STARK SHIFT AND BROADENING OF ATOMIC LINES AS OBSERVED ON OPTOGALVANIC SPECTRA OF NOBLE GASES

HAL Id: jpa-00223306

https://hal.archives-ouvertes.fr/jpa-00223306

Submitted on 1 Jan 1983

HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

STARK SHIFT AND BROADENING OF ATOMIC LINES AS OBSERVED ON OPTOGALVANIC

SPECTRA OF NOBLE GASES

T. Nakajima, N. Uchitomi, Y. Adachi, S. Maeda, C. Hirose

To cite this version:

T. Nakajima, N. Uchitomi, Y. Adachi, S. Maeda, C. Hirose. STARK SHIFT AND BROADENING OF

ATOMIC LINES AS OBSERVED ON OPTOGALVANIC SPECTRA OF NOBLE GASES. Journal

de Physique Colloques, 1983, 44 (C7), pp.C7-497-C7-504. �10.1051/jphyscol:1983749�. �jpa-00223306�

JOURNAIL DE PHYSIQUE

C o l l o q u e C7, s u p p l 8 m e n t a u n o l l , T o m e 44, novernbre 1983 _{page C7-497}

STARK SHIFT AND BROADENING OF ATOMIC LINES AS OBSERVED ON OPTOGALVANIC SPECTRA OF NOBLE GASES

T. Nakajima, N . Uchitomi, Y . Adachi, S . Maeda and C . Hirose

Research Laboratory o f Resources U t i l i z a t i o n , Tokyo I n s t i t u t e of TechnoZogzg, 4259 Nagatsuta-cho, Midori-ku, Yokohama 227, Japan

Resume

-

Nous rapportons l'effet stark des raies atomiques des gaz rares, dQ au champ electrique present pr6s de la cathode (dans La ri5gion de chute du potentiel), que nous avons observ6 dans une d6- charge de cathode creuse 2 l'aide de la spectroscopie optogalva- nique

.

Abstract

-

We report on the Stark effect, caused by the electric field present inside the cathode-fall region, of atomic lines of noble gases as observed by optogalvanic spectroscopy of hollow cathode discharge.

In the course of our preliminary experiment on the ~ptugalvanic(0G) spectrum of Kr-filled hollow cathode lamp which we wanted to use as a frequency standard, we noticed that the transitions whose lower levels are 2p levels in Paschen notation exhibit splittings and broadenings of the spectral lines. More specifically, a transition from a 2p level consists of a single line at lower discharge current, e.g., 1-2 mA, but as one increases the current it gives rise to one or more components which depart from the original one. The linewidth of the departing components is about two times broader than that of the remaining component and the magnitude of the shift is linear with the discharge current. Typical examples of the feature on Kr 2p6-7d transition is shown on Fig. 1. Apparent independence of the peas frequencies to the position of the path of laser beam through the cathode cylinder was also observed.

To verify the origin of the observed features we carried out further experiment using the Ne- and Kr-filld hollow cathode lamps whose cathodes are of open-ended(see-through) cylinder whereas the lamp used in the previous experiment had stop-ended@-shaped) cathode. We hereafter denote the lamp of the former type as ST-HCL(see-through hollow cathode lamp) and the latter as U-HCL(U-shaped cathode HCL).

The experiment has shown that the shift is observed only when the beam passes near the cathode surface and thus it is caused by the Stark effect from the electric field which is present in the cathode-fall region. The main features of our previous observation on U-HCL apparently came from the OG effect by the light which is reflected back from the bottom end of the cathode. The spatial profile of the OG signal in Ne ST-HCL is much different from that in Kr ST-HCL in the intensity distribution, and this explains why similar feature was not observed in Ne U-HCL.

I

-

Experiment

Experimental set-up(Fig. 2) is basically the same as that used in our previous study of the temporal behavior of the OG effect in Ne U-HCL l . The laser source was a ring dye laser pumped by an Ar ion laser of 4W power, and the output power was typically 140 mW with the

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1983749

JOURNAL DE PHYSIQUE

-40 0 +20 GHz 0 5

F.ig.1

-

Result of Preliminary Experiment: spectral Profiles of Kr 2p6-7d3 transition at various discharge current(1eft) and the plot of

- -

the frequencies of the shifted component against the discharge current (right) are shown. Hollow cathode lamp with a stop-ended cathode was used in the experiment.

frequency width of 20 MHz. A small fraction of the beam was led into an iodine cell to monitor the frequency. The main beam was chopped at 1 k ~ z by an E.O. modulator and was focused at the middle point of the cathode by a lens whose focal length was 25 cm so that theconfocal parameter is about the same as the length of the cathode. The

experiment on the U-HCL in our previous observation had been done using a lens of 8 cm focal length. The hollow cathode lamps purchas- ed from Hamamatsu Photonics Ltd. were mounted on an XYZ-slider and the radial position of the beam's path were varied by micrometers which drive the slider. The inner diameter and the length of the U-HCL are 3mm and 14 mm, respectively, and those of the ST-HCL are 4mm and 17 mm, respectively. The lamps are factory-sealed at the filling pressure of 6 torr. The operating condition of the lamps were determined by the stabilization of the discharge. The OG signal was picked up at cathode which is grounded through the resistance of 25 kQ while the anode was connected to a voltage-regulated power source.

ischaxg

recorder

Fig. 2

-

Experimental setup:

Signal was picked up at the cathode. 12;iodine cell used as frequency standard, E.O.;

electro-optical modulator, H.V.; voltage-regulated d.c.

power source, 3;ballast resistor (25 kfl)

,

C; output capacitor (0.025 pF)

The t r a n s i t i o n s chosen f o r t h e p r e s e n t s t u d y were 2p6-7d11 and 2p6-7d3 of K r which a p p e a r a t 17384 cm-' and 17299 cm-', r e s p e c $ i v e l y , and t h e 2pg-4d4; and 2pg-4d11 of Ne a t 17342 cm and 17391 cm

,

r e s p e c t i v e l y . The 2p P -3d 3 D l i n e s of He d i d n o t show o b s e r v a b l e s h i f t and t h e

11 2

A r - and X e - f i l l e d ST-HCL f a i l e d t o s t a b i l i z e .

I1

-

R e s u l t and A n a l y s i s : K r ST-HCL and U-HCL

The OG e f f e c t o f K r ST-HCL was examined a t t h e d i s c h a r g e c u r r e n t of 8.7mA, 10.1 mA, and 10.8 mA f o r t h e 2p -7d

'

t r a n s i t i o n and a t 9.8 mA f o r t h e 2p6-7d3 l i n e . Unlike i n o u r $ r e l & m i n a r y e x p e r i m e n t on K r U- HCL t h e OG s i g n a l s observed by u s i n g t h e ST-HCL gave r i s e t o a s i n g l e component whose l i n e w i d t h d i d n o t show t h e broadening a s observed on t h e s h i f t e d component i n t h e c a s e o f U-HCL, b u t t h e f r e q u e n c i e s of t h e observed l i n e s v a r i e d depending on t h e d i s c h a r g e c u r r e n t and a l s o on t h e p o s i t i o n of beam's p a t h . The s p a t i a l and c u r r e n t dependent v a r i a t i o n was a l s o o b s e r v e d on t h e s p e c t r a l i n t e n s i t y .

We show i n F i g . 3 , t h e f r e q u e n c i e s and i n t e n s i t i e s o f t h e OG s i g n a l s which a r e o b t a i n e d f o r t h e K r 2p6-7d t r a n s i t i o n a g a i n s t t h e r e a d o u t of t h e micrometer a l o n g w i t h t h e e l e E t r i c f i e l d d e r i v e d from t h e observed s h i f t s . The c a t h o d e i s l o c a t e d a t t h e l e f t h a n d s i d e of t h e f i g u r e and i t s p o s i t i o n a s c a l c u l a t e d from t h e a n a l y s i s of t h e s h i f t s i s i n d i c a t e d by a n arrow a t 6.22 mm. The v o l t a g e a c r o s s t h e

e l e c t r o d e s and t h e d i s c h a r g e c u r r e n t a r e i n d i c a t e d i n t h e f i g u r e . I t s h o u l d b e mentioned t h a t t h e s h i f t s i n t h e p r e s e n t c a s e d i d n o t change l i n e a r l y w i t h t h e c u r r e n t . The s i g n of t h e OG e f f e c t i s s o d e f i n e d t h a t a p o s i t i v e s i g n a l c o r r e s p o n d s t o t h e c a s e when t h e i r r a d i a t i o n i n c r e a s e s t h e p i c k u p v o l t a g e a t t h e c a t h o d e , t h a t i s , t h e i n t e r - e l e c t r o d e p o t e n t i a l d e c r e a s e s by i r r a d i a t i o n . The r e s u l t i n F i g . 3 was o b t a i n e d when t h e p o l a r i z a t i o n o f t h e l a s e r was s e t perpen- d i c u l a r t o t h e r a d i a i d i r e c t i o n of t h e c a t h o d e c y l i n d e r w h i c h c o i n c i d e s w i t h t h e d i r e c t i o n of t h e movement of t h e beam's p a t h w i t h r e s p e c t t o t h e c y l i n d e r a x i s . The e x p e r i m e n t u s i n g t h e l i g h t beam of i t s p o l a r - i z a t i o n p a r a l l e l t o t h e r a d i a l d i r e c t i o n and of c i r c u l a r p o l a r i z a t i o n gave e s s e n t i a l l y t h e same r e s u l t a s t h a t shown i n F i g . 3 .

v (cm-') 17384.70

-,60 F i g . 3

-

S p a t i a l p r o f i l e o f t h e OG s i g n a l o f K r 2p -7d11

17384.60 l i n e a s o b t a i n e d by u s f n g

ST-HCL a t t h e d i s c h a r g e c u r r e n t of 1 0 . 1 mA. On a b s c i s s a i s p l o t t e d t h e

17384.50 r e a d o u t of t h e micrometer

;20 and t h e p o s i t i o n of t h e c a t h o d e s u r f a c e i s i n d i c a t e d by a n arrow. Peak f r e q . (b)

,

e l e c t r i c f i e l d ( a )

,

and t h e O peak i n t e n s i t y ( v e r t i c a l l i n e )

a r e shown.

C7-500 JOURNAL D€ PHYSIQUE

The S t a r k e f f e c t o f t h e K r l i n e s h a s been measured by F o s t e r and Horton u s i n g t h e Lo Surdo method/2/ and t h e s h i f t s a t t h e e l e c t r i c f i e l d of 10 kV/mm have been g i v e n by them t o b e 65.9 cm-' and 65.7 cm -1 f o r t h e 2p6-7d3 and 2p6-7d18 t r a n s i t i o n s , r e s p e c t i v e l y . The e l e c t r i c f i e l d i n s i d e t h e c a t h o d e c y l i n d e r were c a l c u l a t e d by u s i n g t h e above v a l u e s and t h e d e r i v e d v a l u e s , which a r e p l o t t e d i n F i g . 3, a r e s e e n t o d e c r e a s e by t h e d i s t a n c e from t h e c a t h o d e s u r f a c e i n t h e s i m i l a r way a s t h a t r e p o r t e d by L i t t l e and von Engel who measured t h e f i e l d s t r e n g t h i n t h e N 2 d i s c h a r g e u s i n g t h e method of e l e c t r o n beam d e f l e c t i o n / 3 / . The d e r i v e d v a l u e s c a n be r e l a t e d w i t h t h e p a r a m e t e r s which c h a r a c t e r i z e t h e c a t h o d e f a l l of a glow d i s c h a r g e s i n c e t h e e l e c t r i c f i e l d a t d i s t a n c e X from c a t h o d e , E ( x ) , i s known t o b e e x p r e s s e d by t h e f o l l o w i n g e q u a t i o n / 3 / .

E(x) = E o ( l - x / d c ) and

E. =2Vc/dc (1)

where, and VC a r e t h e f i e l d s t r e n g t h and t h e p o t e n t i a l a t t h e c a t h o d e , r e s p e c t i v e l y , and d i s t h e l e n g t h of t h e c a t h o d e f a l l r e g i o n . The v a l u e s of t h e C p a r a m e t e r s f o r t h e hollow c a t h o d e d i s c h a r g e of K r i n t h e ST-HCL a r e d e r i v e d from t h e d a t a a s g i v e n i n T a b l e 1.

T a b l e 1

-

P a r a m e t e r s d e r i v e d from t h e S t a r k e f f e c t i n K r hollow c a t h o d e d i s c h a r g e

t r a n s i t i o n V c ( v o l t ) I(mA) dc ON") E. (kV/mm)

2 ~ 6

-

7d3 315.0 9 . 8 0.94+0.05

-

-0.6720.03

2p6

-

7d11 311.6 8.7 0.95+0.05

-

-0.66+0.03

-

316.8 1 0 . 1 1.00+0.05

-

-0.6320.03

320.5 10.8 1.00+0.05 -0.6420.03

I t s h o u l d b e n o t i c e d from F i g . 3 t h a t i n t h e c a s e of K r t h e OG s i g n a l s o b t a i n e d by i r r a d i a t i n g w i t h i n 0.3 mm from c a t h o d e s u r f a c e i s a b o u t t e n t i m e s s t r o n g e r t h a n t h o s e one g e t s from p a s s i n g t h e l a s e r beam t h r o u g h t h e c e n t r a l r e g i o n . F u r t h e r m o r e , t h e i r r a d i a t i o n o f t h e c e n t r a l r e g i o n g i v e s t h e s i g n a l which i s o p p o s i t e i n s i g n from t h a t o b t a i n e d from t h e n e a r - c a t h o d e r e g i o n a l t h o u g h t h e d i s t a n c e a t which t h e s w i t c h o v e r t a k e s p l a c e might depend on t h e d i s c h a r g e c u r r e n t a s i s t h e c a s e i n Ne d i s c h a r g e , and p r o b a b l y on t h e s i z e of t h e c a t h o d e . Re-examination of t h e K r - f i l l e d U-HCL r e v e a l e d t h a t w h e n o n e i r r a d i a t e s a t t h e d i s t a n c e o f l e s s t h a n 0.5 mm from t h e c a t h o d e t h e s i g n a l

c o n s i s t s of two broadened components. One of them behaves s i m i l a r l y b o t h i n i t s s h i f t and i n t e n s i t y t o t h a t we o b t a i n e d on ST-HCL and g i v e s t h e v a l u e s of d =0.48 mm and-E =1.02 kV/mm. The o t h e r com- ponent, on t h e o t h e r gand, s t a y s a t t h e same i n t e n s i t y and f r e q u e n c y by t h e movement o f t h e beam p o s i t i o n and we a s s i g n it t o t h e one which had been o b s e r v e d i n o u r p r e l i m i n a r y e x p e r i m e n t . An unbroaden- ed s i g n a l a p p e a r s when we i r r a d i a t e a t t h e d i s t a n c e of more t h a n 0.5

Fig.4

-

Result of re-examined Kr U-HCL.

Spatial profile of the electric field derived from the OG signals is shown.

Vertical lines along the abscissa indicates the peak intensities of the shifting component. The position of cathode is shown by an arrow.

E.

mm from the cathode. The result for the 2p6-7d line in U-HCL is shown in Fig. 4 where the field strength and sigaal intensity are plotted against the distance from the cathode. The distance is resticted to where significant shift is observed.

In the case of U-HCL we have the incident beam reflected back at the bottom end of the cathode and it is not an easy job to give some definite explanation to the observation but it seems reasonable to assign the shifting component to the one which comes from the

incident beam, and the broadening of the signal by the factor of about two is probably due to the collisions with electrons and Kr atoms.

Kr 2p6-7d3

\ 10 mA

I11

-

Result and Analysis: Ne ST-HCL

The spatial variation of the frequencies and intensities of the peaks of the OG signals of Ne 2pg-4d4' at 17342 cm-I and those of the 2pg- 4d11 at 17389 cm-' are shown in Figs. 5 and 6, respectively. The

c k v ~'\246~O O O Q O ~ V

observation of the latter transition was carried out at the discharge current of 1.9 mA, 2.9 mA, 3.9 rnA, 4.9 mA, 6.0 mA, and 7.1 mA.

-0.5-

i I

^{I . .}

0.5 nun

(cm-') 1 7 3 4 3 ~ 0 @

17342.95

Ne 2p9-4d4' 4.4 m A

173.7 V

a a @ a @ a a O * a * a a a e a a a a e @ a a Fig.5 -Spatial profile of OG signal of Ne 2pg -4d4' transition.

I I 1 1 1 The peak frequencies ' ' ' ' l ]

I l l '

^I are shown by and the

peak intensities are indicated by vertical lines. The cathode is

I I i l

-

located on the left-

9.0

q'

^{10.0 l . m m} hand side as indicated by an arrow.

JOURNAL DE PHYSIQUE

ad

;E

_m

I a m

W a;

a a, N Ll U N

m d ) r l

a, m

z-mcl

m 4 7 w a , 0 0 c w n

-4 0 m

4 4

m c w C 4 c l 0

t r m m

-4 U acl

m -4 3 cl4 0 U Ll m a o a.4 m

+ a a, a , - m Ll

c m ~

@ a , h

.d a, a, w c l c c l

0 . 4 Q a,

m E

cl C m o

C a , a , h a c l 3 U Ll C . 4 - 4 h . 4 Ll E

3 a

U Y a,

@ . c c a , a , u c l P a.4

Q c . G

m a 3 c l

c C -4

; m 2 3 4 - - c , a, a z a , a m m o m E 7 a L l

0.d U 5

.4 U .4 C u c E m m a , c

> 3 w l 0'0.I-J c l @ c m h c l tr W 7 . 4 a, O M

d ~ a

- 4 a m c l u - c a ) a ) m

o a u

Q a

a d ) @ a , A c c l 4 E I c l m

m U

.d cl 0 .I-J m 4

m . G a a . 4 m m 0 m . 4

.d b, I c l r d a ,

.4 a

W_{P m}

.

^C ~^3s _{o c l} C O .c( L l c m

h u m u

While no shift is observed on the 2p8-4d4' transition,there exist shifts of about an order smaller than those on Kr 2p6-7d18 lines on the transition 2p9-4dl'. As seen from Figs. 5 and 6 the spatial profile of the strength of OG effect differs from that in Kr and the switch- over of the signal takes place twice along the radial direction.

The first switchover arises at about 0.4 mm from the cathode and this distorts the spectrum considerably to invalidate the measurement of the frequencies around this position. Thus it is not possible to carry out the analysis similar to that we have done on Kr. It has been reported by Foster that the Stark shifts of Ne 2pn-4dA1 and 2pg- 4d, lines at the electric field of 70.6 kV/cm is -8.8cm-l' and 33.2 cm =l

,

respectively/4/, and from these values and the magnitude of the observed shifts we conclude that the length of cathode fall, dc, is about 1 mm and the field at the cathode E. is around-0.35 kV/mm for the hollow cathode discharge of Ne in the ST-HCL.

Another characteristic feature of the OG effect of Ne in ST-HCL is its spatial profile(Figs. 5 and 6). Rather weak and positive signal is observed when the close vicinity of the cathode is irradiated, the intensity of the signal decreases as the irradiated position moves away from the cathode and switches over to become negative at about 0.4 mm above the cathode surface, the negative signal grows in its magnitude until it reaches an maximum and then decreases to pass the point where the second switchover takes place, and the final positive signal keeps growing until the beam reaches the center of the cathode.

The distance at which the second switchover takes place is strongly dependent on the discharge current and its value decreases from 1.9 mm at 1.9 mA to about 0.8 mm at 7.1 mA, while the position of the first switchover is virtually constant at all discharge current except

at 2 rnA. The discharge at 2 mA is rather special because the stable

discharge has just commenced at that condition. The maximum intensi- ty for each phases of the profile also shows a strong dependence on the current. These characteristics undoubtedly is closely related with the mechanisms of the OG effect and thus with the microscopic processes within the discharge plasmas although the authors are not ready to present them at the moment.

In conclusion, the present authors have demonstrated that the frequen- cy shifts caused by the Stark effect, of which the electric field is the one present in the cathode fall region of the glow discharge, are observed by the laser optogalvanis spectroscopy, and thus the method offers a unique technique for probing the electric field inside the discharge plasmas/5/, The method should be especially useful in the study of the discharges at the pressures of more than 1 torr since beyond that pressure the conventional method of electron beam deflect- ion becomes inapplicable because of multiple scattering unless one raises the beam energy. Combination with the spatial profile of the OG effect should provide a unique viewpoint of the microscopic

processes of glow discharge.

It has also been shown that the OG spectroscopy is a useful method in atomic and molecular spectroscopy too in observing the Stark effect of the transitions from excited states or of unstable species produced in the discharge. It should be noticed that the values of dc and E. which areneeded to derive the electric field E (X) by Eq. 1 can be obtained experimentally without the knowledge of the coefficient of

JOURNAL D€ PHYSIQUE

Stark effect because the plot of the square-root of the shift,(Av) 1/2

,

V.S. the distance from the cathode surface gives the value of dc which in turn gives the value of E. through the measured value of VC.

The authors are grateful to Komatsu Ltd. for the lease of the ring dye laser system which was an essential tool in the present investigation.

REFERENCES

1. Uchitomi N., Nakajima T., Maeda S., and Hirose C., Opt. Commun.

44

(1983) 154

2. Foster J. S. and Horton C. A., Phyl. Trans. Ray. Soc. =(l9381 473

3. Little P. F. and von Engel A., Proc. Roy. Soc. London, ~ z ( 1 9 5 4 ) 209

4. Foster J. S., Proc. Roy. Soc. London, A E ( 1 9 2 8 ) 80

5. Francis G., Handbuch der Physik, 22(1956) 53: Various ways of measurering the quantities defining the glow discharge are reviewed in this article.