DESORPTION YIELDS USING keV POLYATOMIC PROJECTILES

(1)

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DESORPTION YIELDS USING keV POLYATOMIC

PROJECTILES

M. Blain, S. Della-Negra, H. Joret, Y. Le Beyec, E. Schweikert

To cite this version:

(2)

Colloque C2, suppl6ment au

Tome 50,

f6vrier 1989

DESORPTION YIELDS USING

keV

POLYATOMIC PROJECTILES

M. G. BLAIN, S

.

DELLA-NEGRA'

,

H. JORET*

,

Y. LE BEYEC*

and

E.A. SCHWEIKERT

Texas A and M University, Department of Chemistry

,

College Station,

TX

77830-3144,

U. S.A.

" ~ n s t i t u t

de Physique NuclBaire, B.P. n O 1 ,

F-91406 Orsay Cedex, France

R B s u n d . :

N o u s a v o n s e t u d i O l e e m i s s i o n d'ions

n e g a t i f s

.4

partir

d e c i b l e s

s o l i d e s o r g a n i q u e s b o m b a r d e e s par d e s i o n s m o l e c u l a i r e s e t d e s a g r e g a t s

atomiques. N o u s p r e s e n t o n s ici

les r e s u l t a t s o b t e n u s a v e c u n e c i b l e d e

phenylalanine. N o u s a v o n s u t i l i s e

l e s p r o j e c t i l e s o r g a n i q u e s d e m a s s e

73 +

CSi ( C H

)+I

,

147CSi (CH3

1

30Si

(CH3)+ I,

309 tion

moleculaire

d u

c o r o n e n e

+3

+

CZ4Hi2

,

598 Cle d i m s r e d u c o r o n e n e

₊

2 ( M - H )

I,

₊

e t

l e s i o n s atomiquffi e t

₊

polyatomiques d e m a s s e 1 3 3 CCsl

,

393

CCsZIl,

653

CCs3IZ1

.

C e s

i o n s

p r i m a i r e s ont e t O p r o d u i t s e n bombardant u n e c i b l e d e c o r o n h e e t d e C s I par

d e s p r o d u i t s d e fission d u 252~f. E n s u i t e

ces ions ont Bt12

a c c e l e r l s

et

f o c a l i s e s s u r l'&chantillon

A etudier, D e s m e s u r e s d e t e m p s d e vol sophisti-

q u b e s d e s ions p r i m a i r e s e t s e c o n d a i r e s o n t &te

execut&es

avec u n s y s t G m e

d'acquisition d e d o n n e e s speciales. TOUS l e s t e m p s d e vol o n t et12

enregistrfis

s i m u l

tanement

.

L e rendement d e s i o n s mol@culaires d e phenylalanine

a etrS e t u d i e e n fonction

d e 1'Bnergie e t d e la m a s s e d e s projectiles.

U n e g r a n d e augmentation

d u

rendement est o b s e r v e e avec l'energie e t la masse.

Abstract

:

W e h a v e studied t h e n e g a t i v e s e c o n d a r y ion e m i s s i o n f r o m s o l i d

organic t a r g e t s bombarded by molecular ions a n d c l u s t e r ions.

A s

a n e x a m p l e w e

present h e r e t h e r e s u l t s obtained w i t h t h e c o m p o u n d phenylalanine-

W e h a v e

+

used organic p r o j e c t i l e s o f m a s s 7 3 CSi ( C H 1 1

,

147CSi (CH31JOSi (CH3)Zl+l

300

+

Cmolecular ion o f c o r o n e n e C

H

3 + , 598

C:oronene

d i m e r 2(M-HI I

,

and atomic

24

iZ

+

and polyatomic i o n s of m a s s 1 3 3 CCsl,

393

CCsZIl

,

653

CCs31Z3

.

T h e s e primary

ions h a v e been produced in t h e bombardment o f t a r g e t s of c o r o n e n e and CsI by

fission fragment from a 2 5 Z ~ f

source. T h e y w e r e accelerated

and

focussed

o n

t h e s a m p l e target. Sophisticated t i m e o f flight m e a s u r e m e n t s of

t h e primary

and secondary ions h a v e been performed with a special d a t a a c q u i s i t i o n system.

All t h e t i m e o f flight m a s s s p e c t r a w e r e recorded a t one.

T h e s e c o n d a r y molecular ion yield of t h e phenilalanine

(M-HI- = 164

h a s been

studied a s a function o f t h e e n e r g y o f impact a n d

of

t h e m a s s o f t h e

projectile. A l a r g e enhancement o f t h e yield with t h e m a s s and t h e e n e r g y

i s

observed.

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C2-148

JOURNAL

DE

PHYSIQUE

I

NTKODUCTI

ON

Recently there h a s been much interest in the secondary ion emission from

materials induced by cluster ion bombardment

/ I / .

The incident cluster ions

have velocities in three different regimes

:

greater than the Bohr velocity,

v

=

0.22 cm/nsec (fast), approximately equal to vo,

and much

less than

0

v

(slow). The reason for the interest in cluster induced ion emission is the

0

increased secondary ion yields when compared with single atom ion projectile

yields of the same charge state. In this paper, w e describe the experimental

procedure for obtaining quantitative yields of negative ions desorbed by slow

CkeV)

incident positive cluster ions and

w e

present some recent results for

the yield of an organic compound phenylalanine desorbed by the molecular

ion

of coronene and by

C s I

cluster ions.

EXPERIMENTAL

The experiments were performed a t the Institut d e Physique Nucleaire in

Orsay, France. The experimental approach for generating and accelerating

various cluster ions for use a s projectiles is described

elsewhere 121.

Briefly, the cluster ions

(as

well

as

single

atom ion

species)

a r e

produced

by

fission fragment induced desorption from the appropriate material deposited on

a metallic surface [aluminized mylar)- T h i s surface

i s

a t a positive potential

U*.

The desorbed ions (called primary ions in the following) are accelerated

by an extraction grid at ground and travel in a field free region of 1 5 cm. At

the end of this region the primary ions bombard a sample surface titled by 20"

with respect to the primary beam direction ion. Under impact, electrons and

negative ions are emitted from the surface (biased a t a negative potential

U 2 ) ,

accelerated into a second field free region, and detected by a set of

channel plates (CEMA2 in Fig.

1).

The start s.igna1 is given by the

complementary fission fragment in the detector CEMAI.

CEMA 1 start signal

T 0. F primary

ions

target CEMA 2

electron

M-

Fiqure

1

:

Experimental set up.

-

u2 secondary ions

The electrons are used a s stop signals for the primary ion time of flight

(TUF) measurements.

I n

addition the stop signals generated by the serondary

negative ions are also recorded. The spectrum shown in Fig.

223,

measured

with

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F i q u r e

2

:

a) T o t a l TOF spectrum : p r i m a r y i o n s from a coronene t a r g e t and secondary i o n s from a p h e n i l a l a n i n e t a r g e t (see t e x t 1 .

5 7 0 b > Secondary i o n TOF spectrum generated

by t h e impact o f mass

M + = 3 0 0

on the

2 IM-HI'= 598 4 2 8

p h e n i l a l a n i n e t a r g e t . A coincidence

g

2 8 5

IM-Hf=164- time window has been s e t (on mass 300) i n t h e spectrum 2a.

143

O 2 0 1220 1620 2020 T irne (lns/channell

-

t h e TOF spectrum o f t h e p r i m a r y i o n s ( s t a r t w i t h f i s s i o n fragments from

252

C f and s t o p s from e l e r t r o n s ) ,

-

t h e TOF spectrum o f t h e secondary i o n s (same s t a r t and s t o p s from secondary i o n impacts on CEM.421.

The f i r s t p a r t o f t h e TOF spectrum l o o k s l i k e a r e g u l a r TOF kpectrum o f

+

coronene desorbed by f i s s i o n fragments w i t h t h e presence o f t h e peaks

H

,

+

HZ

W9

. .-

M+

=

300 and

Z(M-H)+

=

598.

However almost every peak can be considered as a time o r i g i n f o r t h e secondary i o n time o f f l i g h t spectrum. For example,

+

the f i r s t peak a f t e r

M

=

300 corresponds t o t h e emissions o f

H-

i o n s due t o

+

the impact o f

M

= 300 on t h e p h e n y l a l a n i n e sample. The time d i f f e r e n c e hetween

M+

= 309 and another peak (see F i g .

2al

i n t h e spectrum a l l o w s a l s o t o i d e n t i f y t h e presence o f t h e molecular i o n s o f p h e n y l a l a n i n e

< c M - H ~ -

=

1 6 4 )

+

desorbed by impact o f

M

= 300- Only a few mass assignments can be made i n t h i s way although t h e spectrum, which is v e r y complex, c o n t a i n s much more i n f o r m a t i o n . Several secondary i o n TOF peaks a r e masked by t h e background o f t h e p r i m a r y i o n TOF.

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C2-150

JOURNAL

DE

PHYSIQUE

Time llns/chonnel l

923

a) Total TOF spectrum

:

primary

i o n s from a C s I target a n d

secondary ions from a pheny-

lalanine target.

b) Secondary ion T O F spectrum

with a time o r i g i n c o r r e s p p -

ding t o t h e impact o f C s I o n

692

t h e sample.

w ~ r ~ m a r y ion

b ,

C h 'I= 393

c h a n n e l s o n a n y peak of

interest

in

t h e total TOF spectrum.

By

t h i s

coincidence counting method, individual s p e c t r a a r e extracted f r o m t h e total

TOF spectrum in real t i m e during t h e d a t a acquisition,

A s

examples, F i g u r e s Z b

and 2c s h o w spectra o f ions desorbed from phenylalanine target d u e t o t h e

+

impact by t h e molecular ions o f c o r o n e n e

(M

=

300)

and d u e t o t h e impact

by

t h e dimer ( ~ c M - H ~ + =

5 9 8 ) .

t h e t w o s p e c t r a a r e similar but

t h e secondary

ion

yields (with respect t o the number o f primary ions bombarding t h e sample) a r e

different.

A m o r e complex e x a m p l e i s s h o w n in Fig.

3 a and 3b.

T h e phenylalanine

target i s bombarded by cesium ions and b y cesium iodide c l u s t e r s (the c o r o n e n e

deposit h a s been replaced b y a C s I deposit -see Fig.

I - ) .

Fig.

3 a s h o w s

a

total T O F spectrum- T h e p e a k s a r e d u e t o t h e detection of e l e c t r o n s a s stop

+

s i g n a l s a s well a s secondary ions generated by impacts o f C s

,

C s I

,...

and

other primary ions o n t h e phenylalanine sample. In Fig. 3 b t h e t i m e origin

i s

+

t h e instant o f impact o f C s

I

o n l y and

t h e corresponding t i m e o f

flight

-

spectrum o f phenylalanine

i s observed.

T h e molecular

ion

(M-HI

of

+

phenylalanine desorbed by C 5 I

i s clearly seen.

T h i s n e w method o f t i m e o f flight measurement a l l o w s t o extract several

spectra from o n e s i n g l e spectrum. T h e experimental conditions a r e kept

the

s a m e during t h e experiment (same target, s a m e detection...).

t h e secondary ion

y i e l d s d u e t o the bombardment o f t h e target by different k i n d s o f projectiles

c a n b e extracted f r o m o n e experiment,

T h e emission yield for a n ion of m a s s M is g i v e n by

:

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measured i n c o i n c i d e n c e w i t h a t i m e window s e t on t h e background c l o s e t o t h e p r i m a r y ions.

NpI and NpIEBGl a r e t h e number o f counts i n t h e t i m e window s e t on t h e peak and t h e backgroung.N pI-NpICBG3 i s t h e number o f s t a r t events f o r a c o r r e l a t e d spectrum. I t i s assumed i n r e l a t i o n ( 1 ) t h a t we have s i n g l e impact

+

on t h e sample. T h i s i s n o t t h e case when Cs i o n s a r e e m i t t e d from t h e CsI d e p o s i t by t h e f i s s i o n fragments and t h e r e f o r e a c o r r e c t i o n must be hade on

+

t h e number NpI t o o b t a i n t h e t r u e number o f p r i m a r y i o n s o f Cs

.

We have

+

determined t h e number o f Cs e m i t t e d by f i s s i o n fragments from t h e CsI t a r g e t . The secondary i o n y i e l d o f (M-H)- due t o t h e impact o f p r i m a r y Cs i o n s has been measured as a f u n c i t o n o f g r i d t r a n s p a r e n c i e s w i t h t h e g r i d s s e t between the CsI t a r g e t and t h e p h e n y l a l a n i n e t a r g e t . T h i s method i s s e n s i t i v e and w e have found a value o f 2.0k0.2 i o n s e m i t t e d p e r + i s s i o n fragment. T h i s r e s u l t i s i n agreement w i t h t h e v a l u e found a t Orsay /4/ and a l s o by t h e Darmstadt group I S , & . / .

I n t h e present experiment t h e p o s i t i v e p r i m a r y i o n s a r e a c c e l e r a t e d i n t h e space between t h e sample and t h e e x t r a c t i o n g r i d . Therefore t h e angle o f i n c i d e n c e B' v a r i e s according t o t h e r e l a t i o n :

U i s t h e a c c e l e r a t i o n p o t e n t i a l o f t h e p r i m a r y i o n s U i s t h e a c c e l e r a t i o n p o t e n t i a l o f t h e secondary i o n s

B

i s t h e t i l t angle between t h e sample s u r f a c e and t h e d i r e c t i o n o f p r i m a r y beam.

The v a r i a t i o n o f B' i s r e l a t i v e l y s m a l l under our experimental c o n d i t i o n . I t has been v e r i f i e d t h a t a t a constant energy o f impact (by v a r y i n g Ui and U2), t h e i n f l u e n c e o f t h e B ' v a r i a t i o n on t h e secondary y i e l d i s . n e g l i g i b l e .

RESULTS

AND Df

SCUSSION

F i g u r e 4 shows t h e e f f e c t o f p r i m a r y i o n v e l o c i t y on t h e (M-H)- y i e l d o f phenylalanine due t o i n c i d e n t coronene molecular and dimer i o n s (Fig. 4a) and due t o i n c i d e n t cesium and cesium i o d i d e c l u s t e r i o n s (Fig. 4b). The x a x i s s c a l e i s given i n keV p e r mass u n i t (=kv2) and t h e r e f o r e comparison o f secondary i o n y i e l d s can be made e a s i l y a t t h e same v e l o c i t y f o r v a r i o u s p r o j e c t i l e s . The bombarding e n e r g i e s ranged from 7.86 keV t o 28 keV. For a l l curves shown, t h e l i n e s drawn through t h e d a t a p o i n t s a r e t o a i d t h e eyes. E r r o r b a r s f o r t h e experimental p o i n t s a r e l e s s than 1 0 % i n a l l cases. For t h e experiment u s i n g coronene as t h e p r i m a r y i o n source, p r i m a r y i o n s o f mass 73 CSi(CH ) J and mass 147 CSifCH OSi(CH ) + 3 were a l s o observed f o r t h e i r

3 3 3 3 3 2

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C2-152

JOURNAL DE PHYSIQUE

I I 1 T a r g e t

a

1 -

₂_{IM-HI' =}₅₉₈ _{Pheny l a l a n i n e -} 147' 7 3' I I I

b

1

P h e n y l o l a n i n e

-

Cs21= 393. Cs*= 133' A A

Fiqure

4 a) Secondary ion yield of the molecular

ion

(M-H)-

of phenylalanine a s a

function of the incident mass unit t=kvZ) o f the projectiles

(coronene ions

and dimers). The secondary ion,

yields can be compared easily at the same

velocity.

b) same a s above. The projectiles are C s ions and CsI cluster ions.

+

enhancement by a factor of around 20 between Cs and Cs

I

a s projectiles. For

cluster ions or molecular ions the rate of increase with

the projectile

velocity is also very important. The observed yields for other secondary

ions

and the trends are the same.

CONCLUSION

The use of polyatomic

ions a s projectiles to induce secondary ion

emission from organic solids is found to b e very interesting. I1 is shown that

single ion counting technique and coincidence methods are particularly useful

for these experiments.

The results presented

here show that increased

secondary ion yields can

be

achieved for the same amount of energy deposited

by simply using a polyatomic, rather than a monoatomic, primary ion. It is not

yet clear whether the increased yields of phenylalanine seen with the cesium

iodide species and the coronene ions are due to increased mass or an increase

in the number of particles in the projectile. In addition, the existence

o f

coherent effects in the desorption process remains to be determined

a s the

correct experimental observable remains to b e identified,

e . g . Y / t #

of

constituents in cluster), Y/(amu of projectile),

Y / C

( d E / d ~ ) ~ ,

etc.

Further

data analysis

is

in progress in order to understand

the role of cluster

velocity, mass, and constituent number in the yields of secondary ions.

R e f e r e n c e s

/ 1 /

See for example

:

J.P.Thomas, P.E.Filpus-Luyckx,

M-Fallavier and

(8)

5. Johar and D.A.Thompson, Surf. Sci.

9 2

(1979)

319

;

W-Reuther, Anal.

Chem.

5 9

(1987) 2081.

/2/

M.G.Blain,

E.A.Schweikert and

E.F.Da Silveira, to be published in these

proceedings.

/ 3 /