EUV SPECTROSCOPY OF HIGHLY IONIZED ATOMS

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EUV SPECTROSCOPY OF HIGHLY IONIZED ATOMS

E. Alexander, B. Fraenkel, S. Goldsmith, S. Hoory, M. Even-Zohar

To cite this version:

E. Alexander, B. Fraenkel, S. Goldsmith, S. Hoory, M. Even-Zohar. EUV SPECTROSCOPY OF HIGHLY IONIZED ATOMS. Journal de Physique Colloques, 1971, 32 (C4), pp.C4-64-C4-66.

�10.1051/jphyscol:1971414�. �jpa-00214615�

JOURNAL DE PHYSIQUE

Colloque C4, supplkment au no 10, Tome 32, Octobre 1971, page C4-64

EUV SPECTROSCOPY OF HIGHLY IONIZED ATOMS

E. ALEXANDER, B. S. FRAENKEL, S. GOLDSMITH, S. HOORY and M. EVEN-ZOHAR.

Hebrew University, Jerusalem

Rksum6. - Les procedes experimentaux pour l'analyse des spectres EUV des atomes hautement ionids sont decrits, et tout particulikrement la differentiation des degres d'ionisation par un chan- gement des paramktres Blectriques de l'ktincelle dans le vide.

Les mkthodes &identification et de classification par extrapolation isoelectronique et Bvaluation th6orique sont expliquees. Comme exemple : des relations d'intensite sont apportees pour des lignes nouvelles de la sequence isoelectronique de Ni I des atomes Y XI + MO XV, a la lumikre des calculs de combinaison de termes.

Abstract. - Experimental procedures for the analysis of EUV spectra of highly ionised atoms are described, especially the differentiation of degrees of ionisation by changing the electrical parameters of the vacuum spark.

Methods of identification and classification by isoelectronic extrapolation and theoretical calcu- lations are explained and exemplified on lines in the Ni I isoelectronic sequence for atoms of the fifth row : Y XI t MO XV. Intensity relations are discussed in the light of caIculations of term mixing.

Space research has excited new interest in vacuum UV spectroscopy in the laboratory in order to supply necessary information on identification and degree of ionization for observed lines.

With this interest in view, the Jerusalem group started spectroscopy of highly ionized atoms in the wavelength range below 300 A.

l. Experimental. - 1.1 SPECTROGRAPH.

Our spectrograph [ l ] is of unconventional design, based on a central axle at the centre of the Rowland circle ; this axle bears arms for the grating, slit system and plate holder. This is contained in a nearly spherical stainless steel container.

Our grating is an original Siegbahn grating of 2 m curvature, ruled area 4'

1 cm and 30 000 grooves/inch.

We are very thankful to Prof. M. Siegbahn for the gift of this grating which proved very satisfying in the wavelength range investigated. It was used at grazing incidence of 50.

In order to achieve maximum resolution with the relatively small radius of the instrument, we were careful to work a t optimal conditions : Optimal width of the grating area, slit width of 1 to 3

1 0 - ~ mm and adjustment of the slit locus. This last adjustment was different for every wavelength range investi- gated. The resolution obtained was 20.000, at 200 A.

1 .2 DIFFERENTIATION

- Spectra of the same atom at different degrees of ionization generally overlap strongly. The differentia-

tion between the degrees of ionization is therefore an almost necessary condition for succesful spectroscopy in this field. The high voltages of 50 t 100 kV applied in the ordinary vacuum spark are unfavourable for this purpose. We used therefore the sliding spark of Vodar [2] and various modifications of the three electrode spark of Ballofet [3]. By changing the electric parameters of the discharge : voltage, capacity and especially inductance, the relative intensities of spectra of different degrees of ionization vary, while the rela- tive intensities of lines of the same degree of ioniza- tion stay practically constant. Comparisons of spectra taken on the same plate with different electrical para- meters made the differentiation of ionization in most cases unambiguous. Voltages were applied up to 20 kV, capacitors used were of 2-20 pF capacity.

The lowest inductance of the whole assembly : space chamber, capacitors and other circuit elements, was 70 nH. External inductances up to 1 000 nH were added. Peak amperages reached 250 kA.

On additional remark in this connection : It proved advantageous to compare spectra taken on the same plate with plate mask fixed and the plate itself heigh- tened or lowered behind the mask. When the plate is fixed and the mask moved, the exposed part of the plate c< sees the spark from different angles at the two exposures. By using pinhole methods [4] we were able to show that the degree of ionization changes along the spark plasma, so that the place of maximum line intensity along the line changes its height on the plate for different degrees of ionization. Moving of the mask may therefore change the relative intensities.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1971414

EUV SPECTROSCOPY OF HIGHLY IONIZED ATOMS C4-65 I t is possible to utilize this behaviour of the sparks

for ascertaining a certain degree of ionization in those cases, where no lines are known for a certain degree of ionization.

Wavelengths were measured by using known refe- rence lines. Least square and other methods were applied.

2. Identification and classification. - The main method for identification used was isoelectronic extrapolation. This extrapolation was generally based on least square fittings along the isoelectronic sequence, using the necessary number of parameters.

Deviations from Russel-Saunders coupling occur of course frequently, resulting in interchange of the order of term levels, deviations from intensity rules, etc., and appearance of forbidden lines (for instance between singlet and triplet terms).

Structure formula, term splitting regularities, cal- culations of Slater integrals and spin-orbit interac- tion by Racah methods and Hartree-Fock calculations were used in order to make the classifications as relia- ble as possible.

Isoelectronic extrapolation methods were also used in order to predict, from the observed and calculated behaviour of configurations along the sequence, those regions, where interactions between configurations disturb the regular isoelectronic behaviour of the terms investigated [g]. In these cases, isoelectronic extrapola- tion had sometimes to be stretched beyond 2-3 inter- mediate atoms, so that the classification across the disturbed region may not always be as reliable as in the cases of an unbroken sequence.

CO I, Ni I, Cu Iland Ga I were investigated for Y, Zr, Nb and MO.

Some of this work (part of a Ph. D thesis of Even- Zohar) will be published elsewhere. In table I1 results for the isoelectronic sequence of Ni I are reproduced, in order to illustrate the kind of theoretical considera- tions applied.

Two strong lines in these spectra were known in this sequence up to Y and correspond to the transitions 3d1° 'So-3 d9 4 p 'P, and 3 d i 0 'So-3 d9 4 p 3D,.

It seemed likely that the high intensity of the forbid- den line 1So-3D, could be explained by a coupling deviating strongly from LS coupling.

We therefore studied the term structure of 3 d9 4 p in the isoelectronic sequence by calculating for Ga IV, Ge V and As V1 the electrostatic and spin-orbit parameters (Fo, F2, G,, G,, t,, 5,) by least square methods and diagonalization for J ⁼ l. These para- meters were then extrapolated as functions of z (the degree of ionization) : F,, F2, G , , G, depend linearly on z ; 5,, changes with z4 and was therefore extrapo- lated from 5:r which was very linear with z. 5,, was extrapolated by a procedure given by EdlCn [13].

We found, that the spin-orbit parameters 5 are larger than the electrostatic ones. Howxer, because of the large coefficients multiplying the electrostatic parame- ters, the energies of the levels are mainly determined by the electrostatic interaction, and correspond very closely to the appropriate scheme of Condon-Shortley for L-S coupling.

However, the intensities should be determined by the t2rm mixing produced by the spin-orbit parameters.

Calculations for Y XI1 to MO XV gave on the average the following composition for 3 d9 4 p :

3. Results. - In table I a list of transitions classi-

73 %

+

12 %

+-

^{15 %}

fied by the Jerusalem group is given. Most of the older 3P1

88 % ,P1 i- 12 % 3D1 work dealt with atoms in the fourth row, in the iso- 'P, -+ 84 % 'P1 f 15 % 3D1 lectronic sequences of Ar I, K I, C a I. Several hundreds

of lines were classified in these spectra [5], [6], 171, The experimental intensities (see Table 11) are [g], [lo], [Ill, [121. therefore only partly explained by this mixing. This Lately, we have been able to extend our research discrepancy may have to be explained by term inter- to atoms of the fifth row. Isoelectronic sequences of action (for instance with 3 d8 4 s 4 p).

fsoelectronic sequence

-

Investigated Spectra Ions

- Transitions

3 p6 3 d-3 p6 nf, (n = 4 + 10) ; 3 p6 3 d-3 p5 3 d 4 s

CO I Y XI11 to MO XVI 3 p6 3 d9-3 p5 3 d l 0

CU I YXI to MO XIV 4 S-5 p ; 4 p-5 d ; 4 p-5

; 4 d-5 f ; 4 S-6 p

C4-66 E. ALEXANDER, B. S. FRAENKEL, S. GOLDSMITH, S. HOORY AND M. EVEN-ZOHAR

Element Transition Int.

- -

Y XI1 3 d1° 'so - 3 d9 4 f

10

Is0

-

5

1

so ^-

^D

3 d10 'S, - 3 d9 4 p

25

1

so -

30

l

so -

5

Zr XIII 3 dI0 'S, - 3 d9 4 f 3 ~ y ⁶

Is0

- lp: 3

3 d10 'S, - 3 d9 4 p

8

lso -

13

1

s o ^-

0

Nb XIV 3 d1° 'S, - 3 d9 4 p

8

Is0

- lp: 10

1

so - 3p: 0

Wave length

(4 Wave number

(cm - l) 2 027 160 1 994 820 1 981 890 1 387 110 1 375 069 1 358 200

References [l] ALEXANDER (E.) and FRAENKEL (B. S.), Rev. Sci. Instr.,

1963,34, 887.

121 VODAR (B.) and ASTOIN (N.), Nature, 1950,166, 1029.

[3] BALLOFET (G.), Ann. Phys., 1960,5, 1249.

[4] ALEXANDER (E.), FELDMAN (U.), FRAENKEL (B. S.) and HOORY (S.), Brit. J. Appl. Phys., 1966,77,265.

[51 ALEXANDER (E.), FELDMAN (U.) and FRAENKEL (B. S.), J. Opt. Soc. Am., 1965,55, 650.

[61 ALEXANDER (E.), FELDMAN (U.), FRAENKEL (B. S.) and HOORY (S.), Nature, 1965, 206, 176.

[71 FELDMAN (U.), FRAENKEL (B. S.) and HOORY (S.), Astrophys. J., 1965, 142, 719.

[8] ALEXANDER (E.), FELDMAN (U.), FRAENKEL (B. S.) and HOORY (S.), J. Opt. SOC. Am., 1966, 56, 651.

[g] FELDMAN (U.) and FRAENKEL (B. S.), ^J. Opt. SOC. Am., 1966,56, 1598.

[IO] FELDMAN (U.) and FRAENKEL (B. S.), Astrophys. J., 1966,145,959.

[l l] EVEN-ZOHAR (M.) and FRAENKEL (B. S.), J. Opt. SOC.

Am., 1968, 58,1420.

L121 HOORY (S.), GOLDSM~H (S.) and FRAENKEL (B. S.), Astrophys. J., 1970, 160, 781.

[l31 EDLEN (B.), Encyclopedia of Physics, Springer Verlag Berlin 1946, Vol. 27, p. 128.

DISCUSSION M. ELTON. - Could you please explain how you

succeeded in removing the astigmatism of your grazing incidence spectrograph to obtain space resolved spectra ?

Your data indicates an axial succession of ion stages.

Our observations of axial plasma dynamics indicate several transits over the electrode gap. Would you then not expect to observe more of a blending of ion states at any particular point ?

M. ALEXANDER. - For different ranges of wave- length, different positions of the slit give maximum resolution. From this we suppose that the

exact

position of the slit does not give the optimal conditions for reduction of astigmatism, as far as line broadening is concerned.

We quite agree, that there is a strong blending of

states at any point of the spark, but the relative densities

change systematically along the spark, as explained in [4].