OPTICAL-OPTICAL DOUBLE RESONANCE EXPERIMENTS WITH GALLIUM AND INDIUM ATOMS VIA OPTOGALVANIC DETECTION

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OPTICAL-OPTICAL DOUBLE RESONANCE EXPERIMENTS WITH GALLIUM AND INDIUM

ATOMS VIA OPTOGALVANIC DETECTION

H.-O. Behrens, G. Guthöhrlein, A. Kasper

To cite this version:

H.-O. Behrens, G. Guthöhrlein, A. Kasper. OPTICAL-OPTICAL DOUBLE RESONANCE EXPER-

IMENTS WITH GALLIUM AND INDIUM ATOMS VIA OPTOGALVANIC DETECTION. Journal

de Physique Colloques, 1983, 44 (C7), pp.C7-239-C7-250. �10.1051/jphyscol:1983720�. �jpa-00223277�

JOURNAL DE PHYSIQUE

Colloque C7, supplkment au nO1l, Tome 44, novembre 1983 page C7-239

O P T I C A L - O P T I C A L DOUBLE RESONANCE EXPERIMENTS W I T H G A L L I U M AND I N D I U M ATOMS V I A OPTOGALVANIC D E T E C T I O N

H.-0. Behrens, G.H. Guthiihrlein and A. Kasper

HochschuZe der Bwzdeswehr Hamburg, 2000 Hamburg 70, HoZstenhofieg 8 5 , F.R.G.

RBsumd

-

Les sequences d e s t r a n s i t i o n s s u i v a n t e s

d a n s l e s p e c t r e atomique d ' i n d i u m e t d e s sgquences s i m i l a i r e s

dans l e s p e c t r e atomique de g a l l i u m o n t & t d S t u d i s e s

a

l ' a i d e de deux l a s e r s .Z c o l o r a n t s monorrodes c o n t i n u s p a r l a mdthode d e d o u b l e rdsonance o p t i q u e . Les atomes s o n t c r d e s d a n s l e plasma de l a c a t h o d e c r e u s e p a r

" p u l v d r i s a t i o n " e t i l l u m i n 6 s p a r l e s deux f a i s c e a u x l a s e r s s e p r o p a g e a n t c o l l i n d a i r e m e n t d a n s l a meme d i r e c t i o n ou e n s e n s i n v e r s e . Pour c e s deux 6ldments l e s e x p 6 r i e n c e s o n t 6 t 6 f a i t e s avec d e s i s o t o p e s d'abon- dance n a t u r e l l e ; d a n s l e c a s de g a l l i u m d e s mesures s u p p l 6 m e n t a i r e s o n t e t 6 f a i t e s avec un 616ment f o r t e m e n t e n r i c h i e n i s o t o p e 71. Les c o n s t a n t e s h y p e r f i n e s d e s niveaux 8p 2p0 e t 6p 2p0 d e s t r a n s i t i o n s s o n t donnses.

A b s t r a c t

-

Looking a t t h e t r a n s i t i o n s e q u e n c e s

i n t h e indium a t o m i c s p e c t r a and a t s i m i l a r sequences

4 s 2 4p 2 ~ ; / 2 A=417.206 n% 4 s 2 5 s 2 ~ 1 / 2 A=641.401 11x4s 2 6p 2 ~ ; / 2

i n t h e g a l l i u m atomic spectrum and u s i n g two s i n g l e mode t u n a b l e cw- dye-lasers,optical-optical d o u b l e r e s o n a n c e measurements were p e r - formed. The atoms were i n t r o d u c e d by s p u t t e r i n g i n a hollow-cathode plasma which c o u l d be i l l u m i n a t e d by t h e two a l l i n e a r l a s e r beams

p r o p a g a t i n g e i t h e r p a r a l l e l o r a n t i p a r a l l e l . W i t h b o t h e l e m e n t s measure- ments were done w i t h t h e n a t u r a l i s o t o p i c m i x t u r e ; i n t h e c a s e o f g a l l i u m a d d i t i o n a l e x p e r i m e n t s w i t h h i g h l y e n r i c h e d gallium-71 were performed. R e s u l t s c o n c e r n i n g t h e h y p e r f i n e i n t e r a c t i o n c o n s t a n t s o f t . 8p 'Pp. and 6p 2 ~ * l e v e l s and t h e i s o t o p i c s h i f t s o f t h e l i n e s w i l l be g i v e n .

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1983720

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Optical-Optical Double Resonance Effect:

Optical-optical double resonance (also called "cross-saturated absorp- tion" or " absorption-line narrowing

" )

in dopplerbroadened three

level-systems in interaction with two quasi-resonant single mode lasers has already been observed in neon /1,2/ and demonstrates the capability of, this method for high resolution dopplerfree spectroscopy. Resonances of this kind can be observed with high signal to noise ratios by moni- toqing the transmission of one of the laser beams or by detecting the laser induced fluorescence from one of the excited levels while tuning the laser frequency. In our case we instead have used the powerful technique of optogalvanic detection to monitor the optical-optical double resonance.This was straightforward, because we already used the hollow-cathode discharge for the sputtering of the cathode material in order to produce free atoms of the species we wanted to investigate.

Furthermore we also used the same discharge to excite the sputtered atoms( which normally are in their ground states) into high lying levels which by this process became accessible for spectroscopic studies. The system under study consists of a number of three level systems (cascade or ladder-type

)

resonantly interacting with two laser beams of very different frequencies (many times the dopplerwidth

)

propagating either in the same (€=+l) direction or in opposite direc- tions

( E =

-1) (Fig. 1)

Fig. l-Energy level diagram and charac- teristics of laser fields

Ao =spectral splitting

Ao

=dopplerwidth Q I D =tuned laser beam

Q2

=fixed frequency laser

beam

The first laser Q l (saturating beam

)

selectively interacts with those atoms on the transition oI2 over a narrow axial velocity range corres- ponding to the homogeneous width, which are dopplershifted into reso- nance,i.e. with one definite velocity class of atoms which ?,'-fill tkL.;

condition:

k ~ 2

V~ =

"I - ^{O12 (v,}

⁼

resonant velocity class)

-.

The second laser R2 probes the resulting velocity selection in level

2

on his transition

o

,allowing thus dopplerfree spectroscopy.The double resonance coadition is met, if the second laser Q2 is resonant on his transition

⁰²³

with the selectively excited veloclty class vR

In our case the probing 1as;;')beam

R

has a fixed frequency and the

frequency of the saturating laser beim Q, is tuned over the doppler-

p r o f i l e o f t h e t r a n s i t i o n o I 2 .Monitoring t h e o p t o g a l v a n i c s i g n a l s t r e n g t h a t t h e f i x e d f r e q u e n c y Q2 f n t h e hollow c a t h o d e d i s c h a r g e v e r s u s t h e t u n e d f r e q u e n c y

n l ,

o p t l c a l - o p t i c a l double r e s o n a n c e s a r e o b s e r v e d a t t h e f r e q u e n c y :

I f l e v e l 3 e x h i b i t s a h y p e r f i n e s p l i t t i n g ( Ao between 3 and 3 ' ) and t h e r e s p e c t i v e e r a n s i t i o n s 0 2 3 and

o22

'

o v e r l a p w i t h i n t h e d o p p l e r - w i d t h , t h e r e i s a n o t h e r r e s o n a n t v e l o c l y c l a s s of atoms v ,which s e e s t h e f i x e d f r e q u e n c y l a s e r d o p p l e r s h i f t e d i n t o r e s o n a n c e w i g t h e t r a n - s i t i o n 0 2 3 , . T h i s g i v e s rise t o a n o t h e r d o u b l e r e s o n a n c e :

Q1 (1+2+3' ) = o I 2

+

^E ( k * ₁₂

/

k23 ) ( Q 2

-

_O23'

The s p e c t r a l h y p e r f i n e s p l i t t i n g o f l e v e l 3 : Ao= o

23'

-

^O23

a p p e a r s i n a double r e s o n a n c e experiment a c t u a l l y a s

T h i s r e l a t i o n shows t h a t it i s advantageous t o u s e such k i n d o f t r a n s i - t i o n s , t h a t t h e t u n a b l e l a s e r Q, h a s t h e s h o r t e r wavelength and t h a t t h e t r a n s i t i o n s d i f f e r v e r y much i n f r e q u e n c y i n o r d e r t o a c t u a l l y enhance a s m a l l s p l i t t i n g o f l e v e l 3 and t o r e s o l v e even v e r y c l o s e s t r u c t u r e s . By s i m i l a r r e a s o n s one can show t h a t t h e i s o t o p i c s h i f t one a c t u a l l y ob- s e r v e s i n s u c h a k i n d o f d o u b l e o p t i c a l r e s o n a n c e e x p e r i m e n t s i s g i v e n by t h e fellowing fornuila :

Q 1 ( I s o t o p e 1 )

-

^{R I} ( I s o t o p e 2 ) = IS 12 - E (k12'k23) IS23 I S l 2 : t r a n s i t i o n i s o t o p i c s h i f t on t r a n s i t i o n o

12 I S 2 3 : t r a n s i t i o n i s o t o p i c s h i f t on t r a n s i t i o n 0 2 3

p r o v i d e d t h e mass number o f i s o t o p e 1 i s g r e a t e r t h a n t h e mass number o f i s o t o p e 2

.

The h i g h e s t s i g n a l s t r e n g t h s f o r d o u b l e o p t i c a l r e s o n a n c e s a r e o b s e r v e d when t h e f i x e d f r e q u e n c y l a s e r R 2 i s t u n e d i n t o r e s o n a n c e w i t h t h e atomic v e l o c i t y c l a s s vR=O.For g r e a t e r d e t u n i n g s t h e signal-litude d e c r e a s e s r a t h e r r a p i d l y , s o t h a t e x p e r i m e n t a l l y o b t a i n e d s i g n a l i n t e n s i t i e s d i f f e r from p r e d i c t e d t h e o r e t i c a l o n e s .

One i m p o r t a n t m e r i t o f t h e d o p p l e r f r e e t e c h n i q u e o f d o u b l e o p t i c a l r e s o n a n c e i s t h e a b s e n c e o f d i s t u r b i n g c r o s s - o v e r r e s o n a n c e s , which under c e r t a i n c o n d i t i o n s can b e r a t h e r annoying.The r e a s o n f o r t h e i r d i s a p p e a r a n c e i s t h a t a l t h o u g h b o t h t r a n s i t i o n s ' h a v e one l e v e l i n common, t h e i r f r e q u e n c y s e p a r a t i o n i s many t i m e s t h e d o p p l e r w i d t h s o t h a t t h e r e a r e no atoms which c o u l d s i m u l t a n e o u s l y be i n r e s o n a n c e w i t h b o t h t r a n s i t i o n s .

E x p e r i m e n t a l S e t Up :

I n t h e e x p e r i m e n t a l arrangement o f o u r d o u b l e o p t i c a l resonance e x p e r i - ments w i t h l a s e r beams p r o p a g a t i n g i n t h e same d i r e c t i o n o r i n o p p o s i t e d i r e c t i o n s ( F i g . 2 and 3 ) we used two a c t i v e l y s t a b i l i z e d ( j i t t e r l e s s t h a n 1 MHz ) cw- s i n g l e - f r e q u e n c y dye l a s e r s . ~ e p e n d i n g on t h e wavelength r e g i o n , S t i l b e n 1 and e i t h e r Rhodamin 6 G o r Rhodamin B were used as-dyes To superimpose b o t h d i f f e r e n t f r e q u e n c y l a s e r beams f o r p a r a l l e l

i r r a d i a t i o n a d i c h r o i c m i r r o r was used.For t h e a n t i p a r a l l e l a l l i n e a r c a s e b o t h l a s e r s had t o b e decoupled by d i s p e r s i n g p r i s m s t o a v o i d any m a l f u n c t i o n o f t h e s t a b i l i z a t i o n c i r c u i t of t h e l a s e r s .

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Fig. 2 -Experimental set up for parallel laser beams

Fig. 3 -Experimental set up for antiparallel laser beams

To use the phase sensitive detection technique the fixed frequency laser beam R 2 was intensity-modulated by a mechanical chopper and the chopper-frequency was fed as reference frequency into a lock-in ampli- fier.The second tunable laser beam R I was unmodulated.

Both laser beams were axially directed through a hollow-cathode dis- charge lamp, which was cooled by liquid nitrogen to ensure a quiet and stable discharge over a wide region of different rare gas

pressures and sustaining currents.The bore of the hollow cathode was 40mm long and had a 3mm inner diameter-The hollow cathode was made out of copper and its cylindrical bore was covered either with a metal foil of indium or with a thin film of gallium.Symmetrically on both sides

o i tile .m;-c i n a d i s t a n c e o f 1 to 2 FT c y l i n d r i c a l 7 locca t i e r e

a d a p t e d t o e n s u r e , t h a t t h e whole l e n g t h o f t h e b o r e was c o v e r e d w i t h t h e n e g a t i v e glow.The d i s c h a r g e was o p e r a t e d by a r e g u l a t e d power s u p p l y o v e r a 10 kR b a l l a s t r e s i s t o r (Rv) and t h e modulated a c -0pto- g a l v a n i c s i g n a l was f e d t h r o u g h t h e c o u p l i n g c a p a c i t o r (Ck) t o t h e l o c k - l n a m p l i f i e r

.

P.leasurements :

A s m a l l p a r t o f t h e indium f i n e s t r u c t u r e l e v e l scheme w i t h r e l e v a n t t r a n s i t i o n s f o r t h e o p t i c a l - o p t i c a l r e s o n a n c e e x p e r i m e n t s i s shown i n F i g . 4 :

Two e x p e r i m e n t a l r e c o r d i n g s o f o p t o g a l v a n i c a l l y d e t e c t e d ( i n d o p p l e r - l i m i t e d t e c h n i q u e ) t r a n s i t i o n s a r e shown i n F i g . 5 and F i g . 6

.

P i g . 5

-

E x p e r i m e n t a l r e c o r d i n g o f t h e t r a n s i t i o n

5 s 2 5p 2

h=410.176 n x 5 s 6 s

'sIl2

JOURNAL DE PHYSIQUE

- .

c l g . 6 - E x p e r i m e n t a l r e c o r d i n g o f t h e t r a n s i t i o n 5 s 2 6 s 2 ~ 1 / 2 h= 572.768 n q 5 s 2 8 p

Indium ( Z = 4 9 ) h a s two s t a b l e i s o t o p e s l 1 5 1 n ( 9 6 % ) a n d ' I 3 1 n ( 4 % ) i n t h e n a t u r a l a b u n d a n t c o m p o s i t i o n - B o t h n u c l e i h a v e I = 9 / 2 a s t h e n u c l e a r s p i n quantum nurnber.Fig.7 e x p l a i n s i n a n o t t o s c a l e h y p e r f i n e and t r a n s i t i o n scheme t h e s p e c t r a l a s p e c t s o f t h e i n d i u m o p t i c a l - o p t i c a l Zouble r e s o n a n c e e x p e r i m e n t s f o r t h e above-mentioned c o u p l e d t r a n s i - t i o n s . S i n c e a l l o f t h e t h r e e f i n e s t r u c t u r e l e v e l s have J = 1 / 2 , b e c a u s e o f h y p e r f i n e i n t e r a c t i o n w i t h t h e n u c l e u s ( I = 9 / 2 ) t h e t o t a l a n g u l a r momentum numbers a r e e i t h e r F=4 o r F=5

.

I-.

The h y p e r f i n e s p l i t t i n g (8.4 GHz) o f t h e i n t e r m e d i a t e s t a t e 6

2 ~ 1 12 i s much w i d e r t h a n t h e d o p p l e r w i d t h o f t h e X=572.768 nm t r a n s i t i o n , s o t h a t a s a consequence t h e f r e q u e n c y o f t h e f i x e d l a s e r c a n b e chosen t o match t r a n s i t i o n s which o r i g i n a t e from e i t h e r t h e F=4 o r t h e F=5 l e v e l of t h e i n t e m d i a t e s t a t e .

W i t h i n t h e d b p p l e r w i d t h o f t h e X=572.768 nm t r a n s i t i o n t h e r e a r e two d i f f e r e n t v e l o c i t y c l a s s e s o f atoms which c a n b e r e s o n a n t w i t h t h e f i x e d f r e q u e n c y l a s e r R 2 b e c a u s e o f d o p p l e r t u n i n g o n d i f f e r e n t hyper- f i n e t r a n s i t i o n s . The l a s e r s e l e c t i v e l y marks d e f i n i t e c l a s s e s o f atoms which g i v e rise t o d o u b l e r e s o n a n c e s i g n a l s , when t h e l a s e r R I i s t u n e d o v e r t h e A=410.176 nm t r a n s i t i o n and r e s o n a n t l y m e e t s marked atoms.1n t h e s p e c i a l c a s e t h i s w i l l happen t w i c e , s o t h a t t h e r e w i l l b e a l t o g e t h e r f o u r o p t i c a l - o p t i c a l d o u b l e r e s o n a n c e s .

Because o f t h e wide h y p e r f i n e s p l i t t i n g ( 1 1 . 4 GHz) -, o f t h e 5 s -, 2 5p 2 ~ ; / 2 l e v e l and t h e n a r r o w s p l i t t i n g (221 MHz) o f t h e 5 s L 8 p 'P' l e v e l

,

t h e s e f o u r d o u b l e r e s o n a n c e s a r e g r o u p e d i n n a r r o w d o u b l e l L 2 w h i c h a r e f a r a p a r t - E x p e r i m e n t a l r e c o r d i n g s o f o p t o g a l v a n i c a l l y d e t e c t e d o p t i c a l - o p t i c a l d o u b l e r e s o n a n c e s a r e shown i n F i g . 8 t o F i g . 1 3 .

Fig. 8

-

Optical-optical double resonance in indium-1 with a n t i p a r a l l e l l a s e r beams on

the

t r a n s i t i o n sequence

2 . 2

5p P , i =410.176 r q 6s S1/2 X=572.768 8p 2~;,2

.

L a s e r R2 i s fixed on t r a n s i t i o n s s t a r t i n g from the %4 interrrediate level.

Fig. 9- The s m t r a n s i t i o n sequence as i n Fig. 8 but with p a r a l l e l l a s e r beams and l a s e r R2 i s fixed t o a lower frequency ;the dopplerfree signals are shifted i n relation t o t h e broad pedestal.

S h a r p d o p p l e r f r e e s i g n a l s w i t h a b o u t 120 MHz FWHM a s y m m e t r i c a l l y s i t on b r o a d p e d e s t a l s m a i n l y c a u s e d b g 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 b e t - ween a c t i v e indium atoms i n t h e 6 S i n t e r m e d i a t e l e v e l and p e r -

t u r b i n g neon a t 0 m s . h c e r t a i n p a r t o f l & e indium atoms which a r e ex- c i t e d t o t h e i n t e r m e d i a t e s t a t e by a b s o r b i n g t h e

n l

l a s e r l i g h t a r e t r a n s f e r r e d by 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 o t h e r e s o n a n t a t o m i c v e l o c i t y c l a s s marked by t h e l a s e r R 2 , t h e r e b y p r o d u c i n g c o l l i s i o n i n d u c e d o p t i c a l d o u b l e r e s o n a n c e s i n n a l s . T h e d i s c h a r g e c o u l d o n l y b e

C7-246 JOURNAL DE PHYSIQUE

run in a quiet mode down to a minimum neon pressure of about 0.5 Torr, so that certainly the main part of the homogeneous linewidth is caused by collision broadening.

In the course of the optical-optical double resonance experiments a discharge current as small as permissible for a sufficient signal- to-noise ratio was used in order to minimize any kind of selfabsorp- tion in the resonance line A=410.176

nm

which could distort the line shapes.

The optical-optical double resonance signals are labeled by the triple of F-quantum numbers of the corresponding levels i.e. (21111) means

(Flower'2 lFintermediate=' lFupper=l

)

-

Fig.lO+ Fig.11 -The same transition sequence as in Fig.8.Laser R2 is fixed on transitions starting from the F=5 intermediate level.

Fig.12-The same transition sequence as in Fig.8.Laser R is detuned to much lower frequencies ;the recording shows ?he pro- nounced relative shift of dopplerfree signals and broad pedestal.

A s is clearly seen the signal-to-noise ratio also diminishes.

As every resolved doublet in the optical double resonance signals

reproduces the hyperfine splitting of the uppermost final level

there is a large overdetermination of the value for this splitting so that very small errors for this value are possible.

Under certain discharge conditions (higher currents) even hyperfine -

state changing collisions (spin-flip-collisions) are detectable Fig.13 The laser R2 was fixed for transitions out of the F=4 hyperfine

level of the intermediate state, so that only atoms in this level can be excited to the final state and can give rise to an optical- optical double resonance signal. While scanning laser R, atoms are also pumped out of the ground state into the

F=5

hyperflne level of the intermediate state. By state changing collisions they are transferred to the F=4 intermediate state into the marked velocity class of atoms and give rise to optical double resonance signals induced by state changing collisions.In cases, where one has to quantitatively analyze velocity changing collisions one has to consider such kind of processes.

Fig.13-Optical-optical double resonance signals induced by

state changing collisions (arrows).The same transition sequence as in Fig.8 .Because of higher discharge currents the doublets are not any more reso1ved.A dopplerlimited optogalvanic recording of the A=410.176 nm transition is underlaid to show the correlation.

In addition optical-optical double resonance experiments have also been performed in the atomic spectrum of gallium Fig.14 .

Gallium (Z=31) has two stable isotopes

:

gallium-69 (60%) and gallium-71 (40%) in the natural abundant composition-The nuclear spin quantum number of both isotopes is

I

=3/2 .Measurements were done with natural gallium and also with a highly enriched isotopic probe of gallium-71 (99,6%) in order to get less complex double resonance spectra and to make the identification of the components much easier.The following two transition sequences have been examined:

4s 4p 2 ~ > / 2 h=417.206 nm, 4s2 5s 2

2~

2

4=639.661 nm, 4s 6p

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For the first transition sequence the spectral aspects of the optical-optical double resonance experiment are explained in a not to scale hyperfine transition and level scheme Fig.15

.

Fig.14-Fine structure level and transition scheme for the lowest levels of gallium

(not to scale)

Fig.15-Wyperfine level and transition scheme (not to scale) for the first transition sequence 4s2 4p 2~:/2 ⁺ 4s 5s 2 ~ 1 / 2 2 ⁺ 4s2 6p 2 ~ 0 1 /2 A dopplerlimited recording of the transition A=417.206 nm obtained with natural gallium is shown in Fig.16.

Fig.16-Dopplerlimited recording of the line h=417.206 nm

Two examples of optical-optical double resonances in the gallium atomic spectrum withh parallel and antiparallel laser beams are shown in Fig.17 and Fig.18 .The additional resonances caused by state changing collisions are marked by arrows.

Fig.17-Optical-optical double Fig.18-Optical-optical double resonance recording with resonance recording with anti- parallel laser beams parallel laser beams

Conclusions:

The optogalvanic detection of optical-optical double resonances has proved to be a sensitive and efficient technique.

The liquid nitrogen cooled hollow cathode discharge allowed to obtain spectra with a good signal-to-noise ratio even in the noisy environ- ment caused by sputtering.

The optical-optical double resonance technique allowed to resolve narrow spectral splittings even in the presence of collisions and to determine these splittings with high precision.

As a result the signs of the hyperfine interaction constants of the

JOURNAL DE PHYSIQUE

2 0

8p 2 ~ > / 2 and 8p P3/2 fine structure levels of indium and of the and 6p 2 ~ 0 3,2 fine structure levels of gallium could be derived to be p0sitive.B~ the principle of the quantum beat method only the absolute values of the hyperfine interaction constants could be obtained /3,4/

.

Since the analysis of the hyperfine measurements is still in progress only preliminary results for the isotope shift of the gallium

transitions h=639.661 nm and h=641.401 nm can be given. We find the isotopic shift (71-69) = 140 (20) MHz .This is in excellent agreement with previously published results derived by the collimated beam technique and stepwise excitation /5/ : 143 (8) MHz for

h=639.661 nm and 149 (8) MHz for h=641.401 nm

.

REFERENCES

/l/ DELSART C.,Optics Communications 15 (1975) 91 /2/ DELSART C. ,Optics Communications 16 (1 976) 388 /3/ NEIJZEN J.H.M.,Physica 98C (1979) 143

/4/ NEIJZEN J.H.M., submitted to Physica

C

/5/ JONSSON G. ,Physics Letters 93A (1983) 121