MOLECULAR DISSOCIATION ON BOMBARDMENT BY ELECTRONS OF AN ENERGY LOWER THAN THE IONIZATION POTENTIAL

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MOLECULAR DISSOCIATION ON BOMBARDMENT BY ELECTRONS OF AN ENERGY LOWER THAN

THE IONIZATION POTENTIAL

I. Morozov, V. Talrose

To cite this version:

I. Morozov, V. Talrose. MOLECULAR DISSOCIATION ON BOMBARDMENT BY ELECTRONS

OF AN ENERGY LOWER THAN THE IONIZATION POTENTIAL. Journal de Physique Colloques,

1979, 40 (C7), pp.C7-109-C7-110. �10.1051/jphyscol:1979754�. �jpa-00219460�

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JOURNAL DE PHYSIQUE

MOLECULAR DISSOCIATION ON BOMBARDMENT BY ELECTRONS OF AN ENERGY LOWER THAN THE IONIZATION POTENTIAL

1.1. Morozov, V.L. Talrose.

I n s t i t u t e o f Chemical P h y s i c s o f t h e Academy o f S c i e n c e s o f t h e U.S.S.R., Moscow, U.S.S.R.

One of t h e fundemental t a s k s i n g a s d i s c h a r g e i s t h e dcterrcirination of t h e s t r i c t l y n e u t r a l dissociative elementary s t e p s i u t h e bombardrrlent of niolecules by low energy e l e c t r o n s ,

O f g r e a t i i l t e r e s t i s t h e bombarding by e l e c t r o n s w i t h e n e r g i e s lower than t h e i o n j z a t i o n p o t e n t i n l of t h e molecule, which e x c l u d e s t h e appearance of p o s i t i . v e i o i ~ s . There i s much i ~ ~ f o r m a t i o n on gene- r a t i o n of n e g a t i v e j-ons, but t h e s t r i c t l y n e u t r a l ways of n o l c c u l t decay a r e unknown.

The e x p e r i ~ n e n t a l a p p a r a t u s f o r de- t e r r a i n a t i o n of t h e depelideilce of t h e r e l a t i v e f r a g r ~ i e n t a t i o n c r o s s - s e c t i o n of gaseous p r o d u c t s on e l e c t r o n e n e r g i e s i s sh.own i n Pig.1. The product y i e l d s of t h i e p r o c e s s can a l s o be measured by t h i s technique.

The e l e c t r o n flow from f i l a m e n t 1 f o c u s s e d by a m i r r o r 3 bombards t h e g a s molecules i n chaniber 4 through t h e . c o n t - r o l l i n g dfaphragrfl? Some of Che n e u t r a l fragments formed ' p e n e t r a t k i n t o chamber 7 where t h e y aFe i o n i z e d by a n e l e c t r o n beam froin t h e second f i l a m e h t 8, The i o n s formed a r e e x t r a c t e d from t h e chamber by e l e c t r o d e 9 and focused by an e l e c t r o s t a -

t i c l e n s ( d i a p h r a e 10 and 1 1 ) .

!:lith t h e e l e c t r o l l energies used t h e p o s i t i v e i o n s -do riot f o& ?II . t h e f i r s t . chamber and t h e :negative- i o n s and , e l e c t - r o n s a r e t r a p p e d by g r i d s 5 and 6. The e l e c t r o n c u r r e n t o f t h e f i r s t chamber i s lnoduiated by -a frequency' of 3 0 ,Hz. The a3npILt~ide -of v o l t a g e accelercitirlg t h e e l e c t r o n s can Ghange--from 3 t o -12 v.arid-- t h e a q e r a g e emission c u r r e n t .from 58 .to 300 _changes

r'

A . The '-ioniki.ily e l e c t r o n

_%ram

'energy

' 1 5 t o 120 v when ' t h e ,2,5 mA emission czirrcnt is constant', '

The niodulated i o n c u r r e n t p a s s e s from t h e mass a n a l y z e r Yo t h e secondary e l e c t r o n multiplier. and i s r e c o r d e d by .a l o c k - i n - a m p l i f i e r .

The f r a g m e n t a t i o n of C H moleCules 3 8

was. s t u d i e d a t f i r s t -as i t i s very '~iinpor- t a n t f o r plasma c1ieniisti.y. Tlie B l t e r n a t i ~ i g corrponent of t h e i o n c u r r e n t on nodula- t i o n of t h e e l @ c : t r o n , c u r r e n t in t h e f i r s t chambe$. carmot be a s c r i b e d , e n t i r e l y ^-t o n e u t r a l fragments. The ~mbdulated.~~elec.t- roils i n t h e f r i r s t . c h % ' o e r cbnvert:so?le

of

t h e i n i t i a l molecule,s t o n e g a t i v e i o n s and n e u t r a l fragments and t h u s modulate t h e p a d i a l p r e s s u r e of t h e i n i t i a l g a s

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979754

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in the first chamber and consequently in the second. This modulation of gas density (IBGD) is very weak compared to the total pressure of the gas. But its absolute value is of the same' order as that for modulation of the partial pressure of neutral fragments. Fragmentation of some of the initial molecules in the first chamber results in a proportional decrea- se in the intensity of all mass-spectral lines of these molecules in the second chamber. Therefore IrlGD is synchronous and antiphase to the pressure modulation of neutral fragments. The UGD contribution to the alternating component of ion current for every mass-spectral line is proportional to the intensity of this line in an ordinary mass-spectrum. The experimental procedure was as follows:

the ratio of the intensities of alternating ( I ~ ) and direct (I= ) ion currents

44

4 4

in molecular propane line, m/e 44, has been determined at a fixed electron energy. This ratio was the measure of propane fragmentation in all the channels.

Fig.2 shows the relative fragmentation cross-section vs electron energy.

The fragmentation cross-section along the channel C3H8 % C3H7

+ ^H

^{as a}

function of electron energyis given in Fig.2..1

(C

/ q r vs electron energy

C3H7

threshold at 5.5 eV). Comparison of the energy dependences (Fig.2.1 and 2.2) shows that C

H

formation is of a reso-

3

7

of the above with the energy dependence of convincingly reveals that the hydrogen yield within the range 6-10 eV is of a resonant nature and its threshold is 6 eV at the maximum of 8 eV. Excitation of the triplet level with an energy of 6 eV seems to be the primary process.

The data given in the paper on pro- pylene dissociation to neutral fragments show that dissociation occurs by excitation of triplet levels,

This technique b s been used also for studying the ionization of argon meta- stable neutral states by electron impact at energies 20-75 eV.

1. N.I.Butkovskaya, lT,I?.Larichev, 1.O.Lei- punskii, I.I.hlorozov, I.Yu.Razuvaev, V.L.!Palrose, Dskl. Akad. Nauk SSSB, v.233, N 2, 398 (1977)-

nant nature over the range 5.5-9.5 _Theelectron energy dependence of eV,

I I F"

%2/Cfr is shown in Fig.2.3. Comparison

1