LOW LYING ODD PARITY AUTOIONISING STATES IN THE 2p-SUBSHELL ABSORPTION SPECTRUM OF A1 III AND Si IV

HAL Id: jpa-00227352

https://hal.archives-ouvertes.fr/jpa-00227352

Submitted on 1 Jan 1987

HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

LOW LYING ODD PARITY AUTOIONISING STATES IN THE 2p-SUBSHELL ABSORPTION SPECTRUM

OF A1 III AND Si IV

J. Mosnier, J. Brilly, E. Kennedy

To cite this version:

J. Mosnier, J. Brilly, E. Kennedy. LOW LYING ODD PARITY AUTOIONISING STATES IN THE

2p-SUBSHELL ABSORPTION SPECTRUM OF A1 III AND Si IV. Journal de Physique Colloques,

1987, 48 (C9), pp.C9-219-C9-222. �10.1051/jphyscol:1987934�. �jpa-00227352�

LOW LYING ODD PARITY AUTOIONISING STATES IN THE 2p-SUBSHELL ABSORPTION SPECTRUM OF A1 I11 AND Si IV

J.P. MOSNIER, J. BRILLY and E.T. KENNEDY

National Institute for Higher Education, GAasnevin, Dublin 9 , Ire1 and

Resume - Le spectre de photoabsorption d'un plasma d'aluminium et d'un plasma de silicium ont ete photographies dans le domaine extreme ultra-violet. La technique d'absorption utilisee fait appel a deux plasmas laser, l'un etant utilise comme source de rayonnement continu, l'autre produisant le milieu absorbant. les spectres correspondant a l'excitation d'un electron de la sous-couche 2p dans les ions de structure sodium AP. I11 et Si IV sont observes dans les conditions experimentales appropriees. Les spectres obtenus sont interpretes a l'aide de resultats de calculs de type multiconfiguration Dirac-Fock.

Abstract

-

The photoabsorption spectrum of an aluminum plasma and of a silicon plasma were photographed in the E W region. The absorption technique used utilises two laser plasmas, one produces the background continuum, the other generates the absorbing medium. Under appropriate experimental conditions the 2p-subshell excitation spectra of the sodium-like ions ARIII and Si IV were observed. The observed spectra are interpreted with the help of multiconfiguration Dirac-Fock type calculations.

The study of photoabsorption spectra of atoms and ions provide valuable information relevant to several different areas such as astrophysics or short wavelength laser studies for example. For purpose of systematising, absorption studies should ideally be carried out over extended isoelectronic and isonuclear sequences.

However, when using conventional atomic or ionic absorption techniques, one encounters severe difficulties obtaining the spectra of highly refractory and/or corrosive elements in the vapor phase and also the spectra of multiply charged ions. The dual laser-plasma absorption technique1 presents some advantages that overcome these difficulties. The latest development based on this techniquez is characterised by a great versatility which allows time resolved absorption studies over a wide range of ionisation stages as well as neutral species.

In this paper we present wavelength measurements of inner-shell transitions in the sodium-like ions ARIII and SiIV. The corresponding spectra are due to the

excitation of a 2p-subshell electron into outer shells. Similar studies were carried out in NaI by several groups over the past years 3*4 s 5 and In MgII by Esteva and Mehlman6. In the following, we will first describe briefly the experimental technique used, outline the main features of the 2p spectrum of Na-like species, and then present and comment on our results.

The experimental technique consisted of equally dividing the output of a 1.55, 30 nanos., Q-switched ruby laser between a cylindrical lens and a spherical lens combination. Two different solid targets in vacuo were positioned at the foci such that a line plasma was formed at the surface of one and a point plasma at the surface of the other. The line plasma (aluminium or silicon) constituted the absorbing medium, the point plasma (tungsten or hafnium) acted as a source of quasi

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987934

JOURNAL D E PHYSIQUE

line free continuum radiation7 .The resulting spectra were recorded on Kodak SWR plates by means of a 2m grazing incidence instrument equipped with a 1200 lines/mm concave grating. Emission lines from Aluminium or Silicon ions were superimposed onto the spectra to provide wavelength calibration. The waveleggth of the absorption features were then calculated with an uncertainty of f 0.03A by fitting a third order polynomial to known wavelengths8 of

ARIV, ARV

or SiV, SiVI, SiVII.

The 2p-subshell excitation spectra of Na-like species for transitions originating from ground state, can be described by the following one-electron excitation scheme:

zp6

3s -> 2p5 3snl with%

-

0 or 2 and @ 3. The accessible u per states (using LSJ notation) are of the type (zP53s 3*P)ns2~ 3 or (2$3s qJh)nd$l

3

and are located well above the first ionisation limib1(2p6 $ O ) . Due to

conservation these states can readily autoionise via Coulomb interaction into the accessible continua (2p6 + ~ p )

.

In the following we shall consider only the first three core-excited confli~&tions, namely 2$ 3s'. 2p5 3s4s and

zp5

3s3d and, following ref. 9 and 10, use the right-to-left coupling of the subshells in which the two outer electrons are coupled first. For brevity, the 2p5 subshell will be omitted from the notation for a configuration. The excitation scheme described above would therefore predict a rather simple photoabsorption spectrum. However, the observed structures appear to be more complex than is suggested by this picture and on the low energy side of the spectra, in particular, several supernumerary features with appreciable intensity can be picked out. Two important points must be considered in order to obtain a satisfactory understanding of the spectra. First, correlation in the motion of the two outer electrons results in substantial mixing of configurations. In particular 3p2 acts as a strong perturber of 3s3d and in this respect the 2p excited levels of the NaI sequence can be compared to the optical levels in the MgI sequence. The mixing of 3p2 b with 3s3d b in the MgI sequence has been known for a long time (see for example1=) and accounts for the

singlet-triplet inversion of 3s3d. We shall see that in the present case the effect of 3p2 on 3s3d is somewhat different. Second, mixing occurs between different terms of the same configuration due to spin-orbit interactions produced by the open core.

In particular, several terms acquire some 'P character which results in appreciable oscillator strength for transitions that would otherwise be LS forbidden.

Experimental results (energies are relative to the ground state) together with our assignements are presented in table I. Let us notice here that all the observed lines exhibit standard absorption profiles indicating little interaction with the underlying continua, although in the case of SiIV some features appear very broad.

Energy levels and electric dipole oscillator strengths were computed by using the average-of-configuration schemes of the MCDF code of Grant et all2

.

This method seems to be well suited to the present study as it allows a simultaneous treatment of correlation and relativistic effects within a variational approach. In such types of calculation all the atomic energy levels are built from a common set of orthonormal one particle orbitals and determined self-consistently in a single calculation. Initially the 2p5 3s' was optimised separately; the 1s ,2s and 2p orbitals obtained in this way were ksed and kept fixed throughout all subsequent calculations. Multiconfiguration calculations were then performed, including the following "non-relativistic" configurations : 2$3s,

zp6

3p, 3s2, 3p2 , 3s3d, 3d2, 3s3p, 3p3d, 3p4p, 3s4s, 3s4d, 3s5d, 3d4s, 3d4d. The oscillator strengths listed in table I were obtained in the Coulomb gauge ("velocity gauge") as devised by ~rant'~

, The calculated energy values were adjusted by about 10000 cmml in the case of A%

I11 in order to obtain a correct position for the 2p5 3s' 9 levels, observed at 591440 cm-'and 588100 cm-I respectively [I] (calculated ost;?lator strengths 3.04x10-~ and 6 . 2 2 ~ 1 0 - ~ respectively). In the case of SiIV this correction

[I] The 2p53sZ 'P levels appear as a distinct pair of lines on the low energy side of the NaI MgII spectra. In the present case we observe several other lines in the vicinity of the 2p53s2 P' doublet which were assigned to

zP6

3p->2pS 3s3p transitions. (to be submitted for publication).

C9-221 S t a t e

(3p2 b ) 2pi

(3p2 ^D) 2P3

O s S d ' D ) ' ^ , (3s3d D)*D3

(3p2 3P) "Ps

2"

(3p2 3P) ^ 3 2 (3s3d D)2D3

O s S d ^ )2? , (3s3d %) 2P^

( 3 s 4 s3S ) " p , ( 3 s 4 s³S ) " P 3 ( 3 p2 1S )2P , ( 3 p2 1S )2P j ( 3 s 4 s3S )2P , ( 3 s 4 s³S )²P , ( 3 s 4 s1S )2P | ( 3 s 4 s¹S )²P j O s S d ^ ) ^ ! OsSdM))*?, O s S d ^ P ,

(a) 694250 695200 720980 722070 723600 724600 726000 727900 732600 733030 734380 736200 737950 743380 745990 748200 751140 752500

(c) 692676 693764 721177 721553 724315 724602 725360 727250 727750 732255 733686 734253 736212 737205 737455 741166 744305 749489 751582 754288

(d) 1 . 4 3 ( - 2 ) 2 . 6 2 ( - 2 ) 1 . 4 5 ( - 2 ) 2.34C-2) 2 . 4 (-4) 1 . 6 0 ( - 3 ) 5 . 7 4 ( - 2 ) 2.77C-1) 1 . 6 9 ( - 1 ) 8.1 (-4) 4.1 (-4) 1 . 2 1 ( - 2 ) 4 . 9 2 ( - 3 ) 2 . 1 2 ( - 3 ) 1.03C-2) 1.65 (-3) 7 . 9 (-4) 2.09C-3) 8 . 8 1 ( - 3 ) 1.35" (-2)

(e)

56,24(3s3d1D) ^ . I S O p2 ¥ ) ^ 5 1 , 2 2 ( 3 s 3 dlD )2P , 1 8 ( 3 p2 *P) 2P 8 8 , 9 ( 3 s 3 d3D )2P

7 8 , 9 ( 3 s 3 d %) 2P,8(3s3d3D) V 3 6 , 3 0 ( 3 p2 lS )2P , 1 8 ( 3 s 3 dlD )2P 5 1 , 2 1 ( 3 s 3 dlD )2P , 1 6 ( 3 p2 lS )2P 7 4 , 1 2 ( 3 s 3 d3D )2P

71 . I O O S S D ^ ) ^

8 5 , 8 (3s 3d3 D^D 86,11 ( 3 s 4 s3S )2P 8 2 , 1 5 ( 3 s 4 s3S )2P

4 2 , 2 4 ( 3 p2 3P )2P , 1 4 ( 3 s 4 slS )2P 36,25 (3s4s 3S) 2P , 14 (3s4s 15) 2P 5 0 , 2 9 ( 3 p2 lS )2P , 1 0 ( 3 s 4 s3S )lT - 5 2 , 2 6 ( 3 s 4 s1S )2P , 9 ( 3 s 4 s3S ) '•P 5 5 , 3 4 ( 3 s 4 s3S )2P , 4 ( 3 p2 1S )2P 8 1 , 7 ( 3 s 4 s3S )2P , 6 ( 3 pe 1S )2P 4 8 , 2 3 ( 3 p2 1D )2D , 6 ( 3 s 3 d1D )2P 40,32 ( 3 p2 lD )2P , 1 1 ( 3 p2 3P)2P 36,27(3p2 lD )2P , 1 0 ( 3 s 3 dlD )2D

(b) 934000 935800 978900

975700 983400 986400

994000 1025100

1012900 1017400

(c) 938918 940332 980367 980502 978464 979350 985096 988454 989719 1018432 1020502 992757 994600 1024502 1024997 1029056 1032763 1016074 1017891 1021899

(d) 1 . 4 5 ( - 2 ) 2 . 5 8 ( - 2 ) 1.99C-2) 3 . 2 8 ( - 2 ) 2 . 9 3 ( - 3 ) 1.37 (-3) 3.54C-2) 3 . 0 9 ( - 1 ) 2 . 1 0 ( - 1 ) 2 . 2 (-4)

< 1 ( - 4 ) 1 . 4 K - 1 ) 3.06C-2) 2 . 4 6 ( - 3 ) 8 . 0 K - 3 ) 2 . 2 ( - 4 )

1.1 (-4) 1 . 8 2 ( - 2 ) 4 . 1 8 ( - 2 ) 6.52 (-2)

(e)

5 7 , 2 0 ( 3 s 3 d¹D )²P , 2 0 ( 3 p² ¥ ) ²P 5 2 , 2 0 ( 3 i2 3P )2P , 1 8 ( 3 s 3 dlD )2P 62,16(3p2 3P )2P , 7 ( 3 s 3 d3D )2P 82 ,7 (3s3d 3D) 1?,? (3s3d %) 2P 44,27 (3p² ^-S) ²P , 14 (3sId¹!)) ²P 3 2 , 2 9 ( 3 s 3 d3D ) ',D , 1 3 ( 3 s 3 dlD )2P 8 4 , 8 ( 3 s 3 d3D )2P

67,11 (3p2 ^ P . S O s S d ^ D 82,9(3p2 :fe)2P,7(3s3d3D)2D 8 2 , 1 3 ( 3 s 4 s3S )2P

7 7 , 1 8 ( 3 s 4 s3S )2P

3 4 , 3 0 ( 3 p2 3P )2P , 1 4 ( 3 s 3 d3D )2P 5 7 , 2 0 ( 3 ^ ³P)²P,8(3s3d^D)2P 7 0 , 2 0 ( 3 s 4 s 3S ) * P , 6 ( 3 s 4 s1S )2P 4 3 , 3 4 ( 3 s 4 s1S )2P , 8 ( 3 s 4 s2S ) " p 4 9 , 4 5 ( 3 s 4 s3S )2P

8 8 , 7 ( 3 s 4 s3S )2P

5 1 , 2 0 ( 3 p2 lD )2D , 1 1 ( 3 s 3 dlD )2P 50,31 ( 3 p2 1D )2P , 9 ( 31p2 3P )2P 34,22(3p2 lD )2P , 1 4 ( 3 s 3 dlD )2D (a) Measured values (cra-i), uncertainty ±160 cm"1.

(b) Measured values (cnrl), uncertainty +200 cm-l (c) Calculated values (cnrl) , see text.

(d) Calculated oscillator strengths(see text).The figure in brackets indicates power of ten.

(e) Leading percentages.

JOURNAL

DE

PHYSIQUE

amounts to about 16000 cm-'

,

the observed positions being 809400 cm-' ('P ) and 804400 cm-'('p ) (calculated oscillator strengths 3.09x10-~ and 6 . 3 2 ~ k O - ~ respectively).

4

As indicated in table I the interaction between 3p2 and 3s3d is indeed very strong, so much so that the eigenvectors composition give (3p2 %)2P as the lowest term

(above 2p5 3s') ; furthermore this interaction does not lead to an inversion of the (3s3d3D) %' and (3s3d1~)*p terms. This conclusion is similar to that of

an sen's^

who extensively discussed the same configuration mixing in MgII. One will notice that some lines have been attributed to (3$ 3P)2P and (3p2 %)2P terms. These exhibit a substantial admixture of either (3s4s3-9%) P' (MIII) or (3s3dS1~)*p (SiIV) which accounts for the magnitude of the oscillator strength for the corresponding transitions. Breakdown of LS coupling is also noticed through the observation of (3s3d3~)

' h ,

(3s3d

b)

^{D' and (3s4s 3 ~ ) *P.}

Comparison of the present results with ublished data in N ~ I ~ " and ~ ~ 1 1and ~ 3 ~ ~ calculated oscillator strengths in NaI1' (unpublished work) allows us to draw the

following concluding remarks. The 3s3d levels tend to lie deeper than the 3s4s levels as the nuclear charge is increased. This restructuring of the system of energy levels is accompanied by an increase of the oscillator strength contained in the 2p->3d excitation channel. This can be readily understood in terms of the gradual collapse of the 3d wavefunction.

The authors wish to thank Dr M.W.D. Mansfield, University College Cork, for communicating his results and helpful discussions.

This work was supported by the National Board for Science and Technology (Ireland) under research Grant N:SC/131/86 and by the Department of Education under a Research Fellowhsip scheme.

REFERENCES

1. P.K. Carroll and E.T. Kennedy Phys.Rev.Lett.38, 1068 (1977) 2. P.K. Carroll and J.T. Costello Phys.Rev.Lett.57,1571 (1987)

3. J.P. Connerade, W.R.S. Garton and M.W.D. Mansfield Astrophys.J.165,203(1971) 4. H.W. Wolff, K.Radler, B.Sonntag and R.Haense1 Z.Physik 257,353 (1972) 5. K. Sommer, Thesis, (unpublished) 1986

6. J.M. Esteva and G. Mehlman Astr0phys.J. 193,747 (1974)

7. P.K. Carroll, E.T. Kennedy and G. O'Sullivan Opt.Lett.2,72 (1978) and Appl.Opt.

19, 1454 (1980) 8. W.C. Martin and R. Zalubas J.Phys.Chem.Ref.data 12,323 (1983); 8,817 (1979) 9. J. Hansen J.Phys.B: Atom.Mol.Phys. 8,2759 (1975)

10. G. Meh1man;A.M. Weiss and J.M. Esteva Astr0phys.J. 209, 640 (1976) 11. C. Froese-Fisher, Can.J.Phys.53,184 (1975) and 53,338 (1975)

12. I.P. Grant, B.J. McKenzie, P.H. Norrington, D.F. Mayers and N.C. Pyper, Comput.

Phys. Commun. 21,207 (1980) 13. I.P. Grant J.Phys.B. 7,1458 (1974)

14. M.W.D. Mansfield, Private communication (1987)