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Experimental stark widths of C(II)u.v. lines
J. Qtrent, Spectrosc. Radiat. Transfer Vol. 20. pp. 477-.479
0022--4073/78/I101-0477/$02.00/0
Pergamon Press Ltd., 1978, Printed in Great Britain
EXPERIMENTAL
STARK WIDTHS
O F C(II)u.v. L I N E S
M. PLArI~A and M. PoPow~ Institute of Physics, 11001 Beograd,P.O. Box 57, Yugoslavia and N. KONJEVIt~ Institute of Applied Physics, l l001 Boegrad,P.O. Box 24, Yugoslavia (Received l May 1978)
Abstract--Measurements of the Stark widths of two singly ionized carbon lines were made in a low pressure pulsed arc plasma of electron density 4.9 x 1026cm-3 at a temperature of 26,300K. The measured half-widths agree within about 16% with calculated widths. INTRODUCTION
S~ARr broadening of singly-ionized atomic lines have been investigated in a number of experiments in order to test various theoretical approaches [see, e.g. Refs. (1) and (2)]. The comparison made in Refs. (1), (3) and (4) between experimental data and the results of semiclassical calculations by Jones et at. ~2) [see also G~EM°)] indicated average agreement within -+20%. An exception to this agreement is observed for the C(II)u.v. lines (multiplet No. 13 u.v.) where the experiment ~5)gave more than six times larger values than the theory. The aim of this paper is to report an experimental study of these C(II) lines in a low-pressure, pulsed arc plasma. EXPERIMENT The plasma source was a low-pressure, pulsed arc. (° It consisted of a pyrex tube of 24 mm internal diameter with the distance between electrodes equal to 20 cm. Holes of 1 mm diameter were located at the centres of both electrodes for laser-interferometric measurements of Ne and for end-on plasma observations. The discharge was driven by a 150/~F condenser bank charged to 1.4 kV. During the experiment, a continuous flow of a nitrogen-carbon dioxide mixture was sustained at a pressure of 0.15 torr. The light from the pulsed arc was observed end-on by a photomultiplier-monochromator system (with 1 m focal length and inverse linear dispersion of 4.16 ,~, mm-~). This instrument has a measured instrumental half-width of 0.046 .~ with 10/zm slit width. Scanning of the C(II) lines was accomplished by repeated pulsing of the arc while advancing the monochromator in steps of 0.02,~. The output of the photomultiplier, together with the discharge-current waveform, were recorded on a dual-beam oscilloscope. All signals were analyzed at the maximum electron density. A helium-neon laser interferometer at 6328 ,~ (with a plane external mirror) was used to determine the axial electron density. The peak electron density was 4.9 × 1016cm -3 with an estimated measurement error not exceeding -+7%. The electron temperature of 26,300 K -+ 10% was determined from the Boltzmann plot of the relative intensities of twelve O(II) lines (3377.20, 3390.25, 3712.75, 3727.33, 3973.26, 4317.14, 4366.90, 4414.91, 4590.97, 4596.77, 4649.14, 4650.84/i,); the transition probabilities were taken from the book of Wms~ et al. (7) For these measurements, the spectral response of the photomultiplier-monochromator system was calibrated by using a standard tungsten coiled-coil quartz-iodine lamp. The quoted electron temperature and density were taken at the peak of electron density. Great care was taken to ensure that line self-absorption did not affect our line-shape determinations. This was achieved by careful examination of the line intensities and line-shapes as functions of the experimental conditions and by checking the optical depths of lines by QSRT Vol. 20, No. 5----D
477
478
M. PLATI~Aet al.
measuring intensity ratios within the multiplet 13u.v. The experimental ratio was then compared with the theoretical predictions of Ref. (7). The ratio of CO~:N2 = 1:20 was determined after performing a number of experiments in which the CO2 was gradually diluted. Two criteria were used to determine the time at which to stop further dilution: (a) the line intensities within the multiplet 13 u.v. agreed to within 4% with values derived from transition probability data <7) and (b) the line intensity started to decrease while the linewidth remained constant. At high dilution ratios, the plasma parameters (electron density and temperature) at constant pressure do not change within the limits of the experimental errors. The experimentally observed line profiles are the result of the superposition of a Stark profile, an instrumental profile and the profiles induced by other broadening mechanisms. On the basis of well known formulas, ~1'8~we have estimated the contributions of various broadening mechanisms to the widths of the C(II)u.v. lines. It was found that only the contribution of Doppler broadening is comparable with electron-impact broadening while natural, van der Waals, resonance and ion broadenings can be neglected under the present experimental conditions. Therefore, the experimental line profile consists of two parts: electron impact broadening (Lorentzian) and Doppler and instrumental broadening (Gaussian). To obtain the Stark profile from the measured profile, it was necessary to use a deconvolution procedure for the Lorentzian and Gaussian profiles, e) An example of experimental measurements fitted with a Voigt profile is given in Fig. 1. RESULTS AND DISCUSSION The experimentally determined full halfwidths of C(II) lines (in Angstroms), at an electron concentration of 4.9 × 1016cm -3 and electron temperature of 26,300 K, are given in Table 1. The estimated measurement error for these linewidths is ---17%. This estimated error does not include the uncertainty in electron density and temperature measurements. For the sake of comparison, the theoretical results of JONES e t ~/.,.2) are also given for our experimental conditions. In the next two columns of Table 1, we list our experimental results (normalized to an electron density of 1 x 1017 cm -3) as well as the results of another experiment ~5~for the same electron density and at an electron temperature of 12,800 K. From the comparison of the data in Table 1, we conclude first that our experimental data agree with the theory ~'2) within 16%. Also, Our results are more than six times smaller that the results obtained in another experiment. ~5)We note that these results were not taken at the same
10 Irel.
/
i , 0.116 , 0.48 , 0.32
i
o
i
instrumental width
i
i
o~6 o:32 o~8
~x (A)
Fig. 1. Experimentaldata (o) for the 2837.60,~ C(II)line fitted with the correspondingVoigt profile, The electron density was 4.9 x 10~6cm-3 and the electron temperature26,300K.
Experimental Stark widths of C(II)u.v. lines
479
Table 1. Experimental full halfwidths for C(II) in .& units W,. compared with the theoretical values (Wth) of JONES et al. "'2~ In the last two columns we show our and the Kuscrl ~5~ experimental results normalized to an electron density of x 1017em -3. .... iti . . . . . . y
2s2p2-3s2(is)3p
Multiplet (No.)
Wavelength [~3
Temperature
Electron Density
Eom-3]
,W /ixlO 17
w~[R] wth[R] e~3~thi~
iWm/iXlO 17
o~3, rro~
experiment) gusch(5)
2S-2p°
2836.71
26300
4.9 × 1016
0.082
0.070
0.167
1.07
(13 UV)
2837.60
26300
4.9 X 1016
0.083
0.070
0.169
1.07
electron temperature as ours. However, the large observed discrepancy cannot be explained by the temperature dependence of the line width, which is very weak for Stark broadening of ion lines in plasmas. The most probable reason for the observed discrepancy is self-absorption, which may not have been negligible in the KUSCH~5)experiment. The test that self-absorption did not exist at the peaks of observed line profiles in the experimental line profiles of Ref. (5) was the fit to a Lorentzian profile, which is expected to hold for Stark-broadened lines. It has been shown that this is a very insensitive method for the determination of spectral line optical depth, t~m especially when a photographic technique is used for the recording of line spectra. REFERENCES H. R. GRIEM, Spectral Line Broadening by Plasmas, p. 32. Academic Press, New York (1974). W. W. JONES,S. BENE~ and H. R. GRIEM,University of Maryland, Tech. Rep. 71-128, College Park, Maryland (1971). W. W. JONES, Phys. Rev. A7, 1826 (1973). N. KONJEVIt~and W. L. WIESE,J. Phys. Chem. Ref. Data 5, 259 (1976). H. J. KUSCH,Z. Astrophys. 67, 64 (1967). N. KONIEVI@,M. PLATI~Aand J. PURI(:,J. Phys. B. Atom. Molec. Phys. 4, 1541 (1971). W. L. WIESE, M. W. SMITHand B. M. MILES, Atomic Transition Probabilities, Vol. 1, Hydrogen Through Neon. U.S. Government Printing Office, NSRDS-NBS 4 (1966). 8. H. R. GRIEM,Plasma Spectroscopy. McGraw Hill, New York (1964). 9. J. T. DAVIESand J. M. VAUGHAN,Astrophys. J. 137, 1302 (1963). 10. N. KONJEVI(~and J. R. ROBERTS,3". Phys. Chem. Re.[. Data 5, 209 (1976). 1. 2. 3. 4. 5. 6. 7.
0022--4073/78/I101-0477/$02.00/0
Pergamon Press Ltd., 1978, Printed in Great Britain
EXPERIMENTAL
STARK WIDTHS
O F C(II)u.v. L I N E S
M. PLArI~A and M. PoPow~ Institute of Physics, 11001 Beograd,P.O. Box 57, Yugoslavia and N. KONJEVIt~ Institute of Applied Physics, l l001 Boegrad,P.O. Box 24, Yugoslavia (Received l May 1978)
Abstract--Measurements of the Stark widths of two singly ionized carbon lines were made in a low pressure pulsed arc plasma of electron density 4.9 x 1026cm-3 at a temperature of 26,300K. The measured half-widths agree within about 16% with calculated widths. INTRODUCTION
S~ARr broadening of singly-ionized atomic lines have been investigated in a number of experiments in order to test various theoretical approaches [see, e.g. Refs. (1) and (2)]. The comparison made in Refs. (1), (3) and (4) between experimental data and the results of semiclassical calculations by Jones et at. ~2) [see also G~EM°)] indicated average agreement within -+20%. An exception to this agreement is observed for the C(II)u.v. lines (multiplet No. 13 u.v.) where the experiment ~5)gave more than six times larger values than the theory. The aim of this paper is to report an experimental study of these C(II) lines in a low-pressure, pulsed arc plasma. EXPERIMENT The plasma source was a low-pressure, pulsed arc. (° It consisted of a pyrex tube of 24 mm internal diameter with the distance between electrodes equal to 20 cm. Holes of 1 mm diameter were located at the centres of both electrodes for laser-interferometric measurements of Ne and for end-on plasma observations. The discharge was driven by a 150/~F condenser bank charged to 1.4 kV. During the experiment, a continuous flow of a nitrogen-carbon dioxide mixture was sustained at a pressure of 0.15 torr. The light from the pulsed arc was observed end-on by a photomultiplier-monochromator system (with 1 m focal length and inverse linear dispersion of 4.16 ,~, mm-~). This instrument has a measured instrumental half-width of 0.046 .~ with 10/zm slit width. Scanning of the C(II) lines was accomplished by repeated pulsing of the arc while advancing the monochromator in steps of 0.02,~. The output of the photomultiplier, together with the discharge-current waveform, were recorded on a dual-beam oscilloscope. All signals were analyzed at the maximum electron density. A helium-neon laser interferometer at 6328 ,~ (with a plane external mirror) was used to determine the axial electron density. The peak electron density was 4.9 × 1016cm -3 with an estimated measurement error not exceeding -+7%. The electron temperature of 26,300 K -+ 10% was determined from the Boltzmann plot of the relative intensities of twelve O(II) lines (3377.20, 3390.25, 3712.75, 3727.33, 3973.26, 4317.14, 4366.90, 4414.91, 4590.97, 4596.77, 4649.14, 4650.84/i,); the transition probabilities were taken from the book of Wms~ et al. (7) For these measurements, the spectral response of the photomultiplier-monochromator system was calibrated by using a standard tungsten coiled-coil quartz-iodine lamp. The quoted electron temperature and density were taken at the peak of electron density. Great care was taken to ensure that line self-absorption did not affect our line-shape determinations. This was achieved by careful examination of the line intensities and line-shapes as functions of the experimental conditions and by checking the optical depths of lines by QSRT Vol. 20, No. 5----D
477
478
M. PLATI~Aet al.
measuring intensity ratios within the multiplet 13u.v. The experimental ratio was then compared with the theoretical predictions of Ref. (7). The ratio of CO~:N2 = 1:20 was determined after performing a number of experiments in which the CO2 was gradually diluted. Two criteria were used to determine the time at which to stop further dilution: (a) the line intensities within the multiplet 13 u.v. agreed to within 4% with values derived from transition probability data <7) and (b) the line intensity started to decrease while the linewidth remained constant. At high dilution ratios, the plasma parameters (electron density and temperature) at constant pressure do not change within the limits of the experimental errors. The experimentally observed line profiles are the result of the superposition of a Stark profile, an instrumental profile and the profiles induced by other broadening mechanisms. On the basis of well known formulas, ~1'8~we have estimated the contributions of various broadening mechanisms to the widths of the C(II)u.v. lines. It was found that only the contribution of Doppler broadening is comparable with electron-impact broadening while natural, van der Waals, resonance and ion broadenings can be neglected under the present experimental conditions. Therefore, the experimental line profile consists of two parts: electron impact broadening (Lorentzian) and Doppler and instrumental broadening (Gaussian). To obtain the Stark profile from the measured profile, it was necessary to use a deconvolution procedure for the Lorentzian and Gaussian profiles, e) An example of experimental measurements fitted with a Voigt profile is given in Fig. 1. RESULTS AND DISCUSSION The experimentally determined full halfwidths of C(II) lines (in Angstroms), at an electron concentration of 4.9 × 1016cm -3 and electron temperature of 26,300 K, are given in Table 1. The estimated measurement error for these linewidths is ---17%. This estimated error does not include the uncertainty in electron density and temperature measurements. For the sake of comparison, the theoretical results of JONES e t ~/.,.2) are also given for our experimental conditions. In the next two columns of Table 1, we list our experimental results (normalized to an electron density of 1 x 1017 cm -3) as well as the results of another experiment ~5~for the same electron density and at an electron temperature of 12,800 K. From the comparison of the data in Table 1, we conclude first that our experimental data agree with the theory ~'2) within 16%. Also, Our results are more than six times smaller that the results obtained in another experiment. ~5)We note that these results were not taken at the same
10 Irel.
/
i , 0.116 , 0.48 , 0.32
i
o
i
instrumental width
i
i
o~6 o:32 o~8
~x (A)
Fig. 1. Experimentaldata (o) for the 2837.60,~ C(II)line fitted with the correspondingVoigt profile, The electron density was 4.9 x 10~6cm-3 and the electron temperature26,300K.
Experimental Stark widths of C(II)u.v. lines
479
Table 1. Experimental full halfwidths for C(II) in .& units W,. compared with the theoretical values (Wth) of JONES et al. "'2~ In the last two columns we show our and the Kuscrl ~5~ experimental results normalized to an electron density of x 1017em -3. .... iti . . . . . . y
2s2p2-3s2(is)3p
Multiplet (No.)
Wavelength [~3
Temperature
Electron Density
Eom-3]
,W /ixlO 17
w~[R] wth[R] e~3~thi~
iWm/iXlO 17
o~3, rro~
experiment) gusch(5)
2S-2p°
2836.71
26300
4.9 × 1016
0.082
0.070
0.167
1.07
(13 UV)
2837.60
26300
4.9 X 1016
0.083
0.070
0.169
1.07
electron temperature as ours. However, the large observed discrepancy cannot be explained by the temperature dependence of the line width, which is very weak for Stark broadening of ion lines in plasmas. The most probable reason for the observed discrepancy is self-absorption, which may not have been negligible in the KUSCH~5)experiment. The test that self-absorption did not exist at the peaks of observed line profiles in the experimental line profiles of Ref. (5) was the fit to a Lorentzian profile, which is expected to hold for Stark-broadened lines. It has been shown that this is a very insensitive method for the determination of spectral line optical depth, t~m especially when a photographic technique is used for the recording of line spectra. REFERENCES H. R. GRIEM, Spectral Line Broadening by Plasmas, p. 32. Academic Press, New York (1974). W. W. JONES,S. BENE~ and H. R. GRIEM,University of Maryland, Tech. Rep. 71-128, College Park, Maryland (1971). W. W. JONES, Phys. Rev. A7, 1826 (1973). N. KONJEVIt~and W. L. WIESE,J. Phys. Chem. Ref. Data 5, 259 (1976). H. J. KUSCH,Z. Astrophys. 67, 64 (1967). N. KONIEVI@,M. PLATI~Aand J. PURI(:,J. Phys. B. Atom. Molec. Phys. 4, 1541 (1971). W. L. WIESE, M. W. SMITHand B. M. MILES, Atomic Transition Probabilities, Vol. 1, Hydrogen Through Neon. U.S. Government Printing Office, NSRDS-NBS 4 (1966). 8. H. R. GRIEM,Plasma Spectroscopy. McGraw Hill, New York (1964). 9. J. T. DAVIESand J. M. VAUGHAN,Astrophys. J. 137, 1302 (1963). 10. N. KONJEVI(~and J. R. ROBERTS,3". Phys. Chem. Re.[. Data 5, 209 (1976). 1. 2. 3. 4. 5. 6. 7.