ION-SOLID INTERACTION

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ION-SOLID INTERACTION

H. Bukow, J. Remillieux

To cite this version:

H. Bukow, J. Remillieux. ION-SOLID INTERACTION. Journal de Physique Colloques, 1979, 40

(C1), pp.C1-356-C1-359. �10.1051/jphyscol:1979178�. �jpa-00218458�

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JOURNAL DE PHYSIQUE Colloque C1, supplPment au no 2,

Tome

40,

fhrier

1979, page C1-356

I O N - S O L I D I N T E R A C T I O N

Summary of a round-table discussion held on September 6, 1978 compiled by

H.H. BUKOW, Ruhr-Universitzt Bochum (RFA)

and J. REMILLIEUX, Institut de Physique Nuclkaire (et INZP3), Universitk Lyon-I (France).

The discussion was organized in five p a r t s each of them introduced by an invited speaker. In this s u m m a r y we t r y to reproduce the essential s t a t e - ments of the session without attempting complete- ness. The speakers who gave the introductory s t a - tements w e r e :

I. H. D. Betz, Univ. Munchen 11. B. Rosner, Technion Harfa 111. J. Remillieux, Univ. Lyon-I IV. S. Datz, Oak Ridge Nat. Lab.

V. H.G. B e r r y , Univ. Chicago.

I. The state-of-the-art in ion-solid interactions The m o s t simple collision consists of encounters between single ions and single atoms with very lit- tle interaction. Then, t h e r e i s a l m o s t no d i s t u r - bance of the collision p a r t n e r s and one can apply perturbation techniques. This works well especially f o r f a s t collisions. In slow collisions,of c o u r s e the atoms a r e strongly disturbed and molecular o r

-

bitals a r e partially formed.

L e t us consider what happens in the c a s e of r e - latively f a s t collisions where "fast1' means that the identity of projectile and t a r g e t i s conserved : Excitation of projectile electrons either to nigher lying empty s t a t e s o r to the continuum, de-excitation of the projectile by Auger, x - r a y o r light emission and finally capture of t a r g e t electrons in- to projectile states. The l a t t e r may be low-lying vacancies but capture into excited s t a t e s , continuum s t a t e s and quantum-mechanical mixtures of s t a t e s i s a l s o possible. The basic p r o c e s s e s a r e the Coulomb interaction and the interaction with the photon field and we a s s u m e that we understand them to a l a r g e extent. Now, what changes if we introduce a solid t a r g e t ?

1) Due to the high collision frequency the time between successive collisions may be s h o r t e r than the lifetime of excited s t a t e s leading t o the formation of such states.

2) We may have a l a r g e disturbance of the projectile s t a t e s inside the solid because of static and dynamic screening, particularly important for r e - latively slow moving projectiles. F o r v e r y f a s t p a r t i c l e s i t i s the p r e s e n t understanding that s c r e e - ning effects disappear. But a s long a s the particle velocities a r e n e a r the Bohr velocity and the s t a t e s we consider have binding energies n e a r the F e r m i energy, screening gives r i s e to a number of questions.

3 ) What kind of bound s t a t e s can exist inside a s o - lid ? Especially, can a proton bind a n electron ? This question h a s been discussed extensively in the l i t e r a t u r e and the a n s w e r s a r e conflicting. In a r e c e n t paper C r o s s showed that the binding of a n electron to a proton (that i s 13 eV) i s shif- ted towards the continuum a s soon a s the H atom i s inside the solid. If we a s s u m e t h a t t h e H a t o m i s sitting s t i l l inside the solid the shift exceeds the actual binding and the state becomes unbound. But a s soon a s the H a t o m moves the dynamic s c r e e - ning reduces the shift and when the a t o m reaches the Bohr velocity (i. e. 25 keV) then the shift has diminuished to the extent that the electron becomes bound again.When the projectile becomes f a s t e r the shift is cancelled even m o r e and a t about 10 vo t h e r e i s a l m o s t no detectable effect on the binding.

Thus if a f a s t proton captures a n electron inside the solid i t makes s e n s e t o talk about bound states.

4) The presence of a l a t t i c e compels us t o say that s i z e s of electronic orbitals of the projectile inside the solid can not exceed certain maximum values.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979178

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5) What i s the charge state of the partially scree- ned projectile and how can we calculate its energy loss ? Indeed, a t present we don't have any conci-

se theory for the energy loss of heavy ions in solids.

6) Finally, we have a number of new phenomena a s the wake potential, the large field of channeling effects and the interactions between the projectile and the electrons and atoms on the exit side of the solid.

The question that we cannot answer in all cases is:

What i s the real physics going on if a particle moves through and exits from a solid ?

11. Approach to ion-solid interaction by use of heavy gas targets

Let us investigate the charge state distributions after collisions of heavy ions (as

on+)

with r a r e gas atoms and select those events that belong to very small impact parameters (E. g. p< K shell radius.

Eion = (30-50) MeV, scattering angle 5O) i. e.' collisions that lead to almost completely over- lapping electronic clouds. If we performe such measurements ( 2 ) we observe that with increasing Z (i. e. going from Ne to Xe) the projectile "loo- ses" the memory of its incoming charge state.

Whereas for a Ne target the average charge of the outgoing particle differs by about 1.2 units of charge depending on whether the incoming ion was 04+ o r 07', this difference is only about 0.2 f o r a Xe target. This means that we may consider the collision complex a s a compound atom completely decoupling incoming and outgoing channels analog to the compound nucleus. The observation that the average outgoing charges a r e a s high a s those mea-

sured after the passage through a solid led to the adoption of Brandt's (3) dynamic screening model.

Replacing the thickness of the foil by the'8tomic radius and calculating the "plasma frequency" of the free many-electron atom using its electron den- sity a s given by Jensenls statistical model we get a formula ( 2 ) describing the average outgoing charge a s a function of Z

target that fits the expe- rimental data without further f r e e parameters. Of

course i t i s questionable to use a frequency c o r r e s - ponding to a period that is of the same order of magnitude a s the passage of the projectile through the interaction region. The success of this approach doesn't mean that we understand the physics, but i t means that we have to think about s a - tistical models.

111. Molecules interacting with solids

The study of the interaction of molecular projectiles with matter opens a new field of experiments and r i s e s a lot of questions. We learn about the partition of energy loss of ions in solids between close and distant collisions. By measu- ring the angle and energy distributions of the molecular break-up products emerging from foils of various thicknesses, one can get information on the charge state of the ions both inside a n d outside the solid. In the vacuum downstream the target the ion cluster generally separates v i a r e p u l s i v e m o l e c u l a r l e v e l s o r via a Coulomb explosion. The study of the

break-up of molecular ions through thin foils has also been used to investigate their stereochemical structure and their vibrational excitation prior to fragmentation. In some cases bound molecules a r e observed among the particles transmitted through the foil. When the foil is very thin the molecule emerges with the same bound electrons and one might say that the molecule survived in the traversal through the foil. But molecules a r e also observed after rather thick targets when the transmission of the projectile electrons is unlike- ly. At present there i s no model to explain why the molecular transmission yield does not vary more strongly with target thickness and why the transmission i s observed with some molecules and not with others.

The most important aspect of experiments using molecules i s to elucidate the role played by the wake potential inside a solid. The polarization of the solid due to the penetration of a fast ion has been described more than 30 years ago,

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c1-358 JOURNAL DE PHYSIQUE

and its recent formulation in t e r m s of a wake i s only one kind of approximation to the f a s t ion-solid interaction (4). The wake potential h a s b e e n shown to be important in two types of experiments, the study of the break-up of molecular projectiles and the observation of S t a r k splitting in the levels of channeled ions (5). The molecular break-up t e s t the wake a t distances of the order of the Ang- s t r o m f r o m the leading positive charge center, whereas the S t a r k splitting m e a s u r e s the wake a t a distance of the K-shell radius f r o m the nucleus.

Many questions remain concerning the correspon- dence between the wake and the plasmon frequency.

F o r example, what i s the r o l e played by the surface in experiments with very thin foils and which frequency correctly represents the electron oscil- lations in the bulk of the solid ? The description of the polarization in t e r m s of wakes i s doubtful when the projectile velocity i s not much l a r g e r than the Bohr velocity. Other consequences of the con- cept of wake have not yet been proved, for instance recent measurements failed to observed any wake- riding electrons ( 6 ) and the role played by the wake in the transmission of molecules through thick foils. F u r t h e r questions a r i s e if one calculate the stopping power of a medium f r o m the wake potential a s suggested by Vager and Gemmell (4c). The stopping powers obtained using conventional values of plasma frequency may not in general a g r e e with measurements of energy loss.

IV. The r o l e of channelin%

What can channeling in the future do to i n t e r e s t ' .

beam-foil spectroscopists ?

1) One long standing problem in beam-foil spec- troscopy i B to make fdils thin enough to bridge the gap between gas and solid target.8. F r o m energy l o s s measurements on ions such a s 07+ passing through a channel in Au we l e a r n that t h e r e i s a group of ions traversing +e (thick) target with extremely reduced probability f o r capture, loss o r excitation. If we evaporate a monolayer o r two of the material we a r e interested in onto this kind of substrate in a random configuration we may c r e a t e

a r b i t r a r i l y thin target.

2) Another standard problem i s the question a s t o where excitations occur i. e. to distinguish bulk and surface effects. Again the evaluation of energy loss measurements of channeled ions indeed a l - lows one to deduce whether a certain group of outgoing particles has undergone a change of charge s t a t e on the front side o r in the bulk o r on the exit

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1

side of the foil

.

3) Normally if we p a s s heavy ions through solids they come out with a lot of inner-shell excitation which has been produced in random collisions. In channeling we can avoid these inner-shell excitations, but there i s no reason why the outer shell should not be excited. An estimate for carbon using reasonable c r o s s sections shows that about

5% of the incoming ions leave the target in an excited state. This effect i s much stronger if we go to high Z and high velocity. In a 5-electron sys- t e m such a s ~i~~~ quite a number of excited states may be populated by collisional excitation with the electrons inside the solid o r by coherent excitation i. e. interaction with the regularly oscilla- ting electrostatic field of the lattice atoms forming the channel (5). Indeed, depending on the symme- t r y we can preferentially populate a certain s t a t e in a moving ion. F o r example, in planar channeling the

]

2p > and

1

2p

>

hydrogenic states a r e

X - Y

no longer degenerate and we can in principle p r e - ferentially populate

1

2px> states.

Inside the foil we have for certain system well- defined states, but what happens if the ion leaves the solid, does it smoothly flow andkeep its state o r i s there a sudden change giving r i s e to mixed states a s we know f r o m quantum-beat experiments ? 4) As in the study of moleqular break-up we may 'profit f r o m .the scaling of energy f r o m the center-

of-mass system to the laboratory system. So the 3 eV Stark splitting of hydrogenic nitrogen (cau- sed by the electron wake) manifests itself by a difference of 450 keV in the kinetic energy of the' channeled ions. If we can make reasonably preci- s e measurements of these spacings we should get information about shape and amplitude of the wak6

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V. The interaction with the surface

We may break-up the ion-solid interaction in two processes, one happening in the bulk, the other a t the surface. Observing the light emission f r o m the outgoing particles, a p r i o r i we cannot expect much information on the bulk interaction. One such opportunity i s the Okorokov experiment that has just been realized by Datz (5) who instead of observing the light emission looked for the t r a n s - mission of different charge states. Another way i s to look in end-on geometry a t the dissociation products of e. g.

NH

3 after passage through a foil In this case we may have l a r g e Doppler shifts (50- 100) and small widths (< I

W)

if we look a t hydrogen emission lines. The Doppler shift yields the particle energy and we can evaluate the influen- ce of the wake potential on the Coulomb explosion.

It appears that the wake force i s much stronger than expected from the Vager formula (4c) if the velocity of the moving hydrogen atom approaches the Bohr velocity.

If we wcint to discuss surface effects we must assume an ideally flat surface. Certainly, experiments with single crystals in a vacuum better than

m b a r would be the best way to realize such a surface.

But even with this simplification we cannot r e a l - l y predict what will happen to a . H atom leaving the bulk with an d e c t r o n in the n = 2 state.

The whole conference s o f a r has pictured the atom a s a point charge and the interaction between two atoms a s the encounter of two billiards balls.

But we know that the a t o m i s not a billiard ball; we know that i t has a flexible, m o r e o r l e s s spherical charge distribution. It i s quite natural to think of an a t o m a s a charge distribution around the nucleus and to ask.what transformation will occur if this lqmooth atom impinges on a flat surface. Any variation of the charge distribution yields a variation of the Fano-Macek (9) p a r a m e t e r s which a r e ea- sily determined f r o m a polarization measurement.

So f r o m measured alignment p a r a m e t e r s we l e a r n

that a spherical charge distribution bouncing from a flat surface has a rotating cigar shape. In a til- ted foil experiment we can think of secondary electrons being knocked out of the surface and captured by the outgoing ion. F r o m simple geometric argu- ments i t i s evident that the electrons below and a - bove the ion's trajectory have different irelocities yielding a net angular momentum just a s i t i s observed.

Bibliography

M. C. Cross, Phys. Rev.

x 5

602 (1977) B. Rosner, Phys. Lett. L A , 97 (1977) W. Brandt, R. Laubert, M. Mourino, A.

Schwarzschild, Phys. Rev. Lett. 30, ₃₅₈ (1973)

N. Bohr, K. Dan. Videnskab. Mat.Fys.

Medd. l8, No 8 (1948)

V.N. Neelavathi, R.H. Ritchie and W.

Brandt, Phys. Rev. Lett.

33,

302 (1974) 2. Vager, D. S. ~ e ~ m e l l , Phys. Rev. Lett.

37, 1352 (1976)

S. Datz, C. D, Moak, O.H. Crawford, H.

F. Krause, P. F. Dittner, J. Gomez del Campo, J.A. Biggerstaff, P.D. Miller,

P.

Hvelplund, H. Knudsen, Phys. Rev. Lett.

4 0 , 843 (1978)

-

R. Laubert, I.A. Sellin, C.R. Vane, M.

Suter, S. B. Elston, G.D. Alton, R. S. Thoe, Phys. Rev. Lett.

41,

712 (1978)

S. Datz, Nucl. Instr. and Methods,

132,

7 (1976)

V. V. Okorokov, D. L. Tolschenkov, I. S.

Khizhnyakov, Yu. N. Cheblukov, Y. Y. Lapi, sky, G.A. Iferov, Yu.N. Zhukova, Phys.

Lett. &A, 485 (1973)

U. Fano,

J.H.

Macek, Rev. Mod. Phys.

4 5 , 553 (1973)