Measured Stark widths and shifts of singly ionized oxygen spectral lines in lower multiplets

Pergamon

J. Quant. Spectrosc. Radial. Transfer Vol. 59, Nos l/2, pp. 71-75. 1998 NC,1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0022-4073/98 $19.00 + 0.00 PII: 80022-4073(97)00134-9

MEASURED IONIZED

STARK WIDTHS AND SHIFTS OF SINGLY OXYGEN SPECTRAL LINES IN LOWER MULTIPLETS

S. DJENIZET, Faculty

of Physics,

V. MILOSAVLJEVIe

University

of Belgrade,

(Received

and A. SREeKOVIe

P.O.B. 368, Belgrade,

14 February

Serbia, Yugoslavia

1997)

Abstract-Stark widths and shifts of nine 0 II lines were measured at electron temperature 54000 K and electron density 2.8 x lO23me3 m . a linear, low-pressure, pulsed arc operated in a nitrogen-oxygen mixture. Our data are compared with values calculated on the basis of semiclassical approximation. Q 1998 Elsevier Science Ltd. All rights reserved

1. INTRODUCTION

The abundance of oxygen in the Earth’s crust and atmosphere and the large number of ionized oxygen spectral lines in stellar plasmas make these lines of great importance for diagnostic purposes. Knowledge of the Stark width and shift dependence on the electron temperature (r) in the plasma is of importance for testing theoretical predictions based on various approximations. In spite of these, however, the experimental results on Stark broadening parameters of singly ionized oxygen (0 II) spectral lines have been reported in only three papers. Stark width values of 21 spectral lines are given by PlatiSa et al’ at 25 900 K electron temperature and 0.52 x 1O23m-3 electron density (N). These lines belong to lower multiplets. Stark widths and shifts of 19 0 II spectral lines in higher multiplets at T = 60 000 K and N = 0.81 x 1O23rnp3 are presented by Djeniie et a1.2 PuriC et al3 report Stark widths of two 0 II spectral lines at T = 43 000 K and N = 1.59 x 1O23rnp3. No experimental Stark shift data exist for the 0 II spectral lines from lower multiplets of the 3s-3p transition to the knowledge of the authors.4 The aim of this paper is to provide some new data on the Stark shift (6) of the singly ionized oxygen spectral lines. The Stark HWHM (halfwidth at half intensity maximum, w) values have also been measured. We have measured & and w, of nine 0 II spectral lines of 3s-3p, 3s’-3p’, 3p-3d and 3p-4s transitions at the electron temperature of 54 000 K. Stark shift values for all of these lines have been investigated for the first time; the same also holds for the Stark width values of the spectral lines belonging to multiplets No. 20 and No. 27. The measured values of Stark widths and shifts were compared to the existing theoretical predictions based on the semiclassical approaches performed by Griem’ and Dimitrijevib.6 2. EXPERIMENT

The modified version of the linear low pressure pulsed arc7 has been used as a plasma source. A pulsed discharge driven in a quartz discharge tube of i.d. 5 mm has an effective plasma length of 5.8 cm (see Fig. 1). The tube has end-on quartz windows. On the opposite side of the electrodes the glass tube was expanded in order to reduce erosion of the glass wall and sputtering of the electrode material onto the quartz windows. The working gas was a nitrogen and oxygen mixture (83% N2+ 17% 02) at 70 Pa filling pressure in a flowing regime. Spectroscopic observations of isolated spectral lines were made end-on along the axis of the discharge tube. A capacitor of 14 PF was charged up to 3.0 kV and supplied discharge currents up to 7.7 kA. The line profiles were recorded by a shot-by-shot technique using a photomultiplier (EM1 9789 QB) and a grating spectrograph (Zeiss PGS-2, reciprocal linear dispersion 0.73 nm mm-’ in the first tTo whom all correspondence

should

be addressed. 71

12

S. Djeniie et al 58mm Pyrex \

\

\

\

..-. \

-..-.

\

K

-../

G? AcM

Brass _ r ‘electrodes

*____

-

A -

Fig. 1. Plasma source.

order) system. The instrumental HWHM of 0.004 nm was obtained by using the narrow spectral lines emitted by the hollow cathode discharge. The recorded profile of these lines has been of the Gaussian type within 7% accuracy in the range of the spectral line wavelengths investigated. The exit slit (10 pm) of the spectrograph with the calibrated photomultiplier was micrometrically traversed along the spectral plane in small wavelength steps (0.0073 nm). The photomultiplier signal was digitized using an oscilloscope, interfaced to a computer. Plasma reproducibility was monitored by the 0 II line radiation and by the discharge current (it was found to be within +6%). The measured profiles were of the Voigt type due to the convolution of the Lorentzian Stark and Gaussian profiles caused by Doppler and instrumental broadening. For the electron density and temperature obtained in our experiment the Lorentzian fraction in the Voigt profile was dominant (over 80%). Van der Waals and resonance broadening were estimated to be smaller by more than an order of magnitude in comparison to Stark, Doppler and instrumental broadening. A standard deconvolution procedure’ was used. The deconvolution procedure was computerized using the least-squares algorithm. The Stark widths were measured within 15% error. Great care was taken to minimize the influence of self-absorption on Stark width determinations. The opacity was checked by measuring line-intensity ratios within the multiplet (No. I).

- 50 3.0 -

40

2.5 -

7

E N^ 0

2.0 -

Z

1.5 -

-30

iti

- - - -T(t)

“0

-NW

G -

I

I

I

I

10

20

30

40

20

Wsec)

Fig. 2. Temporal evolution of the electron density (IV) and temperature (7) in the decaying plasma.

73

Stark widths and shifts of 0 I1 lines

Table 1. Experimental Stark HWHM (w,) and shift (d,,,) values at electron temperature T = 54000 K and electron density 2.8 x 1O23m-‘. Transitions, wavelengths and multiplets are also presented. The positive shift is towards the red. Transition

Multiplet

L (nm)

3s-3p

4P-4Do (1)

3s’-3p’

4P-4so (3) 2P-2Po (6) 2D-2Fo (15)

463.885 464.181 374.949 395.437 459.600 459.617

0.032 0.031 0.019 0.027 0.019 0.021

0.005 0.003 0.000 0.000 0.000 0.000

d (nm)

w (nm)

3p-3d

4P0-4D (20) 4So-4P (28)

410.500 489.093

0.041 0.047

0.003 0.000

3p-4s

‘D’-‘P (27)

347.042

0.050

0.009

The values obtained were compared with calculated ratios of the products of the spontaneous emission probabilities’ and the corresponding statistical weights of the upper levels of the lines. It turns out that these ratios differed by less than +12%. The Stark shifts were measured relative to the unshifted spectral lines emitted by the same plasma.” The Stark shift of the spectral line can be measured experimentally by evaluating the

3p-4s

3s-3p No.6 h=396.7nm

0.20

0.15

0.10

8 x

G

XL-

0.35

-.

0.30 0.25

---

0.20

--

+ 0.05

I

I

I

I

I

L I

I

I

No.27 %=346.9nm

G

I

I

I

I

3s-3p No.3 h=373.6nm

0.20

0.15 -------

0.10

0

--

0.25

--

No.28 h=491.3nm

0

-------

A

0.30

x q

+

0.05

I

I

I

I

%

I

I

I

o-

I

3s-3p

I

I

I

I

3p-3d No. I h=465.2nm

0.25

0.25

No.20 1=41l.lnm

--

0.20

0.1s

0.10

I I

I

I

I

I

I

2

3

4

5

6

T(104K)

I

I I

I

I

I

I

I

2

3

4

5

6

T( 104K)

Fig. 3. Theoretical Stark HWHM dependence on the electron temperature scaled to the electron density of 1 x 1O23rnw3.a, This work; A, PlatiSa et al;’ 0, Djeniie et al;’ 0, Puric et ak3 G and D denote values obtained on the basis of the semiclassical approximation performed by Griem’ and DimitrijeviC.6 respectively. x and @, theoretical predictions by Hey and Breger;13 +, theoretical calculations by Hey and BryanI at plasma parameters obtained in PlatiSa et al.’ I is the mean wavelength for the multiplet. The error bars include the uncertainties of the width and electron density measurements.

S. Djeniie et al

74

position of the spectral line centre recorded at two different electron density values during the plasma decay. In principle, the method requires recording of the spectral line profile at the high electron density (Ni) that causes an appreciable shift and then later when the electron concentration has dropped to the value (NJ lower by at least an order of magnitude. The difference of the line centre positions in the two case is Ad, so that the shift d, at the higher electron density N1 is: dl = NI . Ad/(N,

- N2)

The Stark shift data was corrected for the electron temperature determined with f0.0015 nm error at a given N and T.

decay.”

Stark shift data are

The plasma parameters were determined using standard diagnostic methods. The electron temperature was determined from the ratios of the relative intensities of the 348.49 nm N IV to 393.85 nm N III and the previous N III to 399.50 nm N II spectral lines, assuming the existence of LTE, with an estimated error of +12%. All the necessary atomic parameters were taken from Wiese et al.’ The electron density decay was measured using a single wavelength He-Ne laser interferometerI for the 632.8 nm transition with an estimated error of +7%. Electron temperature and density decays are presented in Fig. 2.

d(lO-‘nm)

-

0.10

-

0.00

I

-0.05

I

I

I

I

It

I 3p-3d

3s-3p 0.10 0.05

-

No.3 i=373.6nm

0.00 G

-

I

0

-0.05

No.28 L491.3nm

f 3p-3d 0.15

No.20 L4ll.lnm

0.15

NO.1 b=465,2nm

-

I 1

2

3

4

T( 104K)

5

6

1

2

3

4

5

6

T( 104K)

Fig. 4. Theoretical Stark shift dependence on the electron temperature scaled to the electron density of 1 x lo*’ m-‘. l, this work; 0, Djeniie et al.* G and D denote values obtained on the basis of the semiciassical approximation performed by Griem’ and Dimitrijevi&,6 respectively. 1 is the mean wavelength for the multiplet. The error bars include the uncertainties of the shift and electron density measurements.

Stark widths and shifts of 0 11lines

15

3. RESULTS

The results of the measured w, (HWHM) and d,,, values at the T = 54 000 K electron temperature and N = 2.8 x 1O23me3 electron density are given in Table 1 together with transition arrays and multiplet numbers. 4. DISCUSSION

The theoretical Stark HWHM and shift dependence on the electron temperature together with the values of the other authors and our experimental results (0) at the electron density N = 1 x 10z3 rnp3 are presented graphically in Figs 3 and 4, respectively, assuming the domination of the electron impact mechanism to the line broadening and shift. Solid lines show the theoretical MIand d values, calculated on the basis of the semiclassical approaches, taken from Griem5 (G) (up to 40 000 K electron temperature), while the dashed lines show the results of DimitrijeviC6 (D) calculations (from 20 000 K electron temperature) for the 3s-3p transition taking into account the emitter structure properly. Theoretical predictions by Hey and Bryani ( +) and Hey and Breger13 (x, @), calculated at plasma parameters obtained in PlatiSa et al’ are also given. The agreement between our w, values and theoretical Stark HWHM values (G and D) in the case of the multiplets 1, 3 and 6 is satisfactory. Within the experimental accuracy and the reliability of theory our w, values confirm the temperature trends of the Stark HWHM, predicted by semiclassical theory.596 This statement can also be applied for the multiplet No. 20. In the cases of the multiplets Nos. 28 and 27 our experimental data fall below the values predicted by theoretical trend? (G). The disagreement between these values is evident for the 347.042 nm spectral line (multiplet No. 27). In the case of the Stark shifts the situation is different. The measured values d,,, are generally very small, within experimental error (+O.OOlS nm) they are close to zero. Definite shifts were found only for the multiplets Nos. 1, 20 and 27. Agreement between our d, values and calculated values exist only in the case of the multiplet No. 28. In other multiplets the trends of the theoretical predictions (G), taken from Griem, ’ lie over our experimental values, especially in the case of the multiplet No. 27. It should be pointed out that disagreement is evident between various calculations of the Stark shift values; namely, Stark shift values calculated by DimitrijeviC6 (D) have the opposite sign to those taken from Griem5 (G) (see multiplet No. I in Fig. 4). REFERENCES Platila, M., Popovic, M. and Konjevic, N., Astron. Astrophys., 1975, 45, 325. Djeniie, S., Sreckovic, A., Labat, J. and PlatiSa, M., Z. Phys. D, 1991, 21, 295. Puric, J., Djeniie, S., SreCkoviC, A., PlatiSa, M. and Labat, J., Phys. Rev. A, 1988, 37, 498. Fuhr, J. R. and Lesage, A., Bibliography on Atomic Line Shapes and Shifts, (July 1978 through March 1992), NIST Special Publication 366, Suppl. 4, US:DC National Institute of Standards and Technology, Washington, 1993. 5. Griem, H. R., Spectral Line Broadening by Plasmas, Academic Press, New York, 1974. 6. Dimitrijevic, M. S., Astron. Astrophys., 1982, 112, 251. 7. Djeniie, S., Sreckovic, A., PlatiSa, M., Labat, J., Konjevic, R. and Puric, J., JQSRT, 1990, 44, 405. 8. Davies, T. and Vaughan, J. M., Astrophys. J., 1963, 137, 1302. 9. Wiese, W. L., Smith, M. W. and Glennon, B. M., Atomic Transition Probabilities, Vol. I, NSRDS-NBS 4. U.S. Government Printing Office, Washington, DC, 1966. 10. Puric, J. and Konjevic, N., Z. Phys., 1972, 249, 440. 11. Popovic, L., Sreckovic, A. and Djeniie, S., Proceedings of the 11th ZCSLS, Carry le Rouet, France, 1992. 12. Ashby, D. E. T. F., Jephcott, D. F., Malein, A. and Raynor, F. A., J. Appl. Phys., 1965, 36, 29. 13. Hey, J. D. and Breger, P., JQSRT, 1980, 24, 349. 14. Hey, J. D. and Bryan, R. J., JQSRT, 1977, 17, 221. 1. 2. 3. 4.