OSCILLATOR STRENGTH MEASUREMENTS IN THE VACUUM-ULTRAVIOLET BY THE EMISSION METHOD

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OSCILLATOR STRENGTH MEASUREMENTS IN THE VACUUM-ULTRAVIOLET BY THE EMISSION

METHOD

C. Goldbach, G. Nollez

To cite this version:

C. Goldbach, G. Nollez. OSCILLATOR STRENGTH MEASUREMENTS IN THE VACUUM-

ULTRAVIOLET BY THE EMISSION METHOD. Journal de Physique Colloques, 1988, 49 (C1),

pp.C1-353-C1-356. �10.1051/jphyscol:1988176�. �jpa-00227590�

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JOURNAL DE PHYSIQUE

C o l l o q u e C1, S u p p l h m e n t a u n 0 3 , Tome 4 9 , M a r s 1 9 8 8

OSCILLATOR STRENGTH MEASUREMENTS IN THE VACUUM-ULTRAVIOLET BY THE EMISSION METHOD

C. GOLDBACH a n d G. NOLLEZ

CNRS, I n s t i t u t d ' A s t r o p h y s i q u e , 986iS ~d m a g o , F-75014 P a r i s , France

R Q s u m Q : Les principes et la rbalisation d'une expkrience de mesure de forces d'oscillateur dans I'ultra-violet lointain par la mkthode d'kmission sont brikvernent pr&ent& .Lea rbsultats obtenus pour les rnultiplets intenses de I'azote et du carbone neutres dans le domaine ^1200- 2000 A conduisent B I'Btablissernent d'une bchelle absolue de forces d'oscillateur en bon accord avec les thbories les plus rkcentes.

A b s t r a c t : The principles and the realization of an experiment devoted to oscillator strength measurements in the vacuum-ultraviolet by the emission method are briefly presented. The results obtained for the strong multiplets of neutral nitrogen and carbon in the 1200-2000 .&

range yield an absolute scale of oscillator strengths in good agreement with the most recent calculations.

With the launch of Copernicus (1972) and IUE (1978), UV astronomy entered the era of spatial observatories, used by several hundreds of astronomers, leading to a large number of discoveries and modifying our understanding of many astronon~ical objects 111. Nevertheless, many atomic and molecular d a t a required to analyse the high-resolution spectra obtained in the vacuum-ultraviolet (VUV) spectral range are still missing; in particular, there is a lack of accurate oscillator strength values, even for elements as abundant as v , O .

By the same time, the development of computer control in d a t a acquisition and analysis, the realisation of high-resolution VUV spectrometers and of VUV radiation standards, the accuracy reached on the lifetime data (required t o normalise to a n absolute scale relative f- values) largely improved the experimental techniques (21. In particular, the potential acc.uracy and the reliability of the classical emission method are increased so that its field can now be extended t o the VUV without any dramatic loss of precision.

Therefore we undertook a systematic program of oscillator strength measurements in the VUV region by the emission method. The first investigations are devoted to the strongest multiplets of neutral nitrogen and of neutral carbon in the 1200

-

2000

A

spectral range.

P r i n c i p l e s of t h e m e t h o d

An advanced version of the emission method using a wall-stabilised arc is set up, avoiding the difficulties associated with a n absolute number density determination in a multi-component plasma (demixing effect) 131.

The resolution of t h e VUV monochromator, together with digital d a t a processing techniques, allows the separation of overlapping lines within multiplets. Self-absorption of radiation in the plasma is taken into account by measuring the optical depth profile of each line r(X) = - In(1

-

I x l B x ) , where Ix is the spectral radiance of the homogeneous plasma and Bx the Planck function. As Jline ~ x d X is proportional to the line oscillator strength and to the emitter density, we measure the ratio of the integrated optical depth of the investigated line t o that of a reference line. The corresponding relative oscillator strength is put on a n absolute scale through the known absolute f-value of the selected reference line for which accurate lifetime d a t a are available (beam-foil techniques).

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1988176

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C1-354 JOURNAL DE PHYSIQUE

T h e e x p e r i m e n t

We use a stationary high-current wall-stabilised arc as the emission source (75 mm length, 5 cm diameter); it burns in argon under LTE conditions (I=80 amps, T=13000 K, n,= 1.1 1 0 ' ~ e n a - ~ ) and it remains stable when a small amount of gaseous compound (COz, Kr,

Nz,

...) is introduced to study neutral and singly ionised species. End-on observations of the homogeneous plasma column are performed using a normal incidence monochromator equipped with a holographic concave grating (2400 lineslmm; MgFz coating); with entrance and exit slits set a t 10 p , the measured resolution is about 0.06

A.

A solar-blind photomultiplier with a CsTe photocathode serves as a detector. The VUV spectroscopic system is isolated from the arc chamber by a MgFz window and is evacuated to 10 -7 mbar through two turbomolecular pumping units. Self-absorption of radiation in the cooler zone between the MgF2 window and the electrodes being the main potential source of error, it is checked before each run: we measure the integrated optical depth distribution of lines within a multiplet for which LS-coupling stands; any deviation from the LS-ratio indicates absorption in the boundary layer.

Depending on the investigated multiplet, the photoelectric signal is measured a t 100 to 700 points separated bgD0.004

19

steps. For each data point, the signal is sampled 200 times a t 4x11s intervals. Many of the investigated transitions appear as multiplets of overlapping and blended lines. An accurate determination of the integrated optical depth of the individual lines (or groups of completely blended lines) is performed by least-squares fitting procedures.

As Gaussian (Doppler, instrumental) and Lorentzian (Stark) line-broadening processes are all of the same order of magnitude, each line profile is represented by a Voigt function (four free parameters: Gauss and Lorentz widths, position of the line peak, integrated optical depth). The analytical function representing the multiplet is obtained by addition of a constant continuum to the sum of the individual Voigt components. The integrated optical depth of each line is obtained as the parameter final value after convergence of the fitting procedure.

As regard the strong multiplets of CI and NI in the investigated 1200-2000

A

range, they can all be saturated by increasing the C 0 2 and N2 concentration so that they closely approach blackbody radiation (41. This provides the radiometric calibration of the spectroscopic system.

The investigation of the weaker lines of the same elements, which is in progress, involves a procedure of calibration based on the absolute emission of the argon plasma continuum [5].

Results

Experimental absolute scales of oscillator strengths (based on selected lifetime data and the present relative measurements) can be established for the 1200, 1243, 1493 and 1743

A

NI uv-multiplets (6) and for the 1260, 1277, 1329, 1463, 1561 and 1657

A

CI uv-multiplets (Goldbach and Nollez, to be published),in both cases with an uncertainty to about f 15%.

These scales, labeled as Uexperimentaln, are compared graphically in Fig.1 and 2 with the semi-empirical results of Kurucz and Peytremann (71 and with the most elaborated calculations avalaible a t the moment.

For NI, if the resonance multiplet a t 1200

A

is excepted,the average deviation between Kurucz and Peytremann (71 and experimental absolute scales is (A loggf),,, = -0.014 dex (-3.2%) with a standard deviation a = f 0.08 dex ( f 18.5%) (Fig.1). For the 1200

A

resonance multiplet the multiconfiguration calculations of Hibbert et al. 181 and the experiment agree within about -0.05 dex (-11.5%).

For CI, the Fig.2 shows excellent agreement between the experimental scale and the scale defined by the multiconfiguration caculations in intermediate coupling of Nussbaumer and Storey [9], with (Aloggf),,, = -0.03 dex (-7%) and a standard deviation o = 3~0.04 dex (+9%). If we except the three more discrepant data points all belonging to the 1329

A

multiplet, the mean deviation between the two scales is reduced to (A log gf)a,, = -0.019 dex (-4.4%) with a standard deviation o = f 0.03 dex (f 7%) for the remaining 1260, 1277, 1463, 1561 and 1657

A

CI uv-multiplets.

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Fig.1: Logarithmic comparivo~~ between theoretical and present experimental Nl gf-values

I I I I 1 I I I I 1

t I

^I

- -

-

o :Kurucz and Peytremann (1975)

- .

:Hibbert e t al. (1905) -

- -

-

- -

-

- n -

0

- 0

-

- u

-

^A

-

- -

- ⁰ ⁰ ^A

-

I I I I I I I I

-

a .5 ^-

-

c - 8

d

5 ^-

V -

M

-

0 _I ^-

0 h -

d - 0 :Kurucz and Peytremann (1975)

;

0 -.5 -

.++

-

M

^- .

:Nussbaumer and Storey (1984)

M

0 -

-

-1 ^- ₀

-

⁰

- _I _I _I _l _l ₁ _I _I _I _I _I _I _I _I

- 2 -1.5 - 1 -.5

log(gfexpe-entd)

I I I I I I I

Fig.2: Logarithmic comparison between theoretical and present experimental CI gf-values

I I I I

I

- 1 - .5 0

log(gfexpcrimental)

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(21-356 JOURNAL DE PHYSIQUE

References

[ I ] Vidal-Madjar A., Encrenaz T., Ferlet R., Hknoux J.-C., Lallement R., Vauclair G.:

Rep. Prog. Phys., 50, 65 (1987)

12) Huber M.C.E., Sandernan R.J.: Rep. Prog. Phys., 49, 397 (1986) 13) Boldt G.: 2. Naturjorschg, 18a, 1107 (1963)

141 Boldt G.: Space Science Rev., 11, 728 (1970) [5] Preston R.C.: J. Phys. B, 10, 1573 (1977)

(61 Goldbach C . , Martin M., Nollez G., Plomdeur P., Zirnrnermann J.-P., Babit D.:

Astron. Astrophys., 161, 47 (1986)

(71 Kurucz,R.L.,Peytremann,E.: A Table of Semi-empirical gf-values, Cambridge, (Mass.);

Smithsonian Institution Astrophysical Observatory, (1975)

181 Hibbert A . , Dufton I1.L., Keenan F.P.: Monthly Notices Roy. Aetron. Soc., 213, 721 (1985) (91 Nussbaumer H . , Storey P.J.: Astron. Astrophys., 140, 383 (1984)