Experimental stark shifts of several HeI and ArI spectral lines

J. Quanr. S,mms(.. Rorlicrr.7-rumjiv Vol. 54, No. 3. pp. 5X1-587. 1995 Copyright t< 1995 Elsevier Science Ltd

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STARK SHIFTS OF SEVERAL He1 AND ArI SPECTRAL LINES

S. DJENI?ZE,t$ Lj. SKULJAN,$ and R. KONJEVIc:TI fFaculty of Physics, University of Belgrade, P.O. Box 368, §Astronomical Observatory, Volgina 7 and IInstitute of Physics, P.O. Box 57, 11001 Belgrade, Serbia (Received 7 July 1994; received .for publication 23 February 1995)

Abstract-Stark shifts of four He1 and six ArI spectral lines have been measured in a linear pulsed arc plasma, superimposed to the glow discharge positive column plasma in helium and argon-helium mixture, respectively. The measured values were compared to the existing data calculated, according to the various theoretical approaches.

INTRODUCTION

Stark parameters (the width and the shift) of spectral lines are important because they can be applied to diagnostic purposes in astrophysical and laboratorical plasmas.‘,* Various optical depths of the emitting plasma result in self-absorption as the basic influence on the line width value (by screening the Stark contribution). On the other hand, Stark shift, being independent of self-absorption, is more reliable for diagnostic purposes. Spectral lines of neutral atoms have more reliably measurable shifts, therefore, they are convenient for diagnostic purpases. There are several experimental methods of determining Stark shift (d), such as the photometric technique, 3.4the method of following the line center position during the decaying state of plasma,5,6 and the comparison of the position of the spectral line center with the position of a line emitted from the reference light source at the same wavelength.7-9 All of these methods have their limits of accuracy, which are controlled by plasma parameters, such as electron density N and temperature T. We present here the construction of a plasma source for improving the accuracy of Stark shift measurements. The impulse (AC) discharge was superimposed on the glow (DC) discharge. This enables simultaneous measurements of the profile and the center position of the investigated spectral lines emitted from the same plasma volume, in the case of a DC discharge when the line is unshifted (because of the small electron density), and in the case of a DC + AC discharge with a higher electron density (> 10” me3), when Stark broadening is the main pressure broadening mechanism. The Stark shift was determined from the difference between spectrum positions of the same spectral line centers registered in DC and DC + AC discharge, at several given electron temperatures and densities in the decaying DC + AC plasma. A large number of experimental papers deal with the broadening of neutral helium and argon spectral lines. “-I3 However, the only published experimental data are those of 587.6 and 388.9 nm14 Stark shifts of He1 lines at T > 22,000 K. We have measured Stark shift values for four He1 lines at the electron temperature two times larger in comparison to the electron temperature in other authors’ experimental data, and six ArI spectral lines, which are important in the diagnosis of astrophysical and laboratorical plasmas. The Stark shifts of 706.7 and 738.4 nm ArI spectral lines do not seem to have been investigated experimentally.‘3 We compared our results to the existing experimental and theoretical data. tTo whom all correspondence

should be addressed. 581

S. Djeniie et al

582

d

Water

Fig. I. Pyrex discharge tube with quartz windows, copper (C, A) glow and ring (0,. 0,) impulse discharge electrodes. APPARATUS

AND

MEASUREMENTS

Plasma source A reliable plasma source has been constructed with a repetitive discharge (AC) superimposed to the continuous glow discharge (DC). The Pyrex discharge tube is shown schematically in Fig. 1. The glow discharge was driven between isolated water-cooled copper electrodes (C and A). The homogenous positive-column plasma was located in the linear part of the discharge tube (i.d. 5 mm), which was separated at the optical axis of the observation. This part was sealed with quartz windows. Two auxiliary ring-shape electrodes 0, and 0, were positioned along the optical axis of the glow discharge positive-column plasma, 80 mm apart from each other. They were used to drive the pulse discharge from condensers of 80 or 0.3 pF, charged up to 1.2 and 10 kV, respectively. The space distribution and the composition of the discharges enable simultaneous recording of the profile of a spectral line in the independent DC and superimposed DC + AC discharges. Stark shift determination Scanning of the spectral line profiles using a shot-by-shot technique in the DC and DC + AC discharges simultaneously enables precise determination of the line center in two, very different, discharges. The difference between the measured positions of the center of the spectral line is the shift in the DC + AC discharge at given T and N. Knowing the electron temperature T and density N, it is possible to standardize the value of the shift. As an example, in Fig 2, we present the time evolution of the positions of the centers of the He1 667.8 nm and ArI 696.5 nm lines in the decaying DC + AC plasmas, up to their unshifted positions in the DC discharge. Diagnostic methods Parameters of the DC + AC discharge were selected in such a way, that the neutral emitters radiation should be above the sensibility threshold of the applied detection system. The existence of the investigated ArI spectral lines in the DC + AC discharge was necessary as well, because of a relatively small ionization energy of the argon atoms, which results in their intensive ionization. Grating spectrograph Zeiss PGS-2 (0.73 nm/mm inverse linear dispersion in the first order), with the system Osma II (Detector Head/IRY 700-6, 1024 pixel/25 mm) and calibrated photomultipliers EM1 9789 QB and 9659 B, were used for the plasma radiation detection. The working conditions were determined in such a way to obtain the spectral isolance and reproducibility of the investigated He1 and ArI spectral lines in DC and DC + AC discharge. The optimal working conditions, such as glow discharge current (Znc), pressure (p) and bank voltage (U), are presented in Table 1.

Experimental Stark shifts

583

Table I. Optimal working conditions for DC and DC + AC discharges in Ar + He (72 + 28%) and He; p-gas pressure, I,-glow discharge current, U-bank voltage, [,-DC + AC discharge current maximum, r-period, L-self-inductance and &-decrement. The maximum obtained electron density N (in 10” m-‘) and temperature T (in IO4K) are also given. Ar+Iie

He

P=200 Pa

P=260 Pa

DC

DC+AC

IDC = 50 mA a) C=SOpF Ud.2 kV I,,, = 5.96 kA TZ104pS L=2.41pH 6=7.75 T=1.6 T= 1.8 N = 6.6 N = o.Ow2

DC I~c=‘lmA b) C = 0.3pF II=10 kV I,,, = 3.98 kA +=5pa L=1.68pH L1.55 T= 1.3 T = 1.4 N = 3.8 N = 0.00001

DC+AC a) b) C=SOpF C = 0.3pF Ud.2 kV U=10 kV I,,, = 7.13 kA I,,, = 3.80 kA T = 80~s r=5ps L = 1.78pH L = 1.52pH 6=3.22 6=1.40 T=4.1 T = 4.1 N = 3.3 N = 1.1

Electrical characteristics of the superimposed discharge, obtained from Rogowski coil signal are: discharge current maximum (I,,,), period (r), self-inductance (L) and decrement (6) are presented also in Table 1. Spectroscopic observations of isolated spectral lines were made end-on along the axis of the linear part of the discharge tube. The line profiles were recorded on a shot-by-shot basis using spectrograph-photomultiplier combination,‘5 which was more reliable than spectrograph-Osma II combination because of its small number of pixels per I mm of the spectrum dictated by the performance of the spectrograph PGS-2. By traversing the spectrograph exit slit with the photomultiplier along the focal plain in small wavelength stepsI (0.0073 nm), all alternative recording of the line profile points in DC and

1.0

1

0.8 I “; 0.6: * z

0.4; 0.2 1 0.0 1 -0.2

/~‘~‘I~~“,‘~~~I’~~~I”‘~l~~~~l -0.2 0.0 O” h(nm)

1.2

b)

1.0

0.8

d

i

Fig. 2. Time evolution of the line center position for: (a) He1 667.8 nm and (b) Arl 696.5 nm spectral lines in the decaying DC + AC plasmas up to their unshifted positions in DC discharge.

S. Djeniie et al

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o-0 0

10

20

30

40

50

60

70

tCLLS1

0

IO

20

30

40

50

60

70

VW)

Fig. 3. Temporal evolution of the electron density (N) and temperature (T) in the decaying (a) He plasma of the slow 80 pF, I.2 kV discharge and (b) Ar + He plasma of the fast 0.3 pF, IO kV discharge.

DC + AC discharge was possible, using a step-by-step technique. Every position in the spectrum was recorded several times, in order to make better statistics. The photomultiplier signal was digitalized using HAMEG-205-2 oscilloscope, interfaced to the computer. From the spectroscopic observations, we have concluded that the reproducibility of DC + AC discharge is up to 93 and 96% in the cases of Ar + He and He plasmas, respectively. The DC discharge electron density was obtained from the electric field strength, measured between 0, and 0, electrodes, using Eq. (111.24) from Ref. 16, and also from the measured width values of the hydrogen H, line,17 which exist as impurities in Ar + He plasma. The necessary data were taken from Refs. 18 and 19. When Nn, values are up to 2 x lOi8m-‘, the spectral line shift is negligible. The DC discharge electron temperature was determined using electrical double probe methods with cylindrical electrostatic probes located on the place of the electrodes 02.20.2’The obtained values were up to T,, = 16,000 K + 20%. The DC f AC discharge electron temperature (T) was found from the ratios of the relative intensities of spectral lines emitted from successive ionized stages of atoms: 500.9 nm ArII and 696.5 nm ArI lines in the Ar + He plasma and 468.6 nm He11 and 388.9 nm He1 lines in the He plasma, assuming the existence of the local thermodynamic equilibrium (LTE) with the accuracy of + 14%. The necessary atomic data were taken from Refs. 22 and 23. The DC + AC discharge electron density (N) was measured by a single wavelength He-Ne laser interferometer24 for the 632.8 nm transition with an estimated error of f8%. In Table 1 are also presented plasma parameters (N, T) at the various working conditions realized in the DC and DC + AC discharge at the moment of its maximal values. Temporal evolutions of N and Tin the decaying plasma of slow (80 p F, 1.2 kV) and fast (0.3 p F, 10 kV) DC + AC discharge, in helium and Ar + He plasma, are presented in Fig. 3. RESULTS

Experiment

The results of the measured Stark shift values d,,, (in 10-l nm) at given T (in lo4 K) and N (in 10Z2m-3) are shown in Table 2.

585

Experimental Stark shifts Table 2. Measured Stark shift values d,,, (in IO-’ nm) at a given electron temperature T (in IO4K) and density N (in 102*m-)) and their ratios to the various calculated total shift values: d, (Griem’), d, (Bassalo et a12’)and dDss (Dimitrijevic and Sahal-Brechot’6,27). Transitions and multiplet numbers are also given. The positive shift is toward the red. Emitt.Tmnaitio0 He1

Multiplet S-“PO (2) Ifj-lpo

28-3~

e4

r

388.86

3.9 1.1

T-

&I

$

tak

2.8 0.9

0.64*10% o.llilo% -0.08*20%

0.75 0.77 0.44

1.08 1.07 0.39

0.81 0.83 0.59

501.57

4.0 4.0

2p-3s

(4) IpoLls

728.14

4.0

0.6

O.lSztlS%

0.77

0.83

0.77

2p-3d

(45) IPO_lD

667.82

4.0

0.9

0.20flO%

0.87

1.16

1.00

(46) ArI

4&p

[lfl;l~w

763.51

iY

3.3

o.zl*lo%

4sc4p’

[l;]“-[lg

738.39

1.3

3.3

O.l2ilO%

706.72

1.3 1.8

O.O9zklO% O.l6flO%

696.54

1.3 1.7 1.6

3.3 6.6 3.3 5.5 2.2

O.lOHO% 0.18+10% O.l2f20%

0.55

1.4

2.6

0.23+15%

0.62

-

-

4s-5p

427.22

4s’-5p’

425.94

Theory

For the evaluation of the He1 and ArI Stark shift values at a required electron density and temperature, we use Eq. (227) from Griem,’ based on the quasistatic-ion approximation. The necessary data, such as: electron impact half-half width (w,), shift (d,) and ion broadening parameter (a), are taken from Griem’ (G), Bassalo et al” (B) and DimitrijeviC and Sahal-Brechot26.27(DSB) for the He1 lines, and from Griem’ (G) for the ArI lines. Ratios of the measured d,,, values to the calculated dG, d, and dDsBStark shift values are also given in Table 2.

DISCUSSION In Fig. 4, we have presented Stark shift d as a function of temperature T for four He1 lines and two ArI lines, predicted through theoretical calculations. Dashed lines show the electron contribution (d,), while solid lines show the combined electron-ion contribution, based on the quasistatic approximation of Griem,’ at the electron density of 1 x 1O22mm3, using w,, d, and a from (G), (B) and (DSB) approximations. In addition to our experimental data (a), experimental results of other authors, obtained at the electron density near 1 x 102’mP3 are shown as well (Refs. 28-30 for He1 and Ref. 32 for ArI). By comparing our experimental values with the existing theoretical data (dth), in case of He1 lines (Table 2) we can conclude that our values follow the theoretical predictions, although they are generally lower. In case of 667.8 nm spectral line, the agreement between d, and dG, de and dDse is within f 16%, which is quite satisfactory, while d, is up to 23% lower compared to dth, for Table 3. Comparison of the Stark shift d,,, (in IO-’ nm) values measured at various T(in IO4K) and N (in 102’m-j) for the 696.5 and 763.5 nm ArI lines. dN (in IO-’ nm) represents the normalized Stark shift values at N = I x IO*’m-j and T = IO4K. Emitt.

( G4

ArI

I

1 T

1 N

696.54

1.18 2.0 1.65 0.50 1.7 1.3

0.6 2.0 6.0 1.0 0.55 0.33

763.51

1.0 1.3

0.20 0.33

I

I

1

d,

1

dN

1 Ref.

)

S. Djeniie

586

41

0.10

Hel

388.87

et al

t.i;m)

nm

Hel 0.40

-

0.10

-

667.81

nm

DSB’ ---

-0.40

/

---__

B’

0.00

,

I,

I

Hel

501.57

0

I,

I,

,o

20

I, 30

I, 40

I 50

,I

1 I,

60

70

I 80

T(

nm

103K)

-0.30 0.15

Arl 0.10

I/

c--------

0.05 0.00

0.60

Hel 0.50

728.11

nm

0.00

DSB

-

-----

I 0

,I, 10

I, 2o

I 30

-_.

G”O

425.94

nm

:

0.20

j

0.10

nm

1

Art

0.20

427.22

( 40

I

(I, 50

I 60

[ 70

I,

I 80

0.15

-

0.10

-

0.05

-

-

--o------

0.00 90

0

I

--__

---_

E*

, , , , , , , , , , , , , , , , , , , , , , , , 10 20 30 40

T( 1 03K)

T( 10’:;

Fig. 4. Stark shifts (d) of He1 and ArI lines vs electron temperature at 1 x IO** mm3electron density. 0, Our and other authors experimental data: n , Biittieher et al;*’ A, Morris and Cooper;29 0, Kelleher?” 0, Bues et al’* and various theoretical calculations for electron impact (4) contribution (---) and additional ion contribution (-) following Griem’ (G), Bassalo et alz5 (B) and Dimitrijevic and Sahal-Brechot26.27(DSB) approximations based on the quasistatic ion theory.

728.1 nm line. In case of 388.9 nm line, our d,,, data, within their accuracy and within the reliability of theoretical predictions, follow (+ 8% and - 19%) the values of $ and dDse. On the other hand, theoretical predictions are extremely higher, up to 2.5 times (de, dG), for a 501.6 nm line. As we can see in Fig. 4, the theoretical data (G), (B) and (DSB), based on different approximations, also differ mutually, especially for HeI 501.6 nm spectral line. It should be pointed out that the theoretical predictions based on the ion-dynamic effect14.3’ predict considerably higher values of Stark shifts than our experimentally measured values (up to the factor 2 in average). In case of ArI lines, the existing Griem’s’ data for 427.2 and 425.9 nm lines are considerably higher compared to our d, values (Table 2). The ion contribution for these lines is only up to 10% of dominant electron contribution to Stark shift. In case of 696.5 and 763.5 nm lines belonging to 4s -4~’ and 4s - 4p transition types, respectively, we have used the norming factor NT’j6 from Eqs. (3) and (4) from Ref. 33, assuming the correctness of the low-lying transitions approximation at low plasma temperatures. By using equations from Ref. 33, we have compared values that we have obtained, with the experimental values of other authors. 33-38Experimental results of Stark shifts normalized to N = 1 x 10z3me3 and T = 10,000 K (dN) are presented in Table 3. Our data

Experimental Stark shifts

587

for 696.5 nm line extremely well agree with the corresponding values from Refs. 35 and 36, while in case of 763.5 nm line, we have obtained approx. I .7 times lower shift compared to Ref. 38. For 738.4 and 706.7 nm lines, to the knowledge of authors, no published Stark shift data exist by now. In Ref. 8, shift data of some At-1 spectral lines are published, but without any information about the electron density, so there is no possibility of normalizing the results. REFERENCES I. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.

14. 15. 16. 17. 18. 19.

20. 21. 22. 23. 24. 25. 26. 27. 28.

29. 30. 31. 32. 33. 34. 35.

36. 37.

38.

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