BEAM-FOIL MEASUREMENTS OF ATOMIC LIFETIMES IN Co I AND Co II

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BEAM-FOIL MEASUREMENTS OF ATOMIC LIFETIMES IN Co I AND Co II

G. Sørensen

To cite this version:

G. Sørensen. BEAM-FOIL MEASUREMENTS OF ATOMIC LIFETIMES IN Co I AND Co II.

Journal de Physique Colloques, 1979, 40 (C1), pp.C1-190-C1-193. �10.1051/jphyscol:1979138�. �jpa- 00218418�

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JOURNAL DE PHYSIQUE Colloque C l , supplt!menl au n ^O2 , Tome 40, fkvrier 1979, page C1-190

BEAM-FOIL MEASUREMENTS OF ATOMIC LIFETIMES IN CO I AND CO I1

Institute of Physics, University of Aarhus, DK-8000 Aarhus C, Denmark Resum6. Des faiscaux de cobalt d'une energie 200-450 keV ont 6t6 excites par des feuilles de carbone. Les fentes du monochromateur (McPherson ~ o d & l e 218) sont inferieures 5 200 um, et par consdquent les mesures "beam-foil" ont 6t5 faites avec une rSsolution spectrale meilleule qu'avant. Quant aux termes y % et y 4 ~ 1 y 2 d B n s Co I les durdes de vie mesurees sont en accord avec les Tgsultats obtenus par l'effect Hanle et par l'excitation laser,Qm Co I douze durdes de vie, dans Co 11, on ze durees de vie ont 6t6 mesur6as.

Abstract. Cobalt ion beams (200-450 keV) of high intensity (approximately 0.5 uA) were focussed to a diameter of 2.5 mm and foil-excited. Spectrometer slits were usually kept below 200 pm, which made possible beam-foil mefsure- ments with better resolution than previously. For the y 4 ~ 9 h and y G l y ,

levels in Co I, lifetimes of 6.620.6 ns and 8.620.6 ns, respectively, were measured, in excellent agreement with zero-field level crossing a d laser excitation but in disagreement with previously published beam-foil lifetimes. Atomic lifetimes of eleven levels in Co I1 have been measured,and compared to previously published beam-foil lifetimes, they are 20-50% shorter.

INTRODUCTION

Due to the limited spectral resolution in beam-foil spectroscopy, this technique has sometimes been considered inadequate for measurements of atomic lifetimes in complex spectra. However, both the comparative study of Kessler for Fe 1') and the compil- ation of transition probabilities in scan- dium and titanium by Wiese and Fuhr2) have shown that beam-foil lifetimes can be of value even in these complex spectra where the drawback of beam-foilspectroscopy often is foil breakage. Complex spectra urge small spectrometer slits and, consequently, high ion currents have to be used. To some extent, these difficulties can be overcome

3 ) by using the sliding-foil technique

,

which permits ion-current densities in the transition-metal region of 0.5-2 v~/cm,.

The present study reports on beam-foil lifetimes in Co I and Co 11. Together with previously measured lifetimes in Co I, the present data show a consistent picture, whereas' for Co 11, the data are more scarce.

EXPERIMENTAL PROCEDURE

The 600-kV heavy-ion accelerator at

the University of Aarhus was used to accel- erate 200-400-keV cobqlt ions. Beams of 0.1-0.4-+A with a diameter of 2.5 mrn were foil-excited with 5-pg/cm2 carbon foils

(Yissum)

.

The intensity variation of the ion beam was extremely small, usually %I %

.

A McPherson Model-218 spectrometer with slits of 100-300- vm viewed the light downstream of the foil, which was measured with a dry-ice-cooled EM1 6256 S photomul- tiplier tube. The light-intensity decay curves were followed 140 .mrn downstream of the foil in order to perform the cascade corrections properly. The energetic ions were assumed to loose an energy of 25 keV for a 5-vg/cm2 carbon foil in the energy range studied. The theoretically estimated energy loss based upon the LSS theory

(Lindhard et ale4) ) was approximately 30 keV. The emission spectrum from the beam- foil source contained several intense multiplets in Co I and Co 11, but the present study did not attempt to identify all the spectral lines observed. The arc spectrum of cobalt has previously been analyzed by Russell et a1 .5)

,

who classified 2725 lines and identified 768 multiplets. The spectral identification of the beam-f oil

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979138

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UPPER L E M L FOR 5PECl-L TPANYTION

F i g . 1: Beam-foil e m i s s i o n s p e c t r u m of a 400-keV Co-ion beam. Lower s p e c t r u m ( t h i s work) shown an improved s p e c t r a l r e s o l u - t i o n compared t o upper s p e c t r u m ( P i n n i n g - t o n e t a 1 . 6 ) .

spectrum was f a c i l i t a t e d by v a r y i n g t h e i o n e n e r g y and c h e c k i n g t h e i n t e n s i t y r a t i o o f t h e v a r i o u s t r a n s i t i o n s . Although t h e p r e s - e n f s p e c t r a l r e s o l u t i o n c o n s t i t u t e s an ap- p r e c i a b l e improvement compared t o e a r l i e r beam-foil s t u d i e s , l i f e t i m e measurements were o c c a s i o n a l l y performed on 2-3 s p e c t r a l l i n e s b e l o n g i n g t o t h e same m u l t i p l e t . The e m i s s i o n spectrum (2250-2450 8,) o b t a i n e d i n t h e p r e s e n t s t u d y i s i n F i g . 1 compared w i t h t h e spectrum r e p o r t e d by P i n n i n g t o n e t a l S 6 ) and shows a n improvement w i t h r e s p e c t t o s p e c t r a l r e s o l u t i o n .

I n F i g . 2 i s shown a s p e c t r a l s c a n around 3410 A , which shows t h a t t h e 3405-A l i n e d e p o p u l a t i n g t h e y4F7A i s a l m o s t re- s o l v e d from t h e 3412-A t r a n s i t i o n , t h e i d e n t i f i c a t i o n of which i s more u n c e r t a i n , a s d i s c u s s e d l a t e r . T a b l e .I l i s t s t h e m e a s - u r e d l i f e t i m e s i n Co I . Due t o t h e complex- i t y of t h e CO I s p e c t r u m , a c a r e f u l spec- t r a l a s s i g n m e n t was n e c e s s a r y , a n a t h e s t u d y o f ~ u s s e f l e t a l . 5 ) f a c i l i t a t e d t h e a s s i g n m e n t . The 3845- and 3894-A t r a n s i - t i o n s a r e e q u a l l y i n t e n s e and can b e as- s i g n e d a s a 2 F x - y 2 ~ g 2 and a 2 F * - y 2 ~ 0 A prominent s p e c t r a l l i n e a t 3732 A h a s F . a b o u t h a l f t h e i n t e n s i t y of t h e t r a n s i t i o n s from t h e y 2 ~ 0 l e v e l s . T h i s i s a s s i g n e d

SPECTRAL SCAN AROUND 3400A

I I

I I I I

3390 3400 3410 3420 WAVELENGTH

F i g . 2. P a r t i a l s p e c t r a l s c a n o f t h e emis- s i o n s p e c t r u m of a 400-keV Co beam. Two s p e c t r a l l i n e s a t 3405 A and 3412 A a r e o b s e r v e d , and arrows i n d i c a t e t h e s p e c t r o - m e t e r r e a d i n g s where t h e l i f e t i m e measurements were c a r r i e d o u t .

t o b e a m i x t u r e o f t h e 'P % - z 4 p $ + and t h e b 4 p %-z'P 5n t r a n s i t i o n s , which c a n n o t b e r e s o l v e d i n t h e beam-foil s p e c t r u n . The

'F7/, t r a n s i t i o n a t 3704 A y i e l d s a l i f e t i m e which, w i t h i n t h e a c c u r a c y , i s i d e n t i c a l t o t h e 'F7/, l i f e t i m e measured by means o f t h e 3569-8, t r a n s i t i o n . R a t h e r weak b u t w e l l s e p a r a t e d s p e c t r a l l i n e s a r e o b s e r v e d a t 3635 8, and 3649 A w i t h u 2 F b and u 2 F $h a s upper l e v e l , r e s p e c t i v e l y . The s p e c t r a l l i n e a t 3587 A i s a l m o s t a s i n t e n s e a s t h e 3569-A l i n e w i t h 'F a s t h e upper l e v e l . I t may t h e r e f o r e b e a s - s i g n e d a s a 'F %-y 'F

,

b u t t h e l i f e t i m e measured (5.5'0.6 n s ) i s much s h o r t e r t h a n

t h a t f o r t h e y 2 ~ ; , + l e v e l (8.2'1.0 n s ) l i s t e d , s o i t i s p r o b a b l y b l e n d e d by an- o t h e r s p e c t r a l t r a n s i t i o n . The t r a n s i t i o n o b s e r v e d a t 3489 A h a s been a s s i g n e d a s a ' F v - y 'D

&

^;t h i s a g r e e s w i t h t h e o b s e r - v a t i o n t h a t a 3518 t r a n s i t i o n from t h e

DO^^

l e v e l h a s an i n t e n s i t y o f 90% o f t h e 3489-8, l i n e . The s p e c t r a l t r a n s i t i o n o b s e r v e d a t 3474 A a p p e a r s t o b e d u b i o u s b e c a u s e m u l t i p l e t s 4 and 23 have s t r o n g t r a n s i t i o n s a t t h i s wavelength, a4Fs/,- z 4 F X and b4Fsh-y4F7h

,

r e s p e c t i v e l y . The 3284-A t r a n s i t i o n h a s been a s s i g n e d

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c1-192 JOURNAL DE PHYSIQUE

as a -w2 FO transition. In khe arc

2D5/2 7/2

spectrum of cobqlt (Russell et ala5)) are tabulated two rather intense spectral lines at 3154 A and 3168 A , the former as- signed as the b'P5h-u4~$2 transition, the latter is unassigned. In the beam-foil spec- truT, these two spectral lines have the same intensity ratio as that in the arc spectrum. This then justifies that the u ~ D $ ~ is the upper level for the 3154-8, transition. As regards the 3472-A transition (Fig. 2)

,

the lifetime measured

Table I

Atomic l i f e t i m e s o f l e v e l s i n Co I

) Present r e s u l t s 2, Other r e s u l t s

a ) Beam-foil, E.H. P i n n i n g t o n e t a l . 6 b, Beam-foil, E.H. P i n n i n g t o n e t a l . 7 ') Phase-shift, J. Marek and J. R i c h t e r 8

Hanle-effect, M. v. H a r t r o t t 9 Hanle-effect, W.O. K l o t z e t a l . 10)

f,

Laser e x c i t a t ~ o n , J. Marek and ti. v o g t l 1 ) g, Laser e x c i t a t i o n , H. F i g g e r e t a l . 12)

(9.3t1.2 ns) shows an obvious discrepancy with previously published data8)

.

^{If, how-}

ever, this transition belongs to the y 2 ~ 0 which besides the 3845 transition, also deexcitates via a 3412-A spectral line, the measurements areconsistent.

In the ultraviolet part of the emission spectrum, a number of intense Co 11 lines are observed. In Table I1 are listed the measured lifetimes of various upper levels with the multiplet number in paren- theses. Apart from inconsistencies as far as multiplets 7 and 9 are concerned, the agreement with the previously reported lifetimes of Pinnington et a1 .6r 7, is reas- onable when differences in ion energy and corrections for energy loss are considered.

The present set of lifetime data for levels in Co I and Co I1 will, hopefully, increase the conficence level as far as absolute transition probabilities are concerned. Unfortunately, the table of relative transition probabilities of Roig and

~iller' 3, contains only wavelenqths beyond Table I 1

Atomic l i f e t i m e s of l e v e l s i n Co I 1

a l p r e s e n t r e s u l t s

b ) ~ i n n i n g t o n e t a l . (Ref. 6)

p inningt ton

e t a l . (Ref. 7)

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3 9 4 2 A, which is outside the wavelength range of the present study. In order to convert the present lifetimes to absolute transition probabilities, relative intensity ratios covering a larger part of the spectral range would be of value. This would allow a search for trends in the

severe discrepancies which exist between the previously published data, as reported

1 3 ) by Roig and Miller

.

REFERENCES

1 ) K. Kessler, Comments on Atomic and

Molecular Physics, Vol. 11, 1 5 1 ( 1 9 7 1 ) 2) W.L. Wiese, J.R. Fuhr, J.Phys.Chem.

Reference Data

4,

263 ( 1 9 7 5 )

3) G. S@rensen, Proc.IV 1nt.Conf.on Beam- Foil Spectroscopy, Vol. I (Plenum Press, New York, 1976) p. 1 6 5

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Dan.Vidensk.Se1sk.Mat.Fys.Medd.

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5) H.N. Russel, R.B. King. C.E. Moore, Phys.Rev.

58,

407 ( 1 9 4 0 )

6) E.H. Pinnington, H.O. Lutz, G.W. Car- riveau, Nucl.Instrum.Methods

=,

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8) J. Marek, J. Richter, ~stron.Astrophys.

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9) M. v-Hartrott, Z.Naturforsch. 2 4 a t 1 2 0 2 ( 1 9 6 9 )

1 0 ) W.D. Klotz, U. Becker, L.H. ~Bbel,

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