EXCITATION OF 3 3P LEVEL OF He AND 4 2F LEVEL OF He+ WITH ELECTRON BEAMS

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EXCITATION OF 3 3P LEVEL OF He AND 4 2F LEVEL OF He+ WITH ELECTRON BEAMS

A. Luches, V. Nassisi, A. Perrone, M. Perrone

To cite this version:

A. Luches, V. Nassisi, A. Perrone, M. Perrone. EXCITATION OF 3 3P LEVEL OF He AND 4

2F LEVEL OF He+ WITH ELECTRON BEAMS. Journal de Physique Colloques, 1979, 40 (C7),

pp.C7-67-C7-68. �10.1051/jphyscol:1979733�. �jpa-00219322�

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JOURNAL DE PHYSIQUE CoZZoque C7, suppZe'rnent au n07, Tome 40, JuiZZet 1979, page C7- 67

EXCITATION ff 3 P' LEVEL OF He AND 4 2~ LEVEL. OF He* WITH ELECTRON BEAMS

A. Luches, V. Nassisi, A. Perrone and M.R. Perrone University of Lecce, Physics Department, Lecce I t a l y .

INTRODUCTION A b o u t 6 0 % o f t h e beam emer:es i n t o t h e

A s a n i n t e n s e e l e c t r o n b e a m d r i f t s i n g a s c h a m b e r . T h e p h o t o m e t r i c e q u i p m e n t d e - r a r e g a s e s , i t s k i n e t i c e n e r g y i s e f f i c i - t e c t s l i g h t e m i t t e d p e r p e n d i c u l a r l y t o t h e e n t l y c o n v e r t e d i n t o e l e c t r o n i c e x c i t a t i o n beam a x i s . I t c o n s i s t s o f a f i l t e r s y s t e m , o f t h e g a s . E x c i t e d r a r e g a s e s p l a y a n i m - a m o n o c h r o m a t o r , a p h o t o m u l t i p l i e r a n d a n p o r t a n t r o l e i n t h e d e v e l o p m e n t o f e x c i m e r HP 1 8 4 s t o r a g e o s c i l l o s c o p e .

l a s e r s . To i m p r o v e e f f i c i e n c y o f l a s e r s y -

EXPERIMENTAL RESULTS A N D DISCUSSION s t e m s . a d e t a i l e d k n o w l e d g e o f t h e v a r i o u s -

r a d i a t i v e a n d k i n e t i c p r o c e s s e s 1 1 - 3 1 w h i c h I n t e n s i t y d e c a y c u r v e s f o r t h e 3 3 ~

-

2 3 s o c c u r i s n e c e s s a r y . a n d 4 F'

-

3 Z D t r a n s i t i o n s w e r e o b t a i n e d

We s t u d i e d t h e p r e s s u r e d e p e n d e n t p e a k a t He p r e s s u r e s r a n g i n g f r o m t o 104 i n t e n s i t y a n d t i m e h i s t o r y o f t h e 3 8 8 . 9 nm f o r r .

r a d i a t i o n e m i t t e d f r o m t h e 3 3~

-

2 3~

t r a n s i t i o n o f He a n d t h e 4 6 8 . 6 nm r a d i a t i o n e m i t t e d f r o m t h e 4 *F

-

³ 2 ^D t r a n s i t i o n o f

~ e a t p r e s s u r e s r a n g i n g f r o m + t o 7 5 0 0 ( ( a . u . ) T o r r . From t h e d e c a y o f r a d i a t i o n we d e t e r -

m i n e d t h e s h o r t - l i v e d a n d l o n g - l i v e d d e c a y

-

c o m p o n e n t s a n d t h e v a l u e s o f t h e e f f e c t i v e d i r e c t e x c i t a t i o n c r o s s - s e c t i o n o f t h e 3 3 ~ l e v e l a t s e v e r a l g a s p r e s s u r e s . A t l o w

p r e s s u r e s ( p < 4 T o r r ) w e a l s o d e t e r m i n e d t h e q u e n c h i n g c r o s s - s e c t i o n o f t h e 3 3~ l e v e l b y g r o u n d s t a t e He c o l l i s i o n s .

EXPERIMENTAL APPARATUS

Our e l e c t r o n s o u r c e i s a T e s l a - t r a n s f o r - mer a c c c l e r a t o r / 4 / , w h i c h d e l i v e r s a b e a m o f 2 5 kA 500 keV e l e c t r o n s . The beam l e n g t h a n d d e n s i t y a r e 1 5 n s e c (fwhm) a n d 3 . 5 k ~ / c m ~ , r e s p e c t i v e l y .

T h e d r i f t c h a m b e r ( F i g . 1 ) i s s e p a r a t e d f r o m t h e f i e l d e m i s s i o n d i o d e , o p e r a t e d b e l o w T o r r , b y a t h i n ( 75 p m ) m y l a r f o i l , s u p p o r t e d b y a p e r f o r a t e d s t e e l p l a t e w h i c h a c t s a s a n o d e .

u

F i g . 1 : E x p e r i m e n t a l a p p a r a t u s . 1 ) C a t h o d e ; 2 ) p r e s s u r e g a u g e ; 3 ) v i e w i n g p o r t ; 4 ) a n o d e ; 5 ) f i l t e r ; R S - r e f e r e n c e s o u r c e ; MC-monodrromator;OS-osci110sc.

F i g . 2 : T r a c e s s h o w i n g p h o t o m u l t i p l i e r o u t - p u t s a t a ) O . 5 T o r r a n d 3 8 8 . 9 nm;

b ) 5 0 0 0 T o r r a n d 3 8 8 . 9 nm; c ) 3 0 0 0 ~ 0 r r a n d 4 6 9 . 6 nm; d ) 5 0 0 0 T o r r a n d 4 6 8 . 6 nm

.

F i g . 3 : - P e a k i n t e n s i t y o f t h e 3 3F

-

²^3~

t r a n s i t i o n a s a f u n c t i o n o f p r e s s u r e I n t h e r a n g e 0 . 0 3

-

0 . 5 T o r r p e a k i n t e n - s i t y e x h i b i t s a s t r o n g p r e s s u r e d e p e n d e n c e . Above 1 0 T o r r i t i s a l m o s t p r e s s u r e i n d e p - e n d e n t . I n c o n t r a s t , t h e 4 2~

-

3 2 ~ t r a h s - i t i o n p e a k i n t e n s i t y i s v e r y w e a k a t p r e s -

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979733

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sures lower than a few hundred Torrs, then

increases rapidly with pressure. is the electron flux and p is the gas density. The values of nX and oe- are lis- ted in Tab .l.

1

5 10-

Fig.4: Peak intensity of the 4

2P -

³ ⁰

transition as a function of pressure The decaying part of each oscilloscope trace of the photomultiplier output was fit to a curve of the form

I(t) = I1 e~p(-t!~ l) + I2 exp(-t/T 2) where T~ and T~ represent the short lived and long lived decay,respectively; I1 and I2 characterize the respective intensity amplitudes. ~2 relates to radiative cascade contributions/Z/.

TABLE 1: time constants and amplitudes of the 3 3~

-

2 3s transition.

We determined the relation between the density o f 3 3~ states and the photomultiplier output to be n ( t = 0 ) = n ~ = 4 ' - 1 0 ~ ~ 1 ~ ~ , where I, is the intensity at t=O (volts) and T i s the radiative meanlife.

The number density of atoms excited by direct excitation, at t=O is

n X = no/Io ( 11+12T1/T2 )

The effective direct-excitation cross-section for populating the 3 3~ states is

U

-

⁼^,^X / ^BPTl

Fig.5 : 1 1 as a function of pressure ~ ~ In the range 0.07C4 Torr, l/rl is a lin- ear function of gas pressure.Such behavi- our could be explained a s follows. For pe0.03 Torr the decay is controlled only by spontaneous emission

He (3 3 ~ ) + H e (2 3s) + hv ( 1 ) At higher pressures (p 4 Torr), collisional deactivation by ground state atomic H e is also important

He (3 3p) + He + He(2 3 ~ ) + He ( 2 ) Since processes (1) and (2) are the most probable for pS4 Torr, 1 / can be expres- ~ ~ sed as 1 / ~ , 5 1 / ~ + kp. k is related to the quenching cross-section uc by the relation k = u,<v>,

-

where <v> is the Boltz- mann average velocity of He atoms.From the slope and the intercept of the graph of Fig.5 we get k=2.6510-10 (cm3/sec), r=98 nsec and hence ~ ~ = 2 . 1 0 - ~ 5 cm2.

For p>4 Torr the last relation does not describe well our experimental results.

However, we succeded in determining the short and long lived decay components and the effective direct excitation cross- sec- tion up to 7500 Torr.

REFERENCES

1 1 1 - R . ~ . A n d e r s o n , R . H . H u g h e s , P h y s . R e v . ~ ,