REFRACTION OF DIRECTED SHOCK ELECTRONS AT PLANAR SOLID SURFACES

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REFRACTION OF DIRECTED SHOCK ELECTRONS AT PLANAR SOLID SURFACES

H. Rothard, K. Kroneberger, M. Burkhard, C. Biedermann, J. Kemmler, O.

Heil, K. Groeneveld

To cite this version:

H. Rothard, K. Kroneberger, M. Burkhard, C. Biedermann, J. Kemmler, et al.. REFRACTION

OF DIRECTED SHOCK ELECTRONS AT PLANAR SOLID SURFACES. Journal de Physique

Colloques, 1989, 50 (C2), pp.C2-105-C2-110. �10.1051/jphyscol:1989219�. �jpa-00229417�

JOURNAL DE PHYSIQUE

C o l l o q u e C2, s u p p l b m e n t au n 0 2 , Tome 50, f b v r i e r 1989

REFRACTION OF DIRECTED SHOCK ELECTRONS AT PLANAR SOLID SURFACES

H. ROTHARD, K. KRONEBERGER, M. BURKHARD(

, C. BIEDERMANN(

, J. KEMMLER, 0. HEIL a n d K.O. GROENEVELD

Institut fiir Kernphysik der Johann-Wolfgang-Goethe-UniversitFit August- Euler-Strasse 6 , 0-6000 FrankfurtIMain 90, F.R.G.

>Shock electrons*: s o n t d e s Blectrons secondaires d e f a i b l e 6nergie (Ee < 20 eV) q u i sont produits par d e s ondes d e choc dans l e plasma d'8lectrons (Wakw) d'un s o l i d e bomdarde avec des ions lourds d e grande v i t e s s e (VP > V ~ o h r ) . 11s ont 6tB observ6s en mesurant l a d i s t r i b u t i o n en angle e t en 6nergie d e s Blectrons secondaires 6mergeant d'une c i b l e mince. Ces Blectrons, q u i traversent le volume du s o l i d e perpendiculaire- ment quant au c6ne d e l'onde d e choc, peuvent 6 t r e u t i l i s 6 s pour Btudier l a d i f f r a c t i o n d e s e l e c t r o n s l e n t s a l a surface du s o l i d e .

Shock e l e c t r o n s a r e ejected i n a d i r e c t i o n perpendicular t o t h e shock f r o n t of t h e wake cone, t h e ion induced c o l l e c t i v e electron d e n s i t y f l u c t u a t i o n i n the e l e c t r o n plasma of t h e s o l i d . They have been observed by measuring doubly d i f f e r e n t i a l secondary e l e c t r o n d i s t r i b u t i o n s from t h i n s o l i d f o i l s in forward d i r e c t i o n . Shock e l e c t r o n s represent a unique i n t e r n a l source of d i r e c t e d secondary e l e c t r o n s and t h u s o f f e r t h e p o s s i b i l i t y t o study the r e f r a c t i o n of e l e c t r o n s a t a s o l i d surface. Nearly planar, smooth s o l i d f o i l surfaces a r e obtained by sputter-cleaning in u l t r a h i g h vacuum. The smoothing can be observed by scanning e l e c t r o n microscopy.

PACS: 79.20 r f , 79.20 nc

The i n t e r a c t i o n of s w i f t ions (Ve L VB) with condensed matter r e s u l t s i n t h e production of i n t e r n a l secondary e l e c t r o n s by t h e primary ions, r e c o i l ions, secondary e l e c t r o n s and, possibly, photons [I]. The i n t e r n a l flux of t h e secondary e l e c t r o n s ( i . e . t h e i r energy- and angular d i s t r i b u t i o n ) depends on both the primary processes and t h e transport of e l e c t r o n s through the s o l i d [2]. The d i s t r i b u t i o n of secondary e l e c t r o n s cutside of t h e s o l i d is strongly influenced by t h e transmission of secondary e l e c t r o n s through t h e surface p o t e n t i a l b a r r i e r . Secondary electron creation is important e . g . f o r ' r a d i a t i o n physics, -chemistry and -biology [I]; fusion research f o r controlled thermonuclear power production (plasma-wall- i n t e r a c t i o n s ) [2]; ion induced desorption of molecules from s o l i d s (Plasma Desorption Mass Spectrometry, PDMS) [3] and nuclear track formation [2]. Therefore, it is d e s i r a b l e t o know both t h e internal and t h e e x t e r n a l energy- and angular d i s t r i b u t i o n of secadarg e l e c t m n s .

The measurement of energy- and angular d i s t r i b u t i o n s of secondary e l e c t r o n s emitted from a s o l i d s u r f a c e y i e l d s valuable information about production, t r a n s p o r t and transmissi- on of secondary e l e c t r o n s . I n p a r t i c u l a r , the measurement of secondary electron

d i s t r i b u t i o n s from t h i n f o i l s in fornard d i r e c t i o n y i e l d s more d e t a i l e d information than measuring secondary electrons emission from t h i c k saxples i n backward d i r e c t i o n . This is due

( 1 ) Now a t Kraftwerk Union (KkU) Offenbach, Germany

2) Present address: U n i v e r s i t y 6f T e m e s s e e K n o x v i l l e , TN 37996 and 6ek Ridge National Laboratory, Oak Ri&e, l d 37831,. USA

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1989219

C2-106 JOURNAL DE PHYSIQUE

t o the following reasons: 1. &electram from binary c o l l i s i o n s a r e mostly ejected in for- ward d i r e c t i o n . 2. Convoy-eleCtra~~ C21 a r e observed strongly peaked in forward direction.

Also, 3. the directed shock electrons a r e ejected in f o r d direction and o f f e r an elegant p o s s i b i l i t y t o study the refraction and transmission of low energy electrons created i n t h e bulk a t the surface potential b a r r i e r .

"It is conceivable t h a t t h e r i c h s p a t i a l shock-wave s t r u c t u r e of electron density fluctuations i n the wake of an ion may have interesting manifestations in experiment and important consequences f o r the behaviour of matter under high-intensity ion-beam bombard- ment." (Ritchie, Echenique, Brandt and Basbas 1979 141)

The collective response of t h e electron plasma of a s o l i d t o a penetrating swift heavy ion manifests as an electron density fluctuation with a x i a l symmetry 141. This dynamic po- larization "wake" sh- t h e c h a r a c t e r i s t i c behaviour of Mach shock waves propagating in cones through t h e s o l i d [53. It has been shown t h a t t h i s leads t o t h e directed emission of shock electrons in a direction perpendicular t o t h e shock wave f r o n t [5,6,7].

Shock electrons can be detected as peak s t r u c t u r e s in the angular d i s t r i b u t i o n s of secondary electrons at energies Eo Z 5 eV, superimposed on t h e "true" secondary electron back ground [7]. Experimental d e t a i l s a r e described in [7,8]. The calculated p r e f e r e n t i a l emission sngle Bem+h-o of shock electrons depends mainly on t h e plasma frequency W P of t h e target and t h e p r o j e c t i l e velocity VP and is given by the Mach relation:

(vo = f(&) is t h e velocity of t h e shock wave). I t has been observed t h a t t h e emissim of shock electrons as well as t h e emission of electrons from the single-electron deexcitation of heavy-ion-induced plasmons, a collective e f f e c t closely related t o wake phenomena C91, depend on surface properties such as t h e coverage with impurities and t h e roughness of the surface [7]. In t h i s paper, we w i l l discuss t h e following topics: 1. Nearly planar s o l i d surfaces obtained by sputtering of t h i n s o l i d f o i l surfaces with noble gas ions, 2. Refrac- tion of low energy electrons a t a planar surface potential b a r r i e r , 3. Application of t h i s model t o the emission of shock electrons: The experimentally observable mean shock electron emission angle becomes a function of 8sm+hgo (given by t h e opening angle of t h e Mach cone of t h e wake) and the height of t h e surface potential b a r r i e r U.

Secondary electron emission depends strongly on both the coverage of s o l i d surfaces with impurities and t h e roughness of t h e surface [8,10]. To study secondary electron emis- sion from clean s o l i d f o i l surfaces, they have t o be sputter-cleaned with noble gas ions (Kr+, 2 MeV) i n u l t r a high vacuum (p S 10-7 Pa). The sputtering process a t t h e entrance surface of t h e beam (backward sputtering) and at t h e e x i t surface in forward direction (transmission sputtering) r e s u l t s i n both a cleaning and a smoothing of the surface by pre- f e r e n t i a l sputtering. The composition of the surface can be supervised by Ion Induced Auger Electron Spectroscopy (IAES), Secondary Electron Spectroscopy (SES), f i t h e r f o r d Forward/

Backwad Scattering Spectroscopy (RFS/FBS) and E l a s t i c Recoil Detection (Em). In addition, it is important t o analyze t h e residual gas with a quadropole m a s s spectrometer, especially t o v e r i f y t h a t a vacuum almost f r e e of hydrocarbons has been obtained [Ill. ABS and SES a r e very s e n s i t i v e methods t o control t h e surface, whereas RFS, FU3S and ERD can be used t o con- t r o l the composition of t h e whole t a r g e t volume. Furthermore, RFS and ERD allow t h e detec- t i o n of l i g h t elements such as H, which can play an important r o l e f o r secondary electron emission [lo]. Before cleaning, t h e f o i l surfaces a r e covered mainly with C, 0 and H, most- l y as hydrocarbons and water [10,121.

The reduction of t h e surface roughness can be observed i n two ways. 1. Visually ( i n

s i t u ) , because the t a r g e t s become mirror-like with a highly enhanced r e f l e c t i v i t y a t the

beam spot, and 2. by Scanning Electron Microscopy (SEM) of t h e t a r g e t surface. In t h e pre-

Fig. 1: &it surface of a Au foil (1000 A) bombard& with no-

a ) Photo ^{l m m} ble gas i m s ( f i + , Z MeV).

a. Photograph, b. S65"1 micrograph of the uncleaned surface region, c. S B micrograph of a sputter- cleaned beam spot.

b)SEM uncleaned surface C) SEM sputter-cleaned surface

Au- Foil (10008.)

sent case, the SEM micrographs have been taken outside t h e s c a t t e r i n g chamber. Therefore, we chose an i n e r t gold f o i l t o demonstrate the smoothing of t h e surface under t h e

assumption t h a t the s t r u c t u r e does not change strongly during the transport from t h e expe- riment t o t h e electron microscope.

F i g . l a shows a photograph of t h e e x i t surface of a 1000 A gold f o i l . Two bright bean spots a r e c l e a r l y v i s i b l e . They have a highly enhanced r e f l e c t i v i t y compared t o t h e envi- ronment. Fig.lb presents a SEM micrograph of t h e uncleaned surface region with pronounced s t r u c t u r e s in t h e ~ l m range. In contrast, F i g . 1 ~ shows a SEM micrograph of a sputter- cleaned beam spot. No s t r u c t u r e s can be detected (resolution b e t t e r than 0.05 w), except f o r micro holes (black, no secondary electron emission). Such micro holes appear when the target has been bombarded with heavy ions f o r an extended period of time. Eventually, the ion bombardment r e s u l t s in t h e destruction of t h e thin f o i l .

The clean, smooth surface can be described i n a g o d approximation by a planar elec- t r o s t a t i c surface potential. When a secondary electron (e-) produced inside t h e s o l i d ap- proaches t h e surface with s u f f i c i e n t kinetic energy t o overcome t h e nearly planar surface potential b a r r i e r U(x), it is subject t o a d i f f r a c t i o n phenomenon (Fig.2). The components of t h e electron velocity vector p a r a l l e l t o t h e surface remain unch8ngd (vi" = ve"), whereas t h e component perpendicular t o t h e surface is reduced according t o the height of the surface b a r r i e r U ( v e l < v i l ) . The kinetic energy of the electron is reduced

(Ee = Ei - U).

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An i n t e r n a l source of d i r e c t e d e l e c t r o n s is needed t o study d i r e c t l y t h e r e f r a c t i o n of low energy secondary e l e c t r o n s produced i n s i d e t h e s o l i d a t t h e i n t e r f a c e between t h e s o l i d and t h e vacuum. Secondary e l e c t r o n s can be produced by a multitude of superimposed produc- t i o n mechanisms. Only two kinds of secondary e l e c t r o n s a r e e j e c t e d and s t r o n g l y peaked i n one d i r e c t i o n :

1. Convoy electrons a r e emitted in t h e d i r e c t i o n of t h e beam, but they are not t h e appro- p r i a t e t o o l t o study surface r e f r a c t i o n phenomena: I t has been shown t h a t t h e i r v e l o c i t y is equal t o t h e p r o j e c t i l e v e l o c i t y [12]. They a r e not accelerated o r decelerated by t h e s u r f a c e p o t e n t i a l b a r r i e r because of t h e i r s t r o n g c o r r e l a t i o n t o t h e p r o j e c t i l e [2,10,

121. In c o n t r a s t , t h e y i e l d of low energy convoy e l e c t r o n s depends on t h e height of t h e surface p o t e n t i a l b a r r i e r [lo]. This resembles t h e r e f l e c t i o n of a wave packet a t a po- t e n t i a l s t e p .

2. Shock elect- move i n a d i r e c t i o n perpendicular t o t h e wake shock f r o n t . The wake is a long-range and nonlocalized phenomenon. Thus, t h e c o r r e l a t i o n between shock e l e c t r o n s and t h e p r o j e c t i l e is n o t s o s t r o n g as i n t h e case of convoy e l e c t r o n s . They represent a unique i n t e r n a l source of d i r e c t e d e l e c t r o n s .

When shock e l e c t r o n s a r e transmitted through t h e surface, they a r e r e f r a c t e d as d i s - cussed i n t h e previous s e c t i o n (Fig.2). The experimentally expected emission angle BCQ- becomes a function of t h e height of t h e s u r f a c e b a r r i e r U, t h e mean energy of t h e shock e l e c t r o n s o u t s i d e t h e s o l i d Ee, t h e " t h e o r e t i c a l " mean emission angle O a t h s o (given by t h e opening angle of t h e wake cone e-th- = f(l)p,vp) ) and t h e t a r g e t tilt angle 6 ( s e e Fig.2):

I t should eyen be p o s s i b l e t o d e t e c t shock e l e c t r o n s from t h e beckside of a s o l i d t a r g e t , i f t h e t a r g e t tilt angle is large, t h e p l a s m frequency is low and t h e p r o j e c t i l e v e l o c i t y is high (say, -t% < 9 eV, vp > 2.5 VB, 6 > 60"). I n t h i s case, 0-- is given by

Fig.3 shows t h e observed mean shock e l e c t r o n emission angle 8- f o r carbon t a r g e t s (h%Z21 eV) a s a function of t h e p r o j e c t i l e v e l o c i t y ^VP (open c i r c l e s , from [7]). The (measured!) mean energy of shock e l e c t r o n s o u t s i d e t h e s o l i d is EoX5 e V (6 = 45"). The dashed l i n e shows Barnthe", neglecting t h e s u r f a c e p o t e n t i a l (U = 0, from [5]). The d o t t e d

l i n e shows 8 e m a w i f U is chosen t o be U = @ + EF (%2O e V f o r c q b o n ) . I n t h e o r e t i c a l con- s i d e r a t i o n s concerning secondary e l e c t r o n emission t h e t o t a l e l e c t r o s t a t i c surface poten- t i a l U is o f t e n chosen as e f f e c t i v e surface p o t e n t i a l [I]. This choice f a i l s in d e s c r i b i n g t h e experimental d a t a , because shock e l e c t r o n s o r i g i n a t e from t h e e l e c t r o n plasma of t h e s o l i d with i n i t i a l e n e r g i e s c l o s e t o t h e Fermi energy, EF. The appropriate choice of an e f f e c t i v e surface p o t e n t i a l i n t h i s case should be U = 1, where @ is t h e e l e c t r o n work function. The same a r m e n t is v a l i d f o r e l e c t r o n s r e s u l t i n g from t h e decay of plasmons, which can be observed a t energies E < - 1 [9,13]. Indeed, t h e expected mean emission angle of shock e l e c t r o n s ( s o l i d l i n e , with @athgo taken from [5] and U = 1 = 5 eV) according t o eq. (2) is i n good agreement with t h e experimental d a t a (earn, open c i r c l e s ) !

The heavy-ion-wake-induced shock e l e c t r o n s can be used t o study t h e r e f r a c t i o n of low energy secondary e l e c t r o n s , which have been produced in t h e bulk of a s o l i d , a t t h e sur- f a c e . There a r e i n t e r e s t i n g questions, as e.g. "Is t h e wake r e f l e c t e d a t t h e surface?",

"How many shock e l e c t r o n s can overcome t h e s u r f a c e p o t e n t i a l b a r r i e r end how many shock

e l e c t r o n s a r e retained by t h e s u r f a c e p o t e n t i a l barrier?". They may p o s s i b l y be addressed

by measuring t h e mean emission angle Bem and energy Ee of shock e l e c t r o n s as a function of

t h e t a r g e t tilt angle 6.

Fig.2: Refraction of shock elec- trons at a planar solid sur- face (see text).

SOLID I w, VACUUM

d---_J

I 1

I

0 2 4 6 8 10

PROJECTILE VELOCITY V p l V B - I

I I I ^I

Another i n t e r e s t i n g experiment may be t h e measurement of t h e a l a r d i s t r i b u t i o n of heavy ion induced photons emitted from s o l i d s . Possibly, photons may be produced by t h e f a s t ion-induced e l e c t r o n d e n s i t y f l u c t u a t i o n s . These photons a r e expected t o be emitted coherently in a p r e f e r e n t i a l d i r e c t i o n depending on t h e opening angle of t h e wake cone

[14]. Their energy is estimated t o be in t h e range 10 eV 5 h Y I 1000 eV. Also, experiments on t h e temperature dependence of secondary e l e c t r o n emission and, i n p a r t i c u l a r , shock e l e c t r o n emission, may give important information about thermal and e l e c t r i c a l t r a n s p o r t p r o p e r t i e s , not only i n metals, but even i n superconductors [Ill.

Fig.3: The observed preferential emission angle 6 ' - of shock

This work has been supported by t h e German Bundesminister fiir Forschung und Techno- l o g i e (BMET) / Bonn, under c o n t r a c t number 06 OF 173/2 Ti476, and DFG / Bonn. One of u s (H.R.) acknowledges a g r a n t from t h e Willkonm S t i f t u n g , Frankfurt am Main.

U=20eV=Uo

- electrons (open circle) as a

o function of the projectile velocity VP. Dashed line:

B-t- neglecting electron refraction at the surface

____---- _{(U =} O), Dotted line: Barn- (U = 20 eV = & + @), Solid line: hkpected preferential mission angle Bern-

@ = @ = 5 eV).

o 6 =45"

JOURNAL DE PHYSIQUE

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