INTERMODULATED OPTOGALVANIC SPECTROSCOPY - A COMPARISON WITH OTHER HIGH RESOLUTION TECHNIQUES

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INTERMODULATED OPTOGALVANIC

SPECTROSCOPY - A COMPARISON WITH OTHER HIGH RESOLUTION TECHNIQUES

Ch. Belfrage, P. Grafström, S. Kröll, S. Svanberg

To cite this version:

Ch. Belfrage, P. Grafström, S. Kröll, S. Svanberg. INTERMODULATED OPTOGALVANIC SPEC-

TROSCOPY - A COMPARISON WITH OTHER HIGH RESOLUTION TECHNIQUES. Journal de

Physique Colloques, 1983, 44 (C7), pp.C7-169-C7-174. �10.1051/jphyscol:1983714�. �jpa-00223271�

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JOURNAL DE PHYSIQUE

Colloque C7, supplement au n O 1 l , Tome 44, novembre 1983 page C7-169

INTERMODULATED OPTOGALVANIC SPECTROSCOPY

-

A COMPARISON WITH OTHER

HIGH RESOLUTION TECHNIQUES

Ch. B e l f r a g e , P . G r a f s t r i j m , S. K r b ' l l and S . Svanberg

Department o f Physics, Lund I n s t i t u t e of TechnoZogy, 5-220 07 Lund, Sweden RQsurn6 - Nous f a i s o n s une comparaison e n t r e l e s s p e c t r o s c o p i e s de f l u o r e s - c e n c e i n t e r r n o d u l g e , o p t o g a l v a n i q u e i n t e r m o d u l e e , POLINEX ( e x c i t a t i o n p a r po- l a r i s a t i o n i n t e r r n o d u l 6 e l e t de p o l a r i s a t i o n cornrne mkthodes de s p e c t r o s c o p i e s a n s e f f e t Doppler d a n s une d Q c h a r g e 2 c a t h o d e c r e u s e . La d i s c u s s i o n e s t p r i n c i p a l e m e n t bas6e s u r d e s e x p e r i e n c e s r Q a l i s 6 e s s u r l e s t r a n s i t i o n s

5 3

~ ~ ' 3 . s 3pZ - Zp 3p pl de N E 2 1-5882

i

e t 4d55s 'S, -

Q u e l q u e s a s p e c t s g e n e r a u x c o n c e r n a n t l a c a t h o d e c r e u s e comrne i n s t r u m e n t en s p e c t r o s c o p i e l a s e r 5 h a u t e r g s o l u t i o n s o n t a u s s i brievernent d i s c u t 6 s . A b s t r a c t

-

A comparison o f intermodulated fluorescence spectroscopy, i n t e r - modulated optogal vanic spectroscopy

,

POLINEX (POLarization INtermodul a t e d E x c i t a t i o n ) spectroscopy and p o l a r i z a t i o n spectroscopy used i n Doppler-free spectroscopy on h o l l o w cathode discharges i s made, mainly based on experiments

5 5 3

performed on the Zp 3s 3 ~ 2 - Zp 3p PI t r a n s i t i o n i n Ne a t h-5882 8, and t h e

5 5 5

46 5s S2

-

46 5p 5 ~ 3 t r a n s i t i o n i n MO a t A=5506

R .

Some general aspects r e - garding the hollow cathode as a t o o l i n h i g h - r e s o l u t i o n l a s e r spectroscopy are a l s o b r i e f l y discussed.

I

-

INTRODUCTION

The extension o f t h e optogalvanic spectroscopy t o a Doppler-free technique by t h e Stanford group i n 1979 [l] opened new p o s s i b i l i t i e s t o the use o f discharges i n h i g h - r e s o l u t i o n spectroscopy. E s p e c i a l l y the hollow-cathode discharge,where f r e e atoms o f r e f r a c t o r y elements conveniently can be obtained by s p u t t e r i n g has been given considerable a t t e n t i o n l a t e l y [2,3,41. A program f o r optogalvanic spectros- c o ~ y on h o l l o w cathodes was i n i t i a t e d i n our group i n 1980. Our f i r s t experiments i n t h i s c o n t e x t using pulsed and CW l a s e r s were performed on a commercial Ne-Ba h o l l o w cathode and a He-Ne l a s e r tube [ 5 ] . A f t e r a d d i t i o n a l p r e l i m i n a r y experiments on Ne [ 6 ] and Cu (unoublished) a comparison between t h e Doopler-free methods i n t e r - modulated optogalvanic spectroscopy (IMOGS) [l], intermodulated fluorescence spectroscopy (IFS) [71 and o o l a r i z a t i o n spectroscopy (PS) [81 was made i n connection w i t h s t u d i e s o f isotope s h i f t s i n Ne 3s-3p t r a n s i t i o n s [91. I n t h e main p a r t o f

t h i s i n v e s t i a a t i o n a h o l l o w cathode w i t h a 6 mm bore, o r i g i n a l l y designed f o r c l a s s i c a l emission s p e c t r o s c o ~ y , was used. I n order t o reduce Stark and pressure broadening a cathode w i t h a l a r g e r bore (2.5 cm) designed by Lawler e t a l . L101 was constructed. For t h e Ne 3s-3p t r a n s i t i o n s t h e new arrangement reduced the homoge- neous l i n e w i d t h by roughly a f a c t o r o f two and l i n e w i d t h s o f 15 MHz have been o b t a i - ned i n subsequent measurements on MO i l l u s t r a t i n g t h e favourable e l e c t r i c f i e l d

5 5 5 5

arrangement i n t h i s h o l l o w cathode. I n MO the 4d 5s S2

-

4d 5p P3,2,1 t r a n s i t i o n s have been studied. Magnetic-dipole and e l e c t r i c quadrupole i n t e r a c t i o n constants a and b f o r the upper s t a t e have been measured f o r t h e two i s o t ~ p e s ~ ~ ~ M o and "MO (15.7%and 9.6% abundance) by Meisel [Ill. For t h e a f a c t o r o f the S2 s t a t e a value i s given i n [12,131.

I n t h e next s e c t i o n our s e t up i n hollow-cathode discharge experiments w i l l be

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1983714

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C7-170 JOURNAL DE PHYSIQUE

d e s c r i b e d . S e c t i o n I 1 1 c o n t a i n s r e s u l t s and d i s c u s s i o n . Our r e s u l t s a r e summarized i n a f i n a l s e c t i o n .

I 1 - EXPERIKNTAL ARRANGEMENT -

I n F i g . 1 a schematic diagram o f o u r s e t up i s shown. An a r g o n - i o n pumped CR 599-21 single-mode dye l a s e r was employed. The main p a r t o f t h e beam was s p l i t i n t o two c o u n t e r p r o p a g a t i n g beams p a s s i n g t h e h o l l o w cathode, and t h e n o n l i n m r i n t e r a c t i o n i n t h e beam o v e r l a p r e g i o n was d e t e c t e d employing IMOGS, IFS, PS o r POLINEX [141.

A t A i n t h e f i g u r e a p o l a r i z e r was i n s e r t e d i n t h e beam i n t h e case o f PS and a Pockels c e l l f o r m o d u l a t i n g t h e beam p o l a r i z a t i o n between r i g h t hand and l e f t hand c i r c u l a r p o l a r i z a t i o n i n t h e case o f POLINEX. I n t h e case o f PS and POLINEX a A/4 p l a t e was a l s o i n s e r t e d a t p o i n t B. A squeezed g l a s s p l a t e was used i n PS f o r com-

LASER SPECTRO- SCOPY ON A HOLLOW CATHODE DISCHARGE

I

^r ^RECORDER^C

FLUORESCENCE

Fig. 1. W r a t a l set-up. I n POLINEX o p t o g a l v a n i c d e t e c t i o n was employed.

p e n s a t i n g f o r b i r e f r i n g e n c e i n c e l l windows. Our matched p a i r o f Glan-Thompson p o l a r i z e r s had an e x t i n c t i o n r a t i o o f 107. I n p r a c t i c a l experiments we g e n e r a l l y a c h i e v e d o n l y 105 because o f t h e n o n u n i f o r m b i r e f r i n g e n c e i n t h e c e l l windows o v e r t h e a r e a i r r a d i a t e d by t h e l a s e r beam. By squeezing t h e c e l l windows t h e i n t e r n a l s t r a i n i n these c o u l d p o s s i b l y have been reduced [81. Two d i f f e r e n t choppers were emoloyed; one a t 850 Hz f o r D o p p l e r - l i m i t e d measurements and one a t 750 and 225 Hz f o r D o p p l e r - f r e e measurements. The c h o i c e o f chopper f r e q u e n c y was made w i t h some c a r e . The o u t p u t n o i s e f r o m o u r CR 599-21 dye l a s e r o p e r a t i n g i n a single-mode peaks a t 127 Hz and I770 Hz which we a t t r i b u t e t o t h e s e r v o systems. O t h e r w i s e t h e n o i s e o f t h e l a s e r , t h e h o l l o w cathode and t h e p h o t o m u l t i p l i e r e s s e n t i a l l y c o n s i s - t e d o f w h i t e n o i s e up t o 50 KHz e x c e p t f o r l o w e r harmonics o f 50 Hz. T y p i c a l d a t a o f o p e r a t i o n f o r t h e h o l l o w cathode i n o u r MO measurements were a p r e s s u r e o f 0.1 t o r r and a c u r r e n t o f 200 mA. A d e t a i l e d d e s c r i p t i o n o f t h e h o l l o w cathode i s g i v e n i n Ref. [IO].

I11 - RESULTS AND DISCUSSION

5 5 3

I n ' F i g . 2 r e c o r d i n g s o f t h e 2p 3s 3~

-

2 ~ 3p P Ne t r a n s i t i o n a t A=5882 A a r e shown as o b t a i n e d w i t h IFS, IMOGS, P~LINEX and

PS.

T a b l e I g i v e s a p p r o x i m a t i v e s i g - n a l / n o i s e r a t i o s o b t a i n e d w i t h t h e d i f f e r e n t methods a p p l i e d t o t h i s Ne t r a n s i t i o n . Here t h e d i s c h a r g e c u r r e n t was 45 mA, t h e p r e s s u r e 0.1 t o r r and t h e l o c k - i n t i m e c o n s t a n t 0.3 S.

150 HHZ FP+-+>-'

@-l&>

'S, DIGITAL WVELENGTH METER

0%

1 5 GHz FP1

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Fig. 2.

Experinental c u r v e s f o r the N e

5 5 3

2p 3 s 3 ~ 2

-

2p 3p PI t r a n s i t i o n (5882 A)

,

recorded w i t h

a) IFS, discharge c u r r e n t (I) = 150 mA, p r e s s u r e (p) =0.6 t o r r , time con- s t a n t ( T ) =3 s.

b) LKGS, I=45 mA, p=70 m t o r r , .r=O.I S

C) WLINEX, 1=45 mA, p=70 mtorr, -r=O.I S.

d) PS, I=45 mA, p=0.1 torr, ~ = 0 . 3 S.

I

signal /noise 1 2 0 100 5 5 30 1500

1

Table I

signal/background

I

^- ^- ^{0 . 3} ^{0 . 1}

- -

The term "background" i n Table I r e f e r s t o t h e Doppler-broadened pedestal due t o velocity-changing c o l l i s i o n s . These a r e inherent i n t h e IFS and IMOGS methods and a s we s h a l l s e e , severely l i m i t t h e usefulness of t h e s e two methods.

An i n t e r e s t i n g a s p e c t concerning spectroscopy on a hollow-cathode discharge i s t h e study of r e f r a c t o r y elements. The theory of the hyperfine s t r u c t u r e i n elements with u n f i l l e d d-shell has received considerable a t t e n t i o n l a t e l y [13,15,16]. The Goteborc group has a l s o r e c e n t l y determined t h e hyperfine s t r u c t u r e i n t h e 4d55p 'P+,3,2

s t a t e s i n MO ( p r i v a t e communication). We have s t u d i e d t h e t r a n s i t i o n s between t h e 4d55s 5 S ~ s t a t e and t h e 4d55p 5 P 3 , 2 , 1 s t a t e s i n MO. MO has a r e l a t i v e l y low s p u t t e - r i n g y i e l d [171. This means t h a t t h e c u r r e n t s necessary t o produce a f a i r amount of f r e e MO atoms in t h e discharge i s r a t h e r hiqh. We have used Ne a s c a r r i e r gas.

Other r a r e oases have a hiqher s p u t t e r i n g y i e l d [17,18] and might be b e t t e r s u i t e d . In Ref. [ l 9 1 a u n i f i e d theory f o r t h e s p u t t e r i n g process in a hollow cathode i s given. TO produce a s u f f i c i e n t amount of MO atoms i n t h e lowest metastable 4d55s 5S2 s t a t e t h e discharge was run a t a c u r r e n t of 200 mA and a pressure of 0.1 t o r r .

Doppler-limited

FS OGS

Fig. 3 shows two recordings of the 4d55s 5S2-4d55p 'P3 t r a n s i t i o n i n MO a t A=5506 A . The upoer t r a c e i s taken with IFS ( l a s e r Dower about60 mW). In the lower t r a c e taken with PS ( l a s e r power < l 0 mW) t r a n s i t i o n s correspording t o t h e f i v e even i s o t o p e s

Doppler-free

IFS IMOGS POLINEX P S

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JOURNAL DE PHYSIQUE

5 5 5 5

Fig. 3 . Ekprirrental curves for the 4d 5s S2 - 4d 5p P3 transition (5506 A ) , recorded. w i t h IFS and PS.

and s e v e r a l o f t h e h y p e r f i n e t r a n s i t i o n s i n t h e two odd i s o t o p e s w i t h n u c l e a r s p i n 5/2 can be seen.

Considering Table 1,Fig. 2 and F i q . 3 t h e f o u r a l t e r n a t i v e t e c h n i q u e s w i l l be a n a l y - zed.

The d e t e c t i v i t y i n IFS i s l i m i t e d by f l u c t u a t i o n s i n t h e broadband p e d e s t a l s u r - r o u n d i n g t h e D o o p l e r - f r e e oeaks. T h i s p e d e s t a l emanates f r o m e l a s t i c v e l o c i t y - c h a n g - i n g c o l l i s i o n s i n t h e l o w e r m e t a s t a b l e s t a t e . The n o i s e i n t h e p e d e s t a l s i g n a l i s a t t r i b u t e d t o f l u c t u a t i o n s i n l a s e r i n t e n s i t y and i n t h e m e t a s t a b l e s t a t e atom popu- l a t i o n . Tuning t h e l a s e r o f f l i n e decreased t h e n o i s e l e v e l w i t h a f a c t o r o f f o u r . As a l s o ~ o i n t e d o u t i n Ref. [IO], t h e D o p p l e r - f r e e s i g n a l c o u l d b e enhanced by ope- r a t i n g t h e h o l l o w cathode a t h i g h e r c u r r e n t s and l o w e r p r e s s u r e . IFS i s more appro- o r i a t e f o r weakly a b s o r b i n g l i n e s [201. However, i n d i s c h a r g e s t h e v e l o c i t y - c h a n g i n g c o l l i s i o n s and f l u o r e s c e n c e a g a i n decrease t h e d e t e c t i v i t y .

I n ICOGS t h e s i t u a t i o n i s s i m i l a r . Here t h e D o p p l e r - l i m i t e d p e d e s t a l i s even more pronounced. T h i s i n d i c a t e s t h a t a l s o o t h e r processes t h a n v e l o c i t y - c h a n g i n g c o l l i - s i o n s g i v e a D o p p l e r - l i m i t e d s i a n a l on t h e sum frequency. Our s u g g e s t i o n i s t h a t atoms e x c i t e d by one l a s e r beam i n d i r e c t l y a f f e c t s t h e process b y which t h e o t h e r l a s e r beam modulates t h e d i s c h a r g e impedance. T h i s can be e x p l a i n e d by e.g. t h e process suagested i n Refs. [21,221, where t h e i o n i z a t i o n i s d e s c r i b e d as a m u l t i s t e p process r e s u l t i n g i n a g e n e r a l h e a t i n g o f t h e plasma. Going t o h i g h e r c u r r e n t s i n o r d e r t o enhance t h e D o p p l e r - f r e e s i g n a l i s l e s s a t t r a c t i v e s i n c e f i r s t l y t h e r e l a - t i v e impedance change due t o l a s e r r a d i a t i o n t u n e d t o 3s-3- t r a n s i t i o n s g e n e r a l l y decreases w i t h i n c r e a s i n g c u r r e n t [22,23,24] and secondly t h e demands on t h e h o l l o w cathode c o n s t r u c t i o n f o r m a i n t a i n i n g a q u i e t d i s c h a r g e i n c r e a s e . The rf d i s c h a r g e m i g h t be a good a l t e r n a t i v e f o r s t u d y i n g gases, see e.g. [ 2 5 ]

.

Work i n t h i s f i e l d i s p r e s e n t l y g o i n g on i n o u r l a b o r a t o r y . The n o i s e i n t h e broadband p e d e s t a l was a fac- t o r o f 15 s t r o n g e r t h a n t h e d i s c h a r g e n o i s e when t h e l a s e r was t u n e d o f f l i n e , where i t was a f a c t o r o f 100 above t h e s h o t n o i s e l i m i t . The h i g h e r c u r r e n t s t h a t had t o be used i n t h e MO measurements caused t h e d i s c h a r g e n o i s e t o r i s e a n o t h e r f a c t o r of f i v e above t h e s h o t n o i s e l e v e l . T h i s f a c t i n c o n n e c t i o n w i t h l o w e r s e n s i t i v i t y f o r t h e metal atoms t h a n f o r t h e m e t a s t a b l e Ne atoms I221 caused IMOGS t o be unfavour- a b l e i n t h i s case. I n o p t o g a l v a n i c d e t e c t i o n t h e l a s e r beam s h o u l d have about t h e same d i a m e t e r a s t h e cathode t o g i v e an outimum i n s i g n a l / n o i s e . I n D o p p l e r - f r e e n o n l i n e a r soectroscopy where t r a n s i t i o n s need t o be s a t u r a t e d and i n h o l l o w cathodes w i t h l a r g e b o r e s s u f f i c i e n t l a s e r oower may n o t r e a d i l y be o b t a i n e d f o r weak l i n e s .

I n PGLINEX t h e atomic o r i e n t a t i o n i s orobed. I f most o f t h e v e l o c i t y - c h a n g i n g C O ] -

l i s i o n s a l s o randomize t h e a t o m i c o r i e n t a t i o n , t h e b r o a d background i s e l i m i n a t e d . I f t h e o r i e n t a t i o n i s kept,the background p e d e s t a l can s t i l l be decreased 126,271 s i n c e atoms, which have undergone v e l o c i t y - c h a n g i n g c o l l i s i o n s on t h e average spend more t i m e b e f o r e i n t e r a c t i n g w i t h t h e second beam. I n Ref. L271 t h i s i s done by mo- d u l a t i n g t h e p o l a r i z a t i o n a t a r a t e where t h e D o p p l e r - l i m i t e d and D o p p l e r - f r e e s i g - n a l s come i n q u a d r a t u r e and can t h u s be s e p a r a t e d w i t h a l o c k i n a m p l i f i e r . I t

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would be i n t e r e s t i n g t o i n v e s t i g a t e i f t h e same procedure c o u l d be used t o suppress t h e pedestal i n IFS and IMOGS. F u r t h e r , i n Ref. [ 2 6 ] i t i s demonstrated how t h e b r o a d component can be p a r t l y e l i m i n a t e d by t h e a p p l i c a t i o n o f a combination o f weak magnetic and rf f i e l d s . The d e t e c t i v i t y i n POLINEX i s l i m i t e d by t h e d i s c h a r g e n o i s e and c o u l d be s u b s t a n t i a l l y improved by u s i n g a more q u i e t discharge. However, f o r s t u d y i n g r e f r a c t o r y elements t h e same problems a r i s e here as f o r INOGS.

I n o u r experiments t h e PS t e c h n i q u e c l e a r l y gave t h e b e s t s i g n a l / n o i s e r a t i o . S t i l l several improvements can be made i n t h e e x p e r i m e n t a l s e t up; e.g. u s i n g a more fa- v o u r a b l e qeometry [281, employing a " n o i s e e a t e r " [D], e l e c t r o n i c a l l y d i v i d i n g o u t t h e l a s e r n o i s e and/or u s i n g p o l a r i z e r s as c e l l windows [301. The d e t e c t i v i t y i s l i m i t e d by b i r e f r i n g e n c e i n o p t i c a l components and l a s e r i n t e n s i t y f l u c t u a t i o n s . A drawback w i t h PS i s t h a t t h e s i g n a l shapes make t h e a n a l y s i s somewhat more d i f f i - c u l t . However, f o r s t r o n g e r l i n e s h e t e r o d y n i n g u s i n g a small o f f - s e t a n g l e 8 may n o t always be necessary as can be seen i n F i g . 4 which shows a r e c o r d i n g o f t h e even i s o t o p e s i n t h e A=5506 A t r a n s i t i o n o f MO w i t h p o l a r i z e r s crossed and t h e e a r t h maanetic f i e l d compensated. The presence o f a small magnetic f i e l d o t h e r w i s e

Fig. 4.

Fxper-ntal curve for the

5 5 5 5

4d 5s S2

-

^{4d 5p} P3 tran- sition (5506

a ) ,

^recorded

with PS. The earth magnetic field has been campensated.

causes a p r e c e s s i o n o f t h e atomic p o l a r i z a t i o n and i n combination w i t h t h e v e l o c i t y changing c o l l i s i o n s a broad background i s o b t a i n e d [311.

I V - CONCLUSION

I n a hollow-cathode d i s c h a r g e we have s t u d i e d two s t r o n g t r a n s i t i o n s o r i g i n a t i n g i n m e t a s t a b l e s t a t e s , one i n t h e c a r r i e r gas (Ne) and one i n t h e cathode m a t e r i a l (MO) For these c o n d i t i o n s , techniques p r o b i n g d i f f e r e n c e s i n atomic p o p u l a t i o n (IFS,

IMOGS) seemed i n f e r i o r t o techniques p r o b i n g atomic o r i e n t a t i o n (POLINEX, PS). T h i s c o n c l u s i o n i s c l o s e l y r e l a t e d t o t h e absence o f t h e Doppler-broadened p e d e s t a l i n t h e l a t t e r techniques as compared t o t h e f o r m e r ones, f o r which v e l o c i t y - c h a n g i n g c o l l i s i o n s have a d e t r i m e n t a l e f f e c t . L a s t l y we want t o p o i n t o u t t h a t o t h e r t e c h - n i q u e s n o t d i s c u s s e d h e r e such as frequency-modulation spectroscopy [32,33] m i g h t prove t o be even more e f f i c i e n t i n c e r t a i n cases.

ACKNOWLEDGEMENTS

The a u t h o r s w i s h t o thank fike B e r g q u i s t f o r d e s i g n i n g necessary e l e c t r o n i c s . T h i s work was supported by t h e Swedish N a t u r a l Science Research C o u n c i l .

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