OPTOGALVANIC SPECTROSCOPY OF COPPER IN A Ne/Cu HOLLOW CATHODE

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OPTOGALVANIC SPECTROSCOPY OF COPPER IN A Ne/Cu HOLLOW CATHODE

R. Shuker

To cite this version:

R. Shuker. OPTOGALVANIC SPECTROSCOPY OF COPPER IN A Ne/Cu HOLLOW CATHODE.

Journal de Physique Colloques, 1983, 44 (C7), pp.C7-455-C7-460. �10.1051/jphyscol:1983744�. �jpa-

00223301�

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JOURNAL DE PHYSIQUE

Colloque C7, supplement au nO1l, Tome 44, novembre 1983 page C7-455

OPTOGALVANIC SPECTROSCOPY OF COPPER IN A N ~ / C U HOLLOW CATHODE

R. Shuker

Physics Deparement, Ben-Gurion Uniuersity, Beer-Sheva, IsraeZ

Rdsum6 - La s p e c t r o s c o p i e optogalvanique d'une vapeur de c u i v r e dans une d6- charge B cathode c r e u s e Ne/Cu met en 6vidence des processus l i 6 s au plasma comme l ' i o n i s a t i o n Penning des atomes de c u i v r e p a r l e s atomes m e t a s t a b l e s du nEon e t , une f a i b l e i n v e r s i o n de population s u r l e s t r a n s i t i o n s l a s e r B 510,6 nm e t 3 578,2 nm du c u i v r e .

A b s t r a c t

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Optogalvanic spectroscopy of copper vapor i n a Ne/Cu hollow cathode s t e a d y d i s c h a r g e r e v e a l s plasma p r o c e s s e s such a s Penning i o n i z a t i o n of copper atoms by m e t a s t a b l e neon l e v e l s and p o s s i b l e small p o p u l a t i o n i n v e r s i o n on t h e copper vapor l a s e r t r a n s i t i o n s a t 510.6 nm and 578.2 nm.

T h e o r e t i c a l model i s p r e s e n t e d .

INTRODUCTION

P r e l i m i n a r y s t u d y o f p u l s e d o p t o g a l v a n i c e f f e c t (OGE) i n s t a b l e neon/copper d i s c h a r g e o b t a i n e d i n a hollow cathode t u b e i s r e p o r t e d . The o p t o g a l v a n i c s i g n a l

(OGS) i s p a r t i c u l a r l y used t o i n v e s t i g a t e t h e e f f e c t o f copper. B r i e f l y , t h e opto- g a l v a n i c e f f e c t i s t h e change i n t h e v o l t a g e a p p l i e d on a d i s c h a r g e and i n t h e c u r r e n t through i t due t o changes i n t h e e f f e c t i v e i o n i z a t i o n r a t e s induced by l a s e r l i g h t tuned r e s o n a n t l y t o a s p e c i f i c atomic t r a n s i t i o n o f t h e d i s c h a r g e medim.

Such a l a s e r i l l u m i n a t i o n u s u a l l y causes an i n c r e a s e i n t h e p o p u l a t i o n o f h i g h l y e x c i t e d l e v e l s which enhances t h e i o n i z a t i o n and causes a v o l t a g e drop.

The o p t o g a l v a n i c e f f e c t i s widely used i n v a r i o u s s p e c t r o s c o p i c t e c h n i q u e s . Most r e c e n t l y o p t i c a l double resonance i n atoms and molecules spectroscopy was suggested and measured by Vidal [ l ] . A s i m p l i f i e d t h e o r y d e s c r i b i n g t h e o b t a i n e d p u l s e d s i g n a l was given by Erez e t a l . [ 2 ] . L a y l e r has d i s c u s s e d a s i m i l a r t h e o r y f o r t h e o p t o g a l v a n i c s i g n a l i n helium d i s c h a r g e [ 3 ] .

Smyth and Schenck s t u d i e d mechanisms o f t h e o p t o g a l v a n i c e f f e c t [ 4 ] . These models d e s c r i b e t h e response o f t h e d i s c h a r g e t o a p e r t u r b a t i o n o f t h e s t e a d y s t a t e p o p u l a t i o n d i s t r i b u t i o n o f t h e l e v e l s i n t h e d i s c h a r g e medium caused by a l a s e r i l l u m i n a t i o n tuned r e s o n a n t l y t o a t r a n s i t i o n between t h e s e l e v e l s . These models assume two-level and f o u r - l e v e l atoms and o n l y two s t e p i o n i z a t i o n . This assump- t i o n i s a s i m p l i f i c a t i o n o f t h e very complicated system o f l e v e l s and

among them encountered i n d i s c h a r g e s . T h i s model d e s c r i b e d t h e OGS o b t a i n e d f o r t h e Penning i o n i z a t i o n case too [6]. Pulsed OGE i n Ne/Cu e x h i b i t s b o t h Penning i o n i z a - t i o n s i g n a l s when t h e l a s e r i s tuned t o neon 1s. + 2p. ( i n Pachen n o t a t i o n ) t r a n s i t i o n s and i n v e r t e d OGS on t h e copper l a s e r l t r a n s l t i o n s . Other Cu t r a n s i t i o n s e x h i b i t t h e expected OGS.

EXPERIMENTAL

The experimental s e t u p i s d i s p l a y e d s c h e m a t i c a l l y i n Figure 1.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1983744

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JOURNAL DE PHYSIQUE

20K

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CHART RECORDE

Fig. 1 - Block diagram f o r OGS measurements.

I t i n c l u d e s a n i t r o g e n l a s e r o f 5 nsec p u l s e width and a power on t h e o r d e r o f 100 kW pumping a nonflowing tunable dye l a s e r . The dyes d e l i v e r tunable l a s e r s i n t h e v i s i b l e and a r e Rhodamine 6G, Rhodamine B, Cownarines, S t i l b e n e s and DCM. The dye l a s e r s a r e HXnsch type, have p u l s e e n e r g i e s o f a few microjoule, p u l s e width o f about 3 nsec and linewidth o f 0.1 cm-l. A s o l i d e t a l o n i s a l s o employed t o narrow the l a s e r linewidth. A CuBr l a s e r i s used a t t h e copper 510.6 nm and 578.2 nm l a s e r t r a n s i t i o n s .

The setup c o n s i s t s o f a hollow cathode discharge tube with DC power supply, s t a b l e t o w i t h i n 1 0 - ~ o f t h e DC v o l t a g e t o allow measurement o f small changes due t o t h e optogalvanic e f f e c t . A Ne/Cu hollow cathode tube i s used, t h e neon b u f f e r gas p r e s s u r e i s about 5 t o r r . The discharge c u r r e n t was i n t h e range o f 1 mA t o 20 mA but sometimes reached over 100 mA. The pulsed s i g n a l was coupled v i a a DC blocking c a p i c i t o r t o an o s c i l l o s c o p e o r a Biomation 805 s i g n a l averager and displayed on an o s c i l l o s c o p e o r a c h a r t r e c o r d e r . The l a s e r ' s wavelength was monitored by a Spex 1401 double monochromator.

THEORY

A model i s given t o d e s c r i b e t h e time dependence o f t h e pulsed OGS f o r t h e usua1,the Penning and t h e i n v e r t e d e f f e c t s . Figure Ashows t h e r e l e v a n t neon, copper and copper ion energy l e v e l diagram [ 7 ] .

The main assumption o f t h i s model i s t h a t a small departure from t h e s t e a d y s t a t e population d e n s i t y decays exponentially with a r e l a x a t i o n time, T, d i c t a t e d by t h e plasma a s a whole, namely, by t h e c o l l e c t i v e i n t e r a c t i o n o f t h e r e l e v a n t l e v e l with a l l t h e o t h e r s v i a t h e e l e c t r o n c o l l i s i o n s and r a d i a t i v e t r a n s i t i o n s . Only f o u r l e v e l s a r e taken i n t o c o n s i d e r a t i o n while a l l o t h e r s a r e represented by t h e i r e f f e c t on t h e r e l a x a t i o n time. The r a t e equations f o r t h e d e p a r t u r e o f t h e l e v e l d e n s i t y o f population, Ani,.from t h e steady s t a t e values a r e given by t h e following equation using m a t r i x n o t a t i o n :

where A;(t) i s & e v e c t o r o f t h e d e p a r t u r e s from t h e s t e a d y s t a t e d e n s i t i e s of population and l? i s t h e r e l a x a t i o n matrix. I t s diagonal elements a r e

r .

= l/Ti while t h e o f f diagonal elements a r e t h e r a t e s connecting t h e l e v e l s i and1 j d i r e c t l y . The d e t a i l e d r a t e equations f o r t h e f o u r l e v e l s o f t h e model a r e

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.An

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1399, i614;nm excitinq 1-

Fig. 2

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P a r t i a l energy diagram o f t h e r e l e v a n t l e v e l s o f Ne, Cu and CU'.

The i n i t i a l values f o r t h e s e equations a r e determined by t h e changes i n t h e d e n s i t i e s caused by t h e l a s e r i l l u m i n a t i o n . For a s h o r t l a s e r p u l s e o f d u r a t i o n s h o r t compared with T i t s , t m e d t o an i -t j t r a n s i t i o n and has an i n t e n s i t y I ( t ) one has An: = - An; = AnO and

where ni and n j a r e t h e steady s t a t e d e n s i t i e s and a i j i s t h e stimulated emission cross-section. For t h e case o f i n v e r t e d population An w i l l be

negative. ⁰

For a l e v e l k , where k # i and j one has ~ n ; = 0.

The optogalvanic s i g n a l , i . e . , t h e v o l t a g e change AV(t) i s given by [ 2 ] AV(t) = - B 1 aiAni(t)

i where

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JOURNAL D€ PHYSIQUE

K i s t h e m u l t i p l i c a t i o n f a c t o r [S]. The c o e f f i c i e n t s a i l s a r e p o s i t i v e , a r e r e l a t e d t o t h e i o n i z a t i o n r a t e o f t h e l e v e l i , and i n c r e a s e with t h e e x c i t a t i o n energy o f t h e l e v e l .

I n t h e copper OGE we u s e a t w o - s t a t e model where t h e m e t a s t a b l e L~ l e v e l s and t h e upper l e v e l s 2~ a r e d e s i g n a t e d 1 and 2 , r e s p e c t i v e l y .

For t h e Penning c a s e t h e f o u r l e v e l s a r e shown i n F i g u r e 3.

Penning Ionization

Ne(l S , )

3z-rrmc-C~

Fig. 3 - Schematic d e s c r i p t i o n o f t h e f o u r - s t a t e s model.

The r e l e v a n t l e v e l s a r e neon l s i and 2pj l e v e l s and copper ground s t a t e Cu( So%) 2 and e x c i t e d copper i o n l e v e l CU+* i n resonance w i t h I s i s t a t e s . These a r e d e s i g n a t e d by numbers 1 t o 4 , r e s p e c t i v e l y .

RESULTS AND DISCUSSION

Two t y p e s of OGE a r e observed i n Ne/Cu d i s c h a r g e . 1 ) OGE on t r a n s i t i o n s o f copper i t s e l f on b o t h t h e d l O d and d 9 d n ' & ' systems, and (2) OGE on neon t r a n s i t i o n s from i t s m e t a s t a b l e l s 5 l e v e l g i v i n g r i s e t o Penning i o n i z a t i o n o f Cu and r e l a t e d OGS.

1. OGE on Copper T r a n s i t i o n

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F i g u r e 4 e x h i b i t s a comparison between OGS f o r a t y p i c a l copper t r a n s i t i o n and t h a t o f a copper l a s e r t r a n s i t i o n . These a r e r e p r e s e n t e d by t h e + t r a n s i t i o n a t 513.5 nm and by t h e 2 ~ 5 / 2 + 'p3/2 t r a n s i t i o n a t 510.6 nm, r e s p e c t i v e l y .

The OGE of t h e 510.6 nm i s i n v e r t e d compared w i t h o t h e r copper t r a n s i t i o n s a s i s c l e a r from Fig. 5 , a f t e r t a k i n g i n t o account t h e s i g n a l due t o p h o t o e l e c t r o n

emission by t h e l a s e r l i g h t a t t h e s e wavelengths. I t s OGS i s a m i r r o r image o f t h e OGS o f t h e 513.5 nm t r a n s i t i o n which i s t y p i c a l t o copper t r a n s i t i o n s on both t h e dlOnQ and d9&n'Qf systems i n c l u d i n g t r a n s i t i o n s c o n n e c t i n g t o a u t o i o n i z i n g l e v e l s . A s i m i l a r behaviour i s e x h i b i t e d by t h e o t h e r l a s e r t r a n s i t i o n a t 578.2 nm However, t h e o t h e r t r a n s i t i o n 2 ~ 2 / 2 + . 2 ~ 3 / 2 h a s v e r y weak s i g n a l and i s probably n o n i n v e r t e d . T h i s may i n d i c a t e t e existence o f a low p o p u l a t i o n i n v e r s i o n on t h e s e

t r m s i t i o n s a s can be i n f e r r e d from t h e model. The i n i t i a l p o p u l a t i o n d i f f e r e n c e , An, i n eq. (3) becomes n e g a t i v e f o r an i n v e r t e d p o p u l a t i o n r e s u l t i n g i n i n v e r t i n g t h e s i g n o f t h e r e l e v a n t OGS. The i n v e r t e d OGS i s v e r y weak a t t h e low d i s c h a r g e c u r r e n t compared t o t h a t o f t h e o t h e r t r a n s i t i o n . For h i g h e r c u r r e n t , however, i t s s i g n a l i s l a r g e r i n comparison w i t h t h e h i g h l y e x c i t e d s t a t e s t r a n s i t i o n a t

513.5 nm. Moreover, a t c u r r e n t s lower t h a n 3.5 mA t h e i n v e r t e d OGS d i s a p p e a r s a l t o g e t h e r , a l t h o u g h o t h e r n o n i n v e r t e d OGS1s a r e p r e s e n t . T h i s f u r t h e r s u p p o r t s t h e e x i s t e n c e o f an i n v e r t e d p o p u l a t i o n on t h i s t r a n s i t i o n . F u r t h e r i n v e s t i g a t i o n

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Fig. 4 - OGS t r a c e s o f copper t r a n s i t i o n s a t two d i s c h a r g e c u r r e n t s of 3.5 mA and 1 8 mA i n o s c i l l o g r a m s A and B , r e s p e c t i v e l y . The lower t r a c e i n both o s c i l l o g r a m s i s t h e OGS o f t h e 2 ~ 1 / 2 + 2 ~ 3 / 2 t r a n s i t i o n a t 513.nm and t h e next on i s t h e OGS o f t h e 2 ~ 5 / 2 + 2p312 t r a n s i t i o n a t 510.6 nm which i s a l a s e r t r a n s i t i o n . The upper t r a c e I n o s c l l l o g r a m A i s t h e s i g n a l due t o t h e e f f e c t o f photoemission from t h e copper cathode.

o f t h i s e f f e c t i s underway t o determine i t s v a l i d i t y and t o examine t h e p o s s i b i l i t y o f a u s e f u l gain a t t h i s d i s c h a r g e c u r r e n t .

2. Penning OGS

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I t has been shown t h a t t h e OGS o f neon l s 5 , I s 3 ⁺2pj o f neon i n t h e c a s e o f Penning i o n i z a t i o n has complex f e a t u r e s a t c u r r e n t where t h i s e f f e c t dominates t h e i o n i z a t i o n o f t h e cathode vapor atoms. Figure 5 shows such an OGS f o r t h e neon t r a n s i t i o n l s 5 + 2p2 a t 588.2 nm, i n a Ne/Cu t u b e a t two c u r r e n t s . The OGS o f 1 mA i s Penning dominated, while t h e h i g h e r c u r r e n t OGS i s dominated by e l e c t r o n c o l l i s i o n i o n i z a t i o n .

The Penning r e l a t e d OGS i n Fig. 5 d i s p l a y s t h e following sequence o f e v e n t s . The l a s e r e x c i t a t i o n from t h e neon metastable l e v e l causes t h e f i r s t n e g a t i v e p u l s e of t h e s i g n a l . A f t e r f a s t r e l a x a t i o n , t h i s i s followed by a p o s i t i v e p u l s e a s a consequence o f t h e d e c r e a s e i n neon m e t a s t a b l e p o p u l a t i o n . These two f e a t u r e s a r e t y p i c a l p u r e neon OGS. The Penning e f f e c t follows and c a u s e s two e x t r a f e a t u r e s i n t h e OGS. F i r s t , t h e r e l a t i v e d e c r e a s e i n t h e neon m e t a s t a b l e p o p u l a t i o n r e s u l t s i n a r e d u c t i o n i n t h e r a t e o f t h e Penning i o n i z a t i o n of n e u t r a l copper and i n t h e production o f CU+. The r e s u l t i n g i n c r e a s e i n Cu d e n s i t y causes a d i p a t about 150 'sec. Secondly, a f t e r a d i f f u s i o n t i m e , t h e d e c r e a s e i n Cu+ d e n s i t y r e s u l t s i n a d r a s t i c f a l l i n t h e s p u t t e r i n g r a t e o f calcium and i t s d e n s i t y i n t h e gas causing a long p o s i t i v e p u l s e . This p a r t o f t h e OGS has a time s c a l e o f a few hundreds microseconds.

T r a c e B i n Fig. 5 shows a t y p i c a l OGS a t h i g h e r c u r r e n t s on t h e same t r a n s i t i o n , when t h e copper i o n i z a t i o n i s dominated by t h e discharge and t h e Penning f e a t u r e diminishes e x h i b i t i n g OGS of neon m e t a s t a b l e s t a t e . An e s t i m a t e o f t h e Penning c r o s s - s e c t i o n may be o b t a i n e d by comparing t h e r e l a x a t i o n t i m e , T1, of t h e m e t a s t a b l e s t a t e with and without t h e Penning p r o c e s s , a t an a p p r o p r i a t e c u r r e n t [ 6 ] . I t i s 1 0 - l ~ cm2. This i s lower by an o r d e r o f magnitude than t h a t estimated f o r neon- calcium. This r e s u l t may be v a l u a b l e f o r k i n e t i c modeling o f copper vapor l a s e r .

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JOURNAL D€ PHYSIQUE

Fig. 5

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OGS o f neon t r a n s i t i o n ls5 ^-t2p2 a t 588.2 m. Oscillogram A e x h i b i t s f e a t u r e s r e l a t e d t o Penning i o n i z a t i o n o f Cu a t times l o n g e r t h a n 200 psec.

Oscillogram B shows OGS f o r h i g h e r c u r r e n t s w i t h much s h o r t e r time s c a l e e x h i b i t i n g normal OGE a t t h e neon t r a n s i t i o n from a m e t a s t a b l e l e v e l .

References

1. Vidal, C . R . , O p t i c s L e t t .

5

(1980) 158 and Phys. Rev. L e t t .

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(1983) 1046.

2. Erez, G . , Lavi, S. and Miron, E . , IEEE J . Quant. E l e c t r .

QE-15

(1979) 1328, Een-Amar, A . , Erez, G . , and Shuker, R., J. Appl. Phys. (1983).

3. Lawler, J . E . , Phys. Rev.

A 2 2

(1980) 1025

4. Smyth, K . C . and Schenck, P.K., Chem. Phys. L e t t .

2

(1978) 466.

5. F r a n c i s , G . , Handbuch d e r Physik X X I I , S p r i n g e r Verlag, B e r l i n 1956, p. 81.

6 . Shuker, R . , Ben-Amar, A . , and Erez, G . , J . Appl. Phys. (1983).

7. Moore, C . E . , Atomic Energy Levels, Vol. 1, NBS C i r c . No. 457 (U.S. Government P r i n t i n g O f f i c e , Washington, D.C., 1949).