ELECTRON ATTACHMENT IN SO2

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ELECTRON ATTACHMENT IN SO2

J. Moruzzi, V.K. Lakdawala

To cite this version:

J. Moruzzi, V.K. Lakdawala. ELECTRON ATTACHMENT IN SO2. Journal de Physique Colloques, 1979, 40 (C7), pp.C7-11-C7-12. �10.1051/jphyscol:1979705�. �jpa-00219064�

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JOUBNAI; DE PHYSIQUE CoZZoque C7, suppZ6ment au n07, Tome 40, JuiZZet 1979, a p e C7- 11

ELECTRON ATTACHMENT IN SOZ:

J.L. Moruzzi and V.K. Lakdawala.

Department o f EZectricaZ Engineering and EZectronics, University of LiverpooZ, LiverpooZ L693BX. U.K.

INTRODUCTION

Although there has been much i n t e r e s t i n the Pure SO,

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e l e c t r i c a l properties of SO2 t h e r e has been very ⁱ ^I >....'.. ... ^m:.:.^t

l i t t l e work carried out on t h e electron attaching

w ...

properties of t h i s gas, or on t h e mobility of the

negative ion species produced as a r e s u l t of t =O

= L V ci'torr-' p = 4.0 t a r

attachment processes.

Time scole 20 ps /dot

Some recent s t u d i e s by Dupuy ( I ) have shown

F 16.1.

t h a t a g r e a t l y increased negative ion concen- t r a t i o n e x i s t s i n t h e atmosphere when SO2 poll- ution is present. The present investigation has measured t h e attachment c o e f f i c i e n t (a / ) f o r

a P

1 < E/p < 80 v.cm-' t o r r - I , and h a s found evidence f o r a t h r e e body attachment process f o r values of E/p c. 10 and f o r a d i s s o c i a t i v e attachment process f o r values of E/p > 10 ~.cm" torr-I. Measurement of t h e mobility of t h e negative ions formed ind- i c a t e t h e presence of a t l e a s t t h r e e ions with reduced mobility values of 0.69, 0.62 and 0.55 cm ² V-" sec-" .

APPARATUS

A conventional pulsed photo cathode d r i f t - tube ( 3, was used t o carry out these measurements.

A t y p i c a l current waveform a f t e r s i g n a l averaging is shown i n Figure 1. The attachment c o e f f i c i e n t f o r the gas i n t h e d r i f t tube can be obtained from such waveforms by estimating the electron and ion components,bf t h e waveforms using t h e a r e a method due t o Pack and Phelps ( 3 ) . Also from d a t a such a s t h a t shown i n Figure 2 i t is possible t o estimate t h e ion t r a n s i t time and thus obtain ion mobility values. Using a d r i f t tube

RESULTS AND CONCLUSIONS

For values of E/p 1 1 0 V cm" t o r r -1 t h e attachment process was found t o be independent of pressure i n d i c a t i n g a two body d i s s o c i a t i v e process

SO2 + ^e ^-c ^0-+ SO (1

or

SO2 + e --c SO- + 0 ( 2 )

The values of , a /p determined f o r t h i s process rose a

from about 0.01 cm" torr-I a t 10 V cm-I torr-I a peak of 0.075 a t an E/p of 4 V cm-I torr-I and gradually f e l l t h e r e a f t e r , s e e Figure 2. Forvalues of E/p < 10 V cm-I torr-I i t was found t h a t a / ²

a P

----a p = 3 torr

I ^-.- ^p⁼¹^torr I

C C C -

* -

FIG. 2.

I 1 1 I 1 1 1 1

was pressure independent i n d i c a t i n g t h a t a t h r e e m a s s spectrometer system(4) t h e ion species body attachment process is occurring.

present under various conditions were determined The only other recent experimental attachment and w i l l be discussed below. study i n t h i s gas has been carried out by

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979705

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Schl~mbohm(~) and he r e p o r t s attachment coef f i c - i e n t s i n general agreement with t h e values reported here. A pressure squared attachment process is a l s o reported although t h e E/p range over which t h i s observed i s not mentioned. E a r l i e r work(6) a l s o in- d i c a t e s an attachment minimum a t 10 V cm 1- torr-'.

The values of t h e ion mobility obtained i n these experiments a r e shown i n Figure 3. It can be seen t h a t a t l e a s t t h r e e d i s t i n c t ion s p e c i e s a r e present. An important point t o note i n these r e s u l t s is t h a t t h e r e is no d i s t i n c t change i n ion

mobility as attachment changes from t h e d i s s o c i a t i v e MOSS

Since all three major ions appear under a given process t o the t h r e e body process. It would t h u s

condition i t must be concluded t h a t i n Figure 1 appear t h a t the i o n s present a r e not the ions prod-

uced by t h e attachment process but r a t h e r ions t h a t s e v e r a l ion species a r e present even though t h e ion a r e c l u s t e r s of SO, and t h e o r i g i n a l ion. I n o r d e ~ current fall-off is sharp. IChus t h e mobility

values quoted c l e a r l y r e f e r t o a family of ion species. A s t h e pressure increases i t is expected t h a t even l a r g e r ion c l u s t e r s w i l l . be formed.

(1 ) Dupuy J. P r i v a t e Communication (1977

( 2 ) Moruzzi J.L. 1967 Rev, Sci. Insts. 38, p 1284 -1285.

(3) Chanin L., Phelps, A.V. and Biondi M.A. 1962 Phys. Rev. 128, 219-230.

(4) Coxon P. and Moruzzi J.L. 1977 3. Phya. D.

10 969-977-

-7

V. cm-' t o r i

'

⁽⁵⁾ schlumbohm H. 1962 z. P ~ Y S . 166, ^192-206'

FIG. 3 . (6) Bradbury N.E. an& T a t e l H.E. 1934 J. Chem t o resolve t h i s SO2 w a s studied on the mass spec- Phys. 2 835-840-

trometer d r i f t tube system used by Coxon and ACKNOWLFDGENENTS

( 4 ) * A s p e c t r a of t h e i o n s obtained a t This work w a s c a r r i e d out a t Liverpool a s poruzzi

-1 p a r t of t h e Franco-British Collaboration on Space p = 0.6 t o r r and an E/p = 8 V cm torr-I i s shown

Charge E f f e c t s i n Breakdown Phenomenon i n Electro- i n Figure 4. Under a l l the conditions investigated

t h e i o n a t mass 144 amu t e n t a t i v e l y i d e n t i f i e d a s negative Gases. We g r a t e f u l l y acknowledge many u s e f u l discussions with Professor J. Dupuy 0-. SO^)^, w a s dominant. Other i o n s t h a t a r e

r e a d i l y i d e n t i f i e d a r e o-.so~(~o m u ) , . o r 0-. (University of Paul with whom t h i s collaborative

-

(SO ) (208 m u ) and SO-. (SO2I2 (176 amu).

2 3

venture is being c a r r i e d out.