ION CLUSTERS IN THE POSITIVE COLUMN OF He-Co AND Ar-Co DISCHARGES

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ION CLUSTERS IN THE POSITIVE COLUMN OF He-Co AND Ar-Co DISCHARGES

Y. Kaufman, P. Avivi, F. Dothan

To cite this version:

Y. Kaufman, P. Avivi, F. Dothan. ION CLUSTERS IN THE POSITIVE COLUMN OF He- Co AND Ar-Co DISCHARGES. Journal de Physique Colloques, 1979, 40 (C7), pp.C7-31-C7-32.

�10.1051/jphyscol:1979715�. �jpa-00219124�

JOURNAL DE PHYSIQUE Colloque C7, suppl6ment au n07, Tome 40, J u i l l e t 1979, nape C7- 31

I O N CLUSTERS IN THE POSITIVE C O L U M N OF He-Co AND Ar-Co DISCHARGES

Y. Kaufman, P. A v i v i and F. Dothan.

Racah I n s t i t u t e o f Physics, The Hebrew University o f JerusaZem, Jerusalem I s r a e l .

The p o s i t i v e i o n s p e c t r a o f He-CO discharges w i t h and l o m t o r r , causes t h e disappearance o f heavy c l u s - and w i t h o u t small a d d i t i o n s o f O 2 have been d e s c r i - t e r s w i t h n>9,n>6 and n>3 r e s p e c t i v e l y . W i t h a par- bed i n ( 1 ) . Here we r e p o r t on p o s i t i v e mass s p e c t r a t i a l p r e s s u r e o f l 5 m t o r r O 2 t h e i o n c l u s t e r s d i s - i n He-CO-0 and Ar-CO glow discharges i n a w i d e r

2 appear, w h i l e 02+ becomes t h e dominant i o n ( F i g . 3 ) .

mass range and show t h e presence o f i o n c l u s t e r s having a mass o f up t o 188 A.M.U..

The experimental setup i s described i n r e f . 1 . A Qua- d r u p o l e Residual Gas Analyser was used as an i o n de- t e c t o r . P o s i t i v e i o n s p e c t r a were taken from t h e po- s i t i v e column o f a d i s c h a r g e maintained i n a tube o f 3.3cm i n s i d e diameter.

F i g . 1 shows t h e p o s i t i v e i o n spectrum from a discha- r g e i n a m i x t u r e o f He-CO=7.6-0.8 t o r r a t a c u r r e n t o f 10.5mA. The ions a r e CO + and c l u s t e r s o f t h e t y p e CO .CO.Cn (n=0,1,. + . . ^,11).

without 0,

M A S S ( A . M . U . )

Fig. 1. He-CO-0 =7.6-0.8-0 torr

I =10.5mA.

2 D

Adding

t o the He-CO m i x t u r e i n small amounts o f 3mtorr, 6mtorr (Fig.2)

with 6p O2

I

M A S S (A.M.U.)

Fiq..2. He-CO-02=7.6-0.8-0.006 torr

ID=10.5mA.

with 15p 0, I

M A S S ( A . M . U . )

Fig. 3. He-CO-0 =7.6-0.8-0.015 torr; I =10.5mA.

2 D

The p o s i t i v e i o n s p e c t r a o f Ar-CO discharges a t a c u r r e n t o f 25mA a r e g i v e n i n F i g s . 4-6. Figs. 4 and 5 show t h a t a d d i t i o n o f Ar increases t h e r e l a t i v e con- c e n t r a t i o n of i o n c l u s t e r s . F i g . 6 shows t h a t coo-

l i n g t h e tube w a l l s ( ~ ~ = 2 2 0 ' ~ ) leads t o t h e appea- ranee o f the ions CO .CO.Cn + (n=4, ..., ^{8 ) .}

A r - 0.8 torr C o - 0 2 0 torr

,"

>

M A S S ( A . M . U )

Fig. 4. Ar-CO=0.8-0.20 torr

I =25mA; T =300

D W

The observed change i n i o n composition when O2 i s added i s i n accordance w i t h o t h e r authors, (2,3), who argue t h a t i n t h e presence o f oxygen t h e r e a c t i o n C O * C+O i s d r i v e n p r e f e r e n t i a l l y t o t h e l e f t , t h u s decreasing t h e carbon c o n c e n t r a t i o n hence p r e v e n t i n g c l u s t e r formation. Another p o s s i b l e process f o r

+ +

f r e e carbon d e s t r u c t i o n i s C+02 +CO + 0 (4,s).

4

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979715

The carbon l o s s i s i n agreement w i t h o u r r e s u l t s o f t h e disappearance o f C2 Swan bands when O2 i s added t o t h e m i x t u r e (6). The dominance o f 0 ; i o n can e x p l a i n t h e decrease o f E/N accompanying O2 a d d i - t i o n ( I , $ ) . The decrease o f E/N r e s u l t s i n a s h i f t o f t h e e l e c t r o n energy d i s t r i b u t i o n t o lower ener- gies, thus l o w e r i n g carbon p r o d u c t i o n by d i s s o c i a - t i o n o f CO.

5 . o o A r - 4.0 torr

MASS (A.M.U.)

F i g .

5. A r - C ~ 4 . 0 - 0 . 2 0 t o r r ; I = D 25mA;T =300°~.

A r - 3 . 0 t o r r CO-0.15 t o r r

MASS [A.M.U.)

F i g . 6.

Ar-CO=3.0-0.15 t o m ; ID=25mA

T =220°~. W The gradual disappearance o f t h e c l u s t e r s , s t a r t i n g w i t h t h e h i g h e r mass c l u s t e r s , and t h e monotonic de- crease o f t h e i r c o n c e n t r a t i o n s w i t h i n c r e a s i n g mass

( F i g . l ) , leads t o t h e assumption t h a t t h e c l u s t e r s a r e c r e a t e d one from an o t h e r i n t h e three-body process: CO+.CO-C~-,+C+M

CO+- CO.C~+M

where M i s an atom o r a molecule. T h i s process i s c o n s i s t e n t w i t h Thomson's t h e o r y about t e r n a r y c o l l i - s i o n s (7,8). T h i s t h e o r y s t a t e s t h a t t h e presence o f a t h i r d p a r t i c l e M i s necessary i n o r d e r t o r e - move t h e excess energy o f two c o l l i d i n g p a r t i c l e s , so t h a t t h e y end up i n a bound s t a t e . T h i s causes t h e growth o f c l u s t e r d e n s i t y upon adding A r t o a Ar-CO m i x t u r e .

T h e o r e t i c a l c a l c u l a t i o n s (8,9) and experiments (10)

supported by comparing Figs. 5 and 6. Lowering t h e gas temperature (by c o o l i n g t h e tube wal I s ) r e s u l t s i n t h e appearance o f f i v e heavy c l u s t e r s because o f two reasons: 1) P r e v e n t i n g thermal d e s t r u c t i o n by detachment o f t h e high-mass c l u s t e r s . 2) I n c r e a s i n g s h a r p l y t h e f o r m a t i o n r a t e c o n s t a n t s o f t h e c l u s t e r s

(8).

T h i s work was performed w i t h t h e h e l p o f a g r a n t by t h e I s r a e l Commision f o r Basic Research o f t h e I s r a e l Academy o f Science which we g r a t e f u l l y ack- now1 edge.

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