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Submitted on 1 Jan 1983
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THE IMPORTANCE OF POSITIVE IONS IN OPTOGALVANIC DETECTION WITH THE
THERMIONIC DIODE AND THE GLOW DISCHARGE LAMP
L. Pendrill, M. Pettersson, U. Österberg
To cite this version:
L. Pendrill, M. Pettersson, U. Österberg. THE IMPORTANCE OF POSITIVE IONS IN OPTOGAL- VANIC DETECTION WITH THE THERMIONIC DIODE AND THE GLOW DISCHARGE LAMP.
Journal de Physique Colloques, 1983, 44 (C7), pp.C7-489-C7-496. �10.1051/jphyscol:1983748�. �jpa-
00223305�
THE IMPORTANCE OF P O S I T I V E IONS I N OPTOGALVANIC DETECTION WITH THE THERMIONIC DIODE AND THE GLOW DISCHARGE LAMP
L.R. P e n d r i l l , M. P e t t e r s s o n a n d U . U s t e r b e r g
Fysiska I n s t i t u t i o n e n , ChaZmers Tekniska HijgskoZa, 5-412
96Giiteborg, Sweden
R6sum6 - L ' i m p o r t a n c e des i o n s p o s i t i f s dans l e f o n c t i o n n e m e n t de deux d6tec- t e u r s o p t o g a l v a n i q u e s : d i o d e t h e r m o i o n i q u e e t lampe a decharge lumineuse c o n t i - nue, e s t d i s c u t e e . Les e f f e t s du temps de r6ponse de ces d e t e c t e u r s p o u r une d e t e c t i o n synchrone s o n t d i s c u t e s e t e x p l i q u e s grace a des r e s u l t a t s e x p e r i - mentaux obtenus s u r l e s signaux o p t o g a l v a n i q u e s en f o n c t i o n du temps e t de l a frequence s p e c t r a l e .
A b s t r a c t - The importance o f p o s i t i v e i o n s i n t h e f u n c t i o n n i n g o f two o p t o - g a l v a n i c d e t e c t o r s , t h e t h e r m i o n i c d i o d e and t h e d.c. glow d i s c h a r g e lamp i s discussed. The r e l e v a n c e o f t h e f i n i t e response t i m e o f t h e d e t e c t o r s t o d e t e c t i o n w i t h l o c k - i n a m p l i f i e r s i s e x e m p l i f i e d w i t h some e x p e r i m e n t a l r e s u l t s on t i m e - and s p e c t r a l l y - r e s o l v e d o p t o g a l v a n i c s i g n a l s .
1. I n t r o d u c t i o n
O p t o g a l v a n i c d e t e c t i o n o f t h e a b s o r p t i o n o f l i g h t by atoms and molecules has become a p o p u l a r a1 t e r n a t i v e t o more c o n v e n t i o n a l d e t e c t i o n methods i n o p t i c a l spectroscopy, such as f l u o r e s c e n c e d e t e c t i o n . However, w h i l e o p t o g a l v a n i c s i g n a l s a r e e a s i l y and s i m p l y d e t e c t e d , t h e p h y s i c a l processes l e a d i n g t o changes i n t h e e l e c t r i c a l p r o - p e r t i e s o f a gas f o l l o w i n g e x c i t a t i o n w i t h r e s o n a n t l i g h t a r e s t i l l l i t t l e understood owing t o t h e i r c o m p l e x i t y .
I n t h i s paper, we p r e s e n t a s t u d y o f two t y p e s o f o p t o g a l v a n i c d e t e c t o r , namely, t h e d.c. glow d i s c h a r g e and t h e space-charge l i m i t e d t h e r m i o n i c diode. S p e c i a l emphasis i s p l a c e d on t h e i m p o r t a n c e o f p o s i t i v e i o n s i n t h e p r o d u c t i o n o f t h e s i g n a l s . E x p e r i - mental r e s u l t s showing unexpected d i s t o r t i o n s o f a t o m i c s p e c t r a l p r o f i l e s m o n i t o r e d o p t o g a l v a n i c a l l y a r e d i s c u s s e d here i n terms o f t h e f i n i t e response t i m e o f t h e gas t o a change i n e x c i t a t i o n c o n d i t i o n s .
2. Comparison o f Two O p t o g a l v a n i c D e t e c t o r s
The t h e m i o n i c d i o d e and t h e d.c. glow d i s c h a r g e lamp a r e two v e r y d i f f e r e n t t y p e s o f o p t o g a l v a n i c d e t e c t o r . The p r i n c i p a l d i f f e r e n c e s i n opei-ation o f t h e s e d e t e c t o r s a r e summarised i n a s i m p l e way i n f i g u r e s 1 and 2. The p h y s i c a l c o n s t r u c t i o n can be, how- ever, q u i t e s i m i l a r .
The t h e r m i o n i c d i o d e i s b e l i e v e d t o work i n t h e f o l l o w i n g way / 1 , 2 / : a p o s i t i v e i o n c r e a t e d somewhere between t h e cathode and anode d r i f t s s l o w l y towards t h e " v i r t u a l "
cathode, t h e minimum i n d i o d e p o t e n t i a l i n t h e n e g a t i v e space charge. The l a t t e r i s formed o f t h e r m i o n i c a l l y - e m i t t e d e l e c t r o n s f r o m t h e h o t w i r e cathode,the temperature o f w h i c h i s a d j u s t e d f o r maximum l i m i t a t i o n o f t h e d i o d e c u r r e n t . The i o n i s t r a p p e d i n t h e space charge and, b e f o r e recombination, can p e r t u r b up t o 106 e l e c t r o n s . The response t i m e o f t h e d i o d e i s o f t h e o r d e r o f t e n t h s o f a second, m a i n l y l i m i t e d b y t h e i o n i c d r i f t v e l o c i t y i n t h e weak e l e c t r i c f i e l d s o f t h e d i o d e ( t y p i c a l l y of t h e o r d e r o f one v01 t ) .
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1983748
JOURNAL
DE PHYSIQUE
I. Thermionic Diode.
I I I l
I
cathode grid anode
F i g u r e 1.
11.
Glow Discharge Lamp,
- - - - - - - - p----
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Space Chargel + ! ,-,
Y A +, ,
/ + , -=- - -
F i g u r e 2.
On i n c r e a s i n g t h e anode-cathode p o t e n t i a l t o s e v e r a l hundred v o l t s ( h e a t i n g o f t h e
cathode i s no l o n g e r a n e c e s s i t y ) a d i s c h a r g e w i l l s t r i k e . A t t h e l o w e s t v o l t a g e s a t
which t h e d i s c h a r g e burns, l i g h t i s p r i n c i p a l l y e m i t t e d f r o m t h e n e g a t i v e glow
r e g i o n c l o s e t o t h e cathode. As shown i n f i g u r e 2, t h e s t r o n g e s t e l e c t r i c f i e l d s i n
t h e d i s c h a r g e a r e t o be f o u n d across t h e cathode d a r k space, T h i s i s a r e s u l t of a
p o s i t i v e space charge which b u i l d s up as many p o s i t i v e i o n s produced f r o m t h e gas on
breakdown accumulate a f t e r b e i n g a t t r a c t e d t o t h e cathode. The c y c l i c a l process o f
e l e c t r o n s produced b y bombardment o f t h e cathode by p o s i t i v e i o n s produced f r o m
d r i f t t i m e ) a f f e c t s t h e p o s i t i v e space charge c l o s e t o t h e cathode.
3. Time-resolved O p t o g a l v a n i c S i g n a l s : I o n M o b i l i t y Measurements
I n s e v e r a l experiments performed r e c e n t l y i n t h e Atomic P h y s i c s group i n Goteborg, s t u d i e s have been made o f t h e s t r o n g o p t o g a l v a n i c s i g n a l s a s s o c i a t e d w i t h t h e orange and r e d o p t i c a l t r a n s i t i o n s 3s t o 3p i n Ne i n commercial hollow-cathode lamps. A d i s c u s s i o n o f t h e s i z e and s i g n o f t i m e - r e s o l v e d s i g n a l s f o l l o w i n g r e s o n a n t e x c i t a - t i o n o f a Ne d i s c h a r g e w i t h s h o r t (nsec) p u l s e s o f l i g h t f r o m NZ- o r excimer l a s e r pumped dye l a s e r s has been i n d e p e n d e n t l y r e p o r t e d / 5 / . Here we c o n s i d e r opto- g a l v a n i c s i g n a l s r e s u l t i n g f r o m a chopped c o n t i n u o u s wave s i n g l e mode dye l a s e r / 6 / .
Signal
Light F i g u r e 3.
F i g u r e 3 shows t h e time-dependence o f t h e o p t o g a l v a n i c s i g n a l a s s o c i a t e d w i t h t h e t r a n s i t i o n 3 s / 3 / 2 1 J = l t o 3p1 1/21J=1 i n neon a t an a i r wavelength o f 724.5 nm (upper t r a c e ) t o g e t h e r w i t h t h e modulated l a s e r i n t e n s i t y ( l o w e r t r a c e ) c o r r e s p o n d i n g t o a 1.3 kHz chopping frequency. The d i s c h a r g e v o l t a g e was 320 V, c u r r e n t 2.0 mA. The e l e c t r i c a l c i r c u i t (shown i n F i g u r e 2 ) measures o n l y changes i n t h e d i s c h a r g e voltage.
The s i g n a l i s t h u s expected t o r e t u r n i n z e r o (equili-after each change i n e x c i t a t i o n c o n d i t i o n s .
F i g u r e 4 ( a ) shows s i g n a l s f r o m t h e same t r a n s i t i o n a t a much l o w e r d i s c h a r g e v o l t a g e (145 V, d i s c h a r g e c u r r e n t 0.1 mA) i n w h i c h t h e neon lamp i s r u n n i n g i n t h e n e g a t i v e glow d i s c h a r g e mode. F i g u r e 4 ( b ) i s drawn i n a way t o i n d i c a t e how a s u p e r p o s i t i o n o f s l o w p o s i t i v e ( l a s e r o n ) and n e g a t i v e ( l a s e r o f f ) components can r e c o n s t r u c t t h e observed s i g n a l .
A s i m i l a r l e n g t h e n i n g of t h e response t i m e o f t h e d i s c h a r g e t o a change i n l a s e r
i n t e n s i t y was a l s o observed when, a t a g i v e n e l e c t r i c f i e l d t h e p o i n t o f l a s e r ir-
JOURNAL DE
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-
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F i g u r e 4.
r a d i a t i o n was s h i f t e d f u r t h e r f r o m t h e cathode.
These o b s e r v a t i o n s s u p p o r t e d t h e i n t e r p r e t a t i o n of t h e p r o d u c t i o n o f o p t o g a l v a n i c s i g n a l s as a r i s i n g f r o m l a s e r - i n d u c e d changes i n p o s i t i v e Ne i o n p o p u l a t i o n . The slow response t i m e o f t h e d i s c h a r g e was o f t h e same o r d e r o f magnitude as expected f o r neon i o n s t o d r i f t ( t h e i o n m o b i l i t y i n neon. i s known t o be 6.5 cm2 V - 1 s - l / 7 / ) i n t h e ( r a t h e r p o o r l y determined) d i s c h a r g e v o l t a g e , f r o m i o n p r o d u c t i o n i n t h e l a s e r beam t o " d e t e c t i o n " i n t h e n e g a t i v e glow around t h e cathode.
O f course, p o s i t i v e i o n p r o d u c t i o n i s n o t t h e o n l y process capable o f c a u s i n g opto- g a l v a n i c s i g n a l s . The v a r i o u s r d l e s o f m e t a s t a b l e atoms (e.g. i n Penning i o n i z a t i o n ) , o f e l e c t r o n s and o f t h e p h o t o e l e c t r i c e f f e c t ( e s p e c i a l l y f r o m t h e UV Ne resonance r a d i a t i o n ) have been d i s c u s s e d b y v a r i o u s workers /8-10/. The response o f t h e d i s - charge d i s c u s s e d h e r e emphasises t h e d i r e c t ( f i r s t o r d e r ) importance o f Ne i o n s i n o p t o g a l v a n i c s i g n a l p r o d u c t i o n .
O f d i r e c t p r a c t i c a l importance i n spectroscopy, t h e s e delayed o p t o g a l v a n i c s i g n a l s
have a p r o f o u n d e f f e c t on t h e f u n c t i o n n i n g o f a l o c k - i n a m p l i f i e r (phase s e n s i t i v e
d e t e c t o r ) . S i g n a l s , such as shown i n F i g u r e 5 ( a ) , a r e delayed i n phase w i t h r e s p e c t
t o t h e r e f e r e n c e s i g n a l ( t h e chopped l a s e r l i g h t ) and a r e i n t e r p r e t e d b y t h e l o c k - i n
a m p l i f i e r as s i g n a l o f o p p o s i t e s i g n , u n l e s s t h e phase o f t h e a m p l i f i e r i s manually
r e a d j u s t e d t o r e g a i n t h e o r i g i n a l s i g n a l . F i g u r e 5 ( b ) shows how an a m p l i f i e d s i g n a l
o f z e r o can r e s u l t f r o m a non-zero o p t o g a l v a n i c s i g n a l delayed b y 900 w i t h r e s p e c t t o
t h e reference. The l o c k - i n a m p l i f i e r can t h u s b e used as an i n d i c a t o r o f t h e t i m e -
dependent b e h a v i o u r o f t h e s i g n a l s . T h i s f a c t i s used i n t h e n e x t s e c t i o n where we
d i s c u s s t h e e f f e c t s o f magnetic f i e l d s on t h e d i s c h a r g e and t h e r m i o n i c diode.
- - - - - - - -
b'
A-zero +
I I sign.I I
_ _ j Time
F i g u r e 5.
4. Magnetic F i e l d E f f e c t s i n Discharge Lamps and Thermionic Diodes
I n t h i s s e c t i o n we show how a h i g h - r e s o l u t i o n s p e c t r o s c o p i c s t u d y o f t h e Zeeman e f f e c t i n atoms may be hampered b y i n i t i a l l y unexpected p e r t u r b a t i o n s o f t h e o p t o - g a l v a n i c s i g n a l " c a r r i e r s " . A t t h e same t i m e , t h e experiments r e v e a l some p o t e n t i a l l y i n t e r e s t i n g e f f e c t s f o r t h e s t u d y o f c o l l i s i o n a l i o n i z a t i o n , and f o r sub-Doppler spectroscopy.
F i g u r e 6 shows t h e l o c k - i n a m p l i f i e d o p t o g a l v a n i c s i g n a l s r e s u l t i n g f r o m t h e e x c i t a - t i o n o f t h e t r a n s i t i o n 3s13/21J=O t o 3p[1/21 J = l a t 743.89 nm i n Ne w i t h l a s e r l i g h t p o l a r i z e d l i n e a r l y p a r a l l e l t o a magnetic f i e l d a p p l i e d p a r a l l e l t o t h e d i s c h a r g e a x i s . The f r e q u e n c y o f t h e s i n g l e mode l a s e r l i g h t i s e l e c t r o n i c a l l y swept t h r o u g h t h e
acomponent o f t h e a t o m i c resonance. The d i s c h a r g e v o l t a g e i s , as above, t h a t c o r r e s p o n d i n g t o j u s t above breakdown, i n t h e n e g a t i v e glow mode.
F i r s t l y , we n o t e t h a t t h e a m p l i f i e d s i g n a l s change s i g n . F o l l o w i n g t h e d i s c u s s i o n i n
t h e p r e v i o u s s e c t i o n , t h i s may be i n t e r p r e t e d as an i n c r e a s i n g d e l a y i n t h e response
o f t h e d i s c h a r g e t o a change i n l a s e r i r r a d i a t i o n w i t h i n c r e a s i n g magnetic f i e l d s .
T h i s i s n o t unexpected, s i n c e p o s i t i v e i o n s o f neon w i l l be o b l i g e d by t h e magnetic
f i e l d t o e x e c u t e Larmor o r b i t s ( o f about a mm r a d i u s ) p e r p e n d i c u l a r t o t h a t f i e l d .
The t i m e t a k e n f o r an i o n t o reach t h e cathode w i l l t h u s be lengthened, s i n c e t h e i o n
w i l l encounter more (charge t r a n s f e r ) c o l l i s i o n s on i t s l o n g e r s p i r a l under t h e
combined e f f e c t s o f t h e magnetic and e l e c t r i c f i e l d s .
JOURNAL DE PHYSIQUE
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