BEAM-FOIL SPECTROSCOPY OF IODINE

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BEAM-FOIL SPECTROSCOPY OF IODINE

J. O’Neill, E. Pinnington, K. Donnelly, R. Brooks

To cite this version:

J. O’Neill, E. Pinnington, K. Donnelly, R. Brooks. BEAM-FOIL SPECTROSCOPY OF IODINE.

Journal de Physique Colloques, 1979, 40 (C1), pp.C1-194-C1-196. �10.1051/jphyscol:1979139�. �jpa-

00218419�

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JOURNAL DE PHYSIQUE Colloque Cl , supplkment au n o 2, Tome 40, fkvrier 1979, page C1-194

BEAM-FOIL SPECTROSCOPY OF IODINE

J.A. O'Neill, E.H. Pinnington, K.E. Donnelly and R.L. Brooks

Department of Physics, University of Alberta, Edmonton, Canada T6G 251

Rgsumg - On presente les rgsultats d'une Etude sur l'iode entre 400 et 13001 avec la technique faisceau- lame.

Abstract

-

We report the results of beam-foil investigations of iodine between 400 and 13002.

1.INTRODUCTION

-

No previous beam-foil investi-

ns15 nplp ns3s np3p n d 3 0

----*

--- --- ----.

gations of iodine have been published. In this

b r l a Ltrn~t 5 2: 592,000 sml ~ ~

paper we report the results of both spectroscopic

and lifetime measurements on foil-excited iodine

'I-

beams, adding new terms to the energy level diagrams of I VI and I VII and extending previous studies of f-value trends along isoelectronic sequ- ences in this region of the periodic table.

2.SPECTRAL ASSIGNMENTS

-

Spectra were obtained from a beam of foil-excited hydrogen iodide ions at energies of 0.9, 1.3 and 1.7 MeV. Manyunclassified lines were found. At these energies we expect to see lines of I V, VI and VII, ;o isoelectronic extrapolation techniques were used in an attempt to associate measured wavelengths with transttio~s i r these ions. Unfortunately, the Moseley diagrams for ions isoelectronic with I V show the effects of strong perturbations, so that extrapolations to I V could not be made with any great accuracy. This prevented positive assignment of any transitions in I V. In I VI, however, the Moseley diagrams are much smoother and accurate wavelength predictions could be made, especially for transitions already known in Xe VII. These wavelength extrapolations enabled us to make seven new assignments. Lines at 600, 612, 613, 549 and 6531 are assigned to 5s5p 3~

-

5s5d 3 ~ , a line at 7841 is assigned to 5s2 'S - O2 1s

-

5s5p 'P and a line at 11218. is assigned to 5s

3 ⁰

5s5p PI. All the assignments given are consistent with lifetime measurements (see below); in addition the splittings of the 3~ level derived from our measurements are in excellent agreement with those of Even-Zohar and Fraenkel. El]

A Grotrian diagram of all known energy levels in I VI is shown in Fig. 1. Our observation of the

1 2

line at 7841 allows the three S terms from 5s ₀

,

5 ~ 6 s - a n d 5s7s to be put on an absolute energy scale.

Assuming the terms follow a Ritz formula, the ioni- zation limit for this series is 73.4 ? 1.0 eV. This is in good a'greement with the Hartree-Fock value of Fraga et al. [2], 72.0 eV. A further consequence of our observation of the 7842 line is that the wave-

Fig. 1 : Grotrian Diagram of I VI length of the transition 5s6p 'P

-

^{557s '}^S^{can be}

calculated using lines at 293 and 3436: from [l] (see diagram). The value found is 12981; a line at this wavelength in our spectra completes the loop and provides valuable confirmation of the other three assignments.

In our region of observation, ten lines of I VII have been classified previously. The five lines given by Moore [3] were all prominent in our spectra, but the identification of the lines given by [I] presented some problems. The two lines at 434 and 4641, assigned in [I] to 5p 2~

-

6s 2~ showed an anomalous intensity ratio, with 4341 much weaker than theory would suggest. In the region 550 -5551, where [l] classifies three lines from 4f 2~ - 5 g 2 G,

only one line was apparent in our spectra. Asyrast transitions are well known to be strong in beam- foil spectra, the non-appearance of these lines is particularly striking.

As a solution of these problems in I VII, we suggest that the 4641 line is a blend of 4f 2~ -5g G 2 and 5p 2~ - 6 s S. This explains the anomalous in- 2 tensity ratio of the lines 434114641, as well as the observations of Sugar [4] who was unable to re- concile the value for the 4f-5g wavelength from [l: with a value extrapolated from the isoelectronic

sequence Nd through Cs. Sugar's extrapolated value

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979139 Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979139

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TAB:,;: 1. Decay Cmve Andy&&

06

Iodine

Ion

(i)

Transition Lifetime Cascades

f-values

his

Expt. Isoelectronic Theory I

(ns) E X ~ ~ S .

VI 600 5 ~ 5 p ~ ~ ~ - i s 5 d 3 D O.lit.06 0.84?.1,9?1.

3

7

VI 612 5 ~ 5 p ~ ~ ~ - 5 s 5 d D 0.22k.02 0.79i.Z,15ill. >O.46?.10 0.51 [9] 1.47 (HF) VI 649 5s5p3P2-5856 D3 3 0.23f.02 0.90i.l,9fS. 2

VT 784 5s2 'So -5s5p 'p1 0.25 ? .03 0.77 t .3 1.10 f .15 1.08 [9] 2.15 (HF)

., 1 2

VI 1121 5sL L ~ O -5s5p 'IP L 24.42 .20 0.33 2 .2

VII 592 5p 'p4 - 5d D 2 ₂ c0.17 t .02 0.95

+

^.2 ^--,

} I >0.66 0.47 [9] 1.25 [7]

<0.16f.02 VII 640 5p :P~

-

5d D

% ^.08²^.04,1.06^=k^.2 VII 834 5s Sl -5p 2~

/

0.34

*

^.03 ^?

2 5 ) ^0.90f^-05 ^{0.73 [9]} ^{1.09 [7]}

VII 953 5s S4 -5p 2P: 0.48t.04 - )

for the wavelength is 457

'

151, so that this reas- signment of 4641 leads to good agreement with his results.

3.LIFETIMES

-

^Acomplete list of lifetimes of classified lines is given in Table I. The uncer- tainties given correspond to one standard deviation in the computed lifetimes plus 0.7% from the beam velocity calibration. Three methods were used in the bean velocity calibrarion; Doppler shifts of spe~tral lines observed at precisely measuredangles to the beam, nuclear resonances of accelerated pro- tons with F" and the observation of quantum beats in emission from a beam of helium. The results of all three methods were consistent within the 0.7%

error.

For most transitions the data were analyzed using the multi-exponential fitting program HOMER. How- ever, for the two lines 834 and 9531 the primary lifetimes were derived using an ANDC analysis [5]

to allow for strong cascading, particularly down the yrast chain. The main cascade will be from the 5d levels, with some contribution from 6s. Both these decays were included in our analysis. Unfor- tunately, our 6s decays were recorded at 464ibefore it was realized that this line is blended with the 4f

-

5g transition. However, the ANDC analysis gives very similar results both with and without this cascade, demonstrating the dominance of the 5p

-

5d cascade route.

The accuracy of our results can be checked bycal- culation of lifetime ratios within multiplets. For situations where branching to more than one lower term can be ignored, the lifetime of any level J' of the upper term should be given by the following proportion:

where L(J,J') is the line strength factor. The re- lative lifetimes for the multiplets 5s S - 5 p 2 P il; 2 I VI and 5 ~3p-5s5d 3~ in 5 ~ I VI are in good agree.

ment with this theory within the quoted uncertain- ties. However, in the multiplet 5p 2~ _-5d D in 2 I VII there is a marked disagreement, the measured ratio of lifetimes being 1.07

-

^-15and the theoretical value being 0.83. We take this as an indica- tion that these lifetimes are approaching the lower limit of our experimental arrangement, and the values given in Table I1 should be regarded asupper

limits.

The only possible decay mode for the level Ss5p 3~

in I VI is an intersystem transition to 5s2 IS, made possible by admixture of 'P character into the 3~

term. The relative lifetimes of the 5s5p singlet and triplet terms is therefore an important test of theories of the interactions causing this mixing.

Fortunately both lifetimes are measurable on our apparatus. Assuming that the mixing is caused by the spin-orbit interaction, the theoretical ratio of lifetimes is:

1 3 3 3

,.A ,31'~( P~)-E( P~)-[E( P~)-E( p0)]/2',, 2

- - 2 s

7 , - \ A 1 : 7 3 3

E( PI)-E( Po)

,i

where E( 1 @ ) = energy of 'P level and so on. The

1 1

comparison with experiment is shown in Fig. 2 for the three members of the isoelectronic sequence that have been measured. The agreement with experi mcnt is quite close at the high Z end but there is a factor of two discrepancy at the neutral end.

This is caused by the omission of the spin-other- orbit interaction. If this interaction is included,

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el-196 JOURNAL D E PHYSIQUE

7

T ( 3 ~ 1 ) / T ( I p1 ) for 5p electron

- -

-

o : experiment

I I I I I I I *

Fig. 2 : Ratio of Lifetimes of 'P and 3~ Levels the theoretical ratio of lifetimes becomes (see Wolfe [6]):

The comparison of this theory with experime.~t is vcry good c t borh ends of the isoelectronic sequence covering a range in T ~ / T I of a factor of 20.

4.f-values - To facilitate comparison withtheory, we hsve converted our lifetime measurements to multiplet f-values, using the following formula to

relate measured line f-values to multiplet f-values for a transition between a lower term i with levels J and upper term k with levels J ' :

and our MCHF results are listed, along with themean experimental f-value for the preceding four members of the isoelectronic sequence. There is markeddis- agreement between our results and theory for I VI.

Our values are supported by the good agreement with previous experiments and the ratio of 'P and P 3

lifetimes described earlier. Possibly better agreement will be achieved when theoretical calculations, including relativistic and polarization effects, be- come available for this ion. In I VII the agreement between theory and experiment is much more satisfac- tory. In Fig. 3, we display the results of Dirac- Hartree-Fock calculations [7] and of beam-foil mea-

surements [8,9] for the Ag I sequence. Both ANDC

IPII S n l P C d l I

0.5

-

RHF Cheng &Kim

-

Aarhus

A Edmonton (M-E)

A Edmonton ( A N D C )

-

0 Î Î Î

-

P r X m XeXlU T e n I n m A g I

l/Z -

Fig. 3: Comparison of Theory and Experiment for

,.

^.-,

5s L~

-

5p L~

where GJ is the appropriate Racah coefficient. This

formula is readily derived using L-S coupling theory and HOMER results are shown for I VII and Xe VIII and the definition of multiplet f-values. In cases to illustrate the effect of cascading. The experi- where branching ratios are necessary to calculate mental trend lies generally about 20% below theory,

line f-values, we have again assumed L-S coupling. however Cheng and Kim [7] estimate that core polar- The experimental multiplet f-values are listed in ization effects may lower the calculated f-value by Table 1. For comparison, Dirac-Hartree-Fock [7] this amount.

References

[I] M. Even-Zohar and B.S. Fraenkel, J. Phys. 2 , proceedings).

1596 (1972).

[6] H.C. Wolfe, Phys. Rev.

41,

443 (1932).

[2] S. Fraga, Handbook of Atomic Data, Elsevier,

[7] K. Cheng and Y. Kim, to be published.

New York, 1976.

[3] C.E. Moore, N.B.S. Circular 467, 1958. [8] T. Andersen and G. Sorensen, J. Quant. Spect.

Rad. Trans.

13,

369 (1973).

[4] J. Sugar, J . Opt. Soc. Am.

67,

1518 (1977).

[9] T. Andersen, A.K. Nielsen and G. Sorensen, [5] E.H. Pinnington and R.N. Gosselin (these Phys. Ser.

5,

122 (1972).