OSCILLATOR STRENGTH DETERMINATIONS FOR Δn=0 TRANSITIONS IN Be-LIKE IONS OF THE THIRD PERIOD ELEMENTS.

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OSCILLATOR STRENGTH DETERMINATIONS FOR

∆n=0 TRANSITIONS IN Be-LIKE IONS OF THE THIRD PERIOD ELEMENTS.

D. Pegg, J. Forester, P. Griﬀin, G. Alton, S. Elston, B. Johnson, M. Suter, R.

Thoe, C. Vane

To cite this version:

D. Pegg, J. Forester, P. Griﬀin, G. Alton, S. Elston, et al.. OSCILLATOR STRENGTH DETERMI- NATIONS FOR ∆n=0 TRANSITIONS IN Be-LIKE IONS OF THE THIRD PERIOD ELEMENTS..

Journal de Physique Colloques, 1979, 40 (C1), pp.C1-205-C1-207. �10.1051/jphyscol:1979141�. �jpa-

00218421�

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JOURNAL DE PHYSIQUE Colloque C1, supplkment au n o 2, Tome 40, fkvrier 1979, page C1-205

OSCILLATOR STRENGTH DETERMINATIONS FOR An=O TRANSITIONS IN Be-LIKE IONS OF THE THIRD PERIOD ELEMENTS (u).

D.J. Pegg, J.P. Forester, P.M. Griffin, G.D. Alton, S.B. Elston, B.M. Johnson ( W ) ,

M. Suter, R.S. Thoe and C.R. Vane,

University of Tennessee, Knoxville, Tennessee, U . S . A . and Oak Ridge National Laboratory, Tennessee, U.S.A.

R69umB. Les zorces d'oscillateurs mesurees pour les transitions El An=o du type 2s -2s2p-2p dans les ions tres charges de la sequence iso~lectronlque ⁱ³Be inter- viennent dans l'interpretation des diaqnostiques des sources B trSs haute temperature de laboratoire et astrophysique. Les durees de vie des niveaux bas dans plusieurs ions 5 4 electrons (13 d 217) ont 6tG mesurees avec la technique faisceau-lame. ~

Abstract. Measured oscillator strengths for An=O E 1 transitions of the type 2sL-2s2p- 2p' in highly ionized members of the Be isoelectronic sequence are valuable in the interpretation of diagnostic measurements from high temperature laboratory and astrophysical sources. Lifetimes of low-lying levels in some 4-electron ions (13<Z<_17) have been measured with the beam-foil technique.

1. INTRODUCTION

The Be atom and its isoelectronic ions provide theorists with an excellent testing ground for calculations based upon m~lticonfi~urational atomic structure models. Electron correlation is important (especially for low Z systems) due to the interpenetrability of the shell-equivalent 2s and 2p valence electrons and consequently the Be sequence has received much theoretical attention Ill,

C21, C33. Relativistic corrections also become significant as one goes to higher Z members of the sequence due to the fact that An=O transitions

2 2

between the low-lying 2s

,

2s2p and 2p configurations represent relatively small differences between large binding energies.

Oscillator strengths (f-values) are particularly sensitive to the appropriateness of the eigenfunctions used in calculations and thus provide a convenient "bridge" between experiment and theory; At the time of the last BFS Conference in Gatlinburg very little experimental f value data was available for few-electron systems with Z>10.

In the interim period a considerable amount of data has been accumulated, particularly for Li-like and Be-like ions. In this paper we concentrate on measurements for Be-like ions with 13SZ117.

Accurately measured f values for highly ionized simple systems find practical application in the interpretation of spectral data from both

laboratory and astrophysical high temperature plas- ma sources. These line strength quantities for example are currently needed in both diagnostic work and investigations of the unwanted cooling of magnetically confined thermonuclear plasmas by radiative energy losses from heavy,multiply-char- ged impurity ions. On the astrophysical side, both rocket-borne and satellite instruments have indicated the presence of Be-like ions of high Z in the quiet-sun and solar flare coronal spectra. The pro- minent resonance lines of ~e-like ions have been used as a diagnostic tool for determining electron temperatures and densities in the solar corona.

Figure 1 shows the possible terms arising from the 2s2, 2s2p and 2p configurations of the 2 Be-like ion, S XIII. The four allowed An=O transitions that are the subject of this paper are indicated. Transition probabilities for intercombination transitions are relatively small and can be neglected in this work. Miihlethaler and Nussbaumer

121 for example, estimate the branching ratio for the 2s2p 3 ~ 2

-

2p2 1 ~ 2 transition to be 161 in the case of S XIII. The sole experimental study of an intercombination transition in the Be-like system was made recently by Dietrich et al. C41 who ob-

tained an f value of 0.0024+0.0007 for the 2s'

-

2s2p P1 transition in Fe XXIII. This forbidden 3 process arises of course as a consequence of singlet-triplet mixing caused by the spin-orbit

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979141

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c

1 -206 JOURNAL D E PHYSIQUE

Fig. 1. Partial energy level diagram showing the possible An=O transitions in the Be-like ion,

S XIII.

interaction. The measurement implies a mixing of a few percent.

Figure 1 also serves to indicate the type of cascading that might be expected for such systems Out-of-shell transitions (i.e. from levels with

n

>

³⁾are negligible for high Z ions except at po-

sitions extremely close to the foil due primarily to the W 4 scaling dependence on the transition rates. Thus the levels associated with the 2p con- 2 figuration are not troubled significantly by cascading and decay curves from these levels are well- fitted hy single exponential functions. In-shell cascading into the 2s2p level from the 2p2

'so

and 2p2 D' levels is however expected since their 2

lifetimes are roughly equivalent and they are com- parably populated. The 2p2 1 ~ 2 level, being some four times longer lived than the primary level, is easy to take account of by standard curve-fitting procedures. The 2p2

'so

level however poses a far more serious problem in the data analysis since its lifetime is approximately two thirds that of the 2s2p PI level which it feeds strongly. 1

2. EXPERIMENT

The time-resolved technique of beam-foil spectroscopy was employed in the measurement of the radiative lifetimes of levels arising from the 2s2p and 2pL configurations of Be-like ions. The standard time of flight apparatus that is used in the present

Fig. 2. Multiplet oscillator strengths for some ions of the Be sequence. Theoretical values are indicated by continuous lines CRef.21 and dotted lines CRef.31. Experimental results are due to:

OCRef.61; A CRef.71; V CRef.81; 0 represents the present work.

investigation has been described in detail pre- viously by Pegg et al. [5]. The specific energy of

the ion beam extracted from the Oak Ridge National Laboratory model EN tandem accelerator was selec- ted to efficiently produce four-electron ions of the element under study in the foil-excited source.

The lifetimes were measured under conditions of

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T A B L E 1.

T r a n s i t i o n

O s c i l l a t o r s t r e n g t h s

A1 X S i X I P X I 1 S X I 1 1 C 1 X I V

( a ) Ref. C61 (b) Ref. C51 ( C ) Ref. C71 Ref. C83

moderate s p e c t r a l r e s o l u t i o n (X/AX Q 10 3 ) and good temporal r e s o l u t i o n ( t / ~ ^Q1 0 -1 ) .

3. RESULTS

T a b l e 1 and F i g u r e 2 s e r v e t o summarize t h e f v a l u e r e s u l t s f o r i o n s w i t h 1 3 2 2 ~ 1 7 . The o n l y s i g n i f i c a n t s y s t e m a t i c d i s c r e p a n c y between e x p e r i - ment and t h e o r y i s i n t h e c a s e of t h e 2s' 'S

-

2 ~ 'P 2 ~t r a n s i t i o n where, a s h a s a l r e a d y be:n men- 1

t i o n e d , s t r o n g c a s c a d i n g from t h e 2p2

'so

l e v e l i s v e r y d i f f i c u l t t o account f o r i n t h e d a t a a n a l y s i s . I n g e n e r a l t h e r e s u l t s a r e i n s l i g h t l y b e t t e r agree- ment w i t h t h e i n t e r m e d i a t e - c o u p l i n g c a l c u l a t i o n s of Miihlethaler and Nussbaumer C21 t h a n w i t h t h e Z-ex- p a n s i o n r e s u l t s o f Safronova e t a l . C32.

REFERENCES

(*) Research s u p p o r t e d by NSF, ONR, Dept. of Energy and Union Carbide Corp. under c o n t r a c t w i t h t h e Dept. o f Energy.

( * * ) ~ e r m a n e n t Address: Brookhaven N a t i o n a l Laborato- r y , Upton, N.Y., U.S.A.

[I] For example, A. H i b h e r t , Beam-Foil S p e c t r o s c o p y e d i t e d by I.A. S e l l i n and D.J. Pegg (Plenum P r e s s , New York) 1976 Volume 1, p.29; C.A. N i - c o l a i d e s , D.R. Beck and 0 . S i n a n o g l u , J . Phys.

B

6,

p.62 (1973); C. Laughlin, E.R. C o n s t a n t i - n i d e s and G.A. V i c t o r , J . Phys. B

2,

p.2243

(1978).

[Z] H.P. Miihlethaler and H. Nusshaumer, Astron. and Astrophys.

48,

p. 109 (1976).

[3] U . I . Safronova, A.N. Ivanova, and V.N. K h a r i t o - nova, Theor. and Expt. Chem.,

5,

p.325 (1969).

B.M. Johnson and D . J . Pegg, Phys. Rev.

18

A t o b e p u b l i s h e d ,

C51 D . J . Pegg, J.P. F o r e s t e r , C.R. Vane, S.B.

E l s t o n , P.M. G r i f f i n , K-0. Groeneveld, R.S.

P e t e r s o n , R.S. Thoe and I.A. S e l l i n , Phys.

Rev. A

2,

p. 1958 (1977).

C61 E. T r a b e r t , P.H. Heckmann, and H.v. B u t t l a r , 2. P h y s i k ,

3,

p. 333 (1977).

[71 B. Denne, D.J. Pegg, E. A l v a r e z , R. H a l l i n , K. I s h i i , J . P i h l and R. S j o d i n , t o be pub- l i s h e d i n t h e s e p r o c e e d i n g s .

C81 K. I s h i i , E. A l v a r e z , R. H a l l i n , J.K. Lind- skog, A. M a r e l i u s , J . P i h l , R. S j i j d i n , B.

Denne, L. Engstrijm, S. H u l t and I . Martinson, P h y s i c a S c r i p t a ,

18,

p. 57 (1978).

C41 D.D. D i e t r i c h , J.A. L e a v i t t , S. Bashkin; J.G.

Conway, H. Gould, D. MacDonald, R. Marrus,