f o r the 3451

8

t r a n s i t i o n (no f o i l ) . (b)with t i t l e d f o i l

I f the beam i s i n i t i a l l y passed through a f o i l w i t h a=oO, an equilbrium charge d i s t r i b u t i o n i s pro- duced which i s then incident on the s o l i d surface. A similar signal i s then observed, but with t:he r e l a t i v e modulation sharply reduced t o 9.3%. How- ever, most of the unpolarized component of t h i s l i g h t a r i s e s from the decay of the r e l a t i v e l y long- lived strongly excited 3451

fi

t r a n s i t i o n from t h e un- neutralized f r a c t i o n of the beam excited a t the f o i l . This level can decay i n t o detection system before the ions s t r i k e the Si surface. That t h i s i s the case can be readily seen in Fig

.

2 ( b ) , where the quantum beats [7] i n the signal are produced by ions oriented by a a=45' inclined f o i l c o l l i s i o n , and the underlying Hanle signal a r i s e s from the surface col- l i s i o n of the (primarily neutral) equilibrium charge d i s t r i b u t i o n incident on the s o l i d surface. The mag- nitude of t h i s f r a c t i o n a l surface signal i n t e n s i t y can be obtained by se?arately c a l i b r a t i n g the r e l a - t i v e amplitude of t h e f o i l s i g n a l , and subtracting i t from the superposition s i g n a l . The r e s u l t i s a r e l a t i v e Hanle-type modulation of about 37%. Similar data a t d i f f e r e n t f o i l angles and f o r d i f f e r e n t tran- s i t i o n s have been taken: some r e s u l t s a r e summa- rized i n Table I .

cl-326 JOURNAL DE PHYSIQUE

TABLE I As an additional consideration, an inclined f o i l

Relative signals i n c i r c u l a r l y polarized l i g h t a r i s - ing from surface s c a t t e r i n g of incident beam f o r d i f - f e r e n t incident charge d i s t r i b u t i o n s . A Boron beam was incident on a s i l i c o n t a r g e t .

Approximate Re1 a t i v e Moduation B/A(%)

Wavelength Upper (a) Ions (b) Charge

fi

1 eve1 Incident Distribution

Incident 3451 BII 2p2 24.6

+

1.8 37 f 9.4

2089 BI 2 ~ 2 4 . 3 f 1 . 7 2 ~ 3 1 + 4 ~ ~ ~ These changes a r e not produced by energy l o s s of the f a s t p a r t i c l e s i n the i n i t i a l f o i l c o l l i s i o n , since t h i s energy change i s small compared t o characteris- t i c energy dependences of the orientation parameter.

Substantially identical B I 1 r e s u l t s were a l s o ob- served a t 135 KeV incident p a r t i c l e energies.

Discussion

Orientation of a given level may be produced a t t h e material surface by charge capture, by exci t a - t i o n , or by more complex i n t e r a c t i o n s . Tf a part- t i c u l a r interaction should dominate, then t h e

-

o r i e n t a t i o n can be expected t o depend on departures of t h e incident beam from charge s t a t e equilibrium.

Such an equilibrium i s determined both by projec-

c o l l i s i o n will impart a net orientation t o the f a s t p r o j e c t i l e [8]. I t i s generally t r u e t h a t most ex- c i t e d levels of the p r o j e c t i l e will have the same sense of orientation [3], and t h a t t h i s orientation wi 11 be p a r t i a l l y preserved through the subsequent r a d i a t i v e decays. Yore importantly perhaps, B I has a P ground s t a t e , which can be oriented [2] by the c o l l i s i o n . I f such an i n i t i a l e l e c t r o n i c orien- t a t i o n plays a r o l e in the s o l i d surface excitation process, then some dependence of the orientation pro- duced i n the subsequent surface c o l l i s i o n on t h e foil angle i s expected. Of course, an average t r a n s f e r of orientation t o the nucleus via the hyperfine i n - teraction [9,10] will a l s o y i e l d a residual elec- t r o n i c orientation a f t e r a second c o l l i s i o n . No s i g n i f i c a n t dependence was observed, implying sur- face penetration, rapid relaxation, o r i n s u f f i c i e n t s e n s i t i v i t y of the measurements.

Concl usi on

The data indicate a dependence of orientation magnitude which i s considerably l e s s then the charge- s t a t e modulation. S t a t i s t i c a l uncertainties a r e l a r g e due t o the background subtraction procedure.

More work is necessary t o d e f i n i t e l y e s t a b l i s h the magnitudes of a charge-state dependence.

Acknowledgment

D .A.C. thanks Professors J . Reading, S.Rountree t i l e energy and by t h e path length in the t a r g e t ma- and R . Shakeshaft f o r useful discussions, Professor t e r i a l . The fractional beam energy directed normal Porter f o r use of the f a c i l i t i e s of the I n s t i t u t e t o the s o l i d surface i s E = E CoS 2 ~ = 1 0 - ~ ~ = - 1 . 4 KeV; of s o l i d S t a t e Electronics a t TAMU, and the Research t h i s i s s u f f i c i e n t t o produce a maximum penetration Corporation f o r supporting t h i s research.

i n Si of about 60

8.

The ions observed by t h e i r l i g h t e m i s s i o n a r e those most e f f i c i e n t l y s c a t t e r - ed back through the s u r f a c e .

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