Ionic E.U.V. branching-ratio measurements using electron-impact excitation

.I. Quant. Spectrosc. Radiat. TransferVol. 49, No. 1, pp. 53-64, 1993 Printed in Great Britain. All tights reserved

0@22-4073/93$5.00+ 0.00 Copyright 0 1993Pergamon Press Ltd

IONIC E.U.V. BRANCHING-RATIO USING ELECTRON-IMPACT

MEASUREMENTS EXCITATION

FENG YANG? and A. J. CUNNINGHAMS Physics Programs and Center for Space Sciences, University of Texas at Dallas, P.O. Box 830688,

Richardson, TX 75083-2848,U.S.A. (Received 4 February 1992)

Abstract-Ratios of transition probabilities of e.u.v. transitions of 0 II, N II, S II, S III, Ne III, and Cl II have been determined using the branching-ratio technique. The transitions studied were excited under optically-thin conditions, using electron-impact excitation of low-pressure gases and vapors. Significant improvements in operating resolution (to 0.04 A) and S/N recovery capabilities of the data-acquisition system over previous studies in this laboratory allowed blended branching emissions to be resolved and weak components to be monitored. A total of 22 ratios including 7 first-time determinations are reported.

1. INTRODUCTION

Reliable transition-probability data of signature emissions are needed for general diagnostic purposes in the fields of astrophysics, space physics, and plasma physics and in such applications as laser-isotope separation, the evaluation of new types of laser systems and in the fabrication of semiconductor microelectronic devices. ‘A For low-2 atoms, both neutral and ionic signature emissions of interest lie in the technically difficult e.u.v. spectral domain and this has hindered routine measurements of the needed spectroscopic parameters. Calculations are also complicated because configuration-mixing effects can distort the wavefunctions of multielectron atoms to varying degrees. Comparisons of measured and calculated transition probability data are useful in gauging which states are most influenced by configuration mixing and identifying optimum approaches for treating the mixing itself. Reported here are new measurements of branching ratios (relative transition probabilities) for e.u.v. emissions of singly- and doubly-ionized atomic oxygen, nitrogen, sulfur, neon, and chlorine. This work represents an extension to and builds upon the findings of earlier studies in this laboratory involving the use of glow-discharge lamps and beam-foil techniques to excite ionic emissions. In this study, electron-impact excitation of low-pressure gases and vapors was employed, and the spectral resolution was enhanced in order to access emissions that were too weak to be monitored previously. A total of 22 e.u.v. ionic branching ratios were examined, including seven first-time determinations. The paper is organized as follows. The measurement strategy is reviewed in Sec. 2, and the experimental setup is presented in Sec. 3. The procedures employed to obtain a calibration of the optical system at e.u.v. wavelengths are reviewed in Sec. 4, and the experimental results are presented and discussed in Sec. 5. 2. MEASUREMENT

SCHEME

The array of ionic branching emissions examined in this work is summarized in Tables l-3. Each table entry indicates the wavelengths A, and 1, of the pair of emissions originating from a common upper level i and terminating on different lower levels j and k. The term value of the upper level is also indicated. Under optically-thin source conditions, the recorded photon intensity of an atomic emission from level i to level j, ZVcan be related to the population Ni of the upper level, to the tP_t

address: Keithley Instruments, Inc., 28775 Aurora Road, Cleveland, OH 44139, U.S.A.

$To whom all correspondence

should be addressed. 53

FENG YANG and A. J. CUNNINGHAM

54

Table 1. Branching ratios used for system calibration. Only integer values of the transition wavelength are shown and this convention is also followed in the text.

0 II

719 797

A

N II

sso 746

A

s II

3

s II

2~2~’ *D

6.13-cO.i 1

4.9’3

1.3

2s2ps ‘PO

1.91to.012

2.5’3

0.76

*P

0.76kO.07

0.66”

1.4

3s23p2(‘D)4s *D

2.420.12

1.5”

1.6

3s*3p*(sP)4s *P

52~~0.4

2.88'4

1.8

li%i s II

transition probability A,, and to the spectral sensitivity S(&), of the optical system (monochromatar-detector combination) at wavelength A,, via the expression Iv = &4&J.

(1)

For branching emissions sharing a common upper-state population Ni, measurements of Ii/ and Zti, the photon intensities of the branching emissions, coupled with knowledge of the spectral sensitivities of the optical system at these wavelengths, can be used to express the ratio of transition probabilities (or branching ratio) of the branching emissions as

(2)=[&I/[&]-

(2)

Transition probabilities A, and oscillator strengthsAY can be formally related using the expression’ J;, = 1.4992 x 10-‘6(lij)‘(gi/gj)A,,

(3)

where gi and gj denote the statistical weights of levels i and j and I, and A, have units of A and set-‘, respectively. 3. EXPERIMENTAL

SETUP

The major components of the experimental configuration used are shown schematically in Fig. 1. Excitation of the test gases was achieved using a compact electron gun located within the electron-impact chamber attached to the entrance slit of the 1 m e.u.v. vacuum monochromator (McPherson model 225). The thoriated filaments used in the source were capable of delivering beam currents up to 5 mA for periods exceeding 50 h. The beam fluctuation, as monitored by a Faraday cup, was less than 3%/h. The normal beam energy employed was between 125 and 200 eV and could be selected throughout the 50-350 eV range. The monochromator was fitted with a

Ionic e.u.v. branching-ratio

measurements

55

1200 l/mm, non-blazed holographic grating and could be operated in higher orders if required. This capability was important in resolving blended emissions. The electron multiplier (Hamamatsu model R595), was attached to the exit slit of the spectrometer and was a nude, 20-stage device operated in the pulse-counting mode. The monochromator-detector combination was maintained at pressures in the 10m5to 10m6torr range using a dedicated turbo pump. A separate turbo pump was used for the target chamber and pure gases (0,, NZ, Ne) or vapors (C&, CC&F,) were allowed to flow slowly through the target chamber. To ensure optically-thin source conditions, the pressure in the target chamber was maintained between 10e4 and 10m5torr during e-beam excitation. Output pulses from the detector were amplified, shaped and stored in a dedicated, computer controlled data acquisition system. Beam current and the chamber pressure were both monitored throughout each run, and the signal intensities were normalized to compensate for small fluctuations. Long accumulation times (7 h) were required for weak signals. The off-line analysis involved background noise subtraction and integration over the line profiles to determine the intensities of individual branching components. 4.

OPTICAL

CALIBRATION

The spectral sensitivity of the monochromator-detector combination required in the use of Eq. (2) for branching ratio determinations was determined as follows. The electron-impact chamber was replaced with a hollow cathode lamp. The emissions listed in Table 1 were excited by introduction of the appropriate host gas (0,, N,, or Ne) or vapor (CS,) into the lamp. Typical lamp operating pressures lay between 5 and 10 mtorr and the lamp current was adjusted in the 20-50 mA range to achieve stable operation. The branching emissions listed in Table 1 were selected for calibration purposes because they were found to be free of blending problems, and reliable Table 2. 0 II, N II, and Ne III branching ratios determined in this study. The transition wavelengths and upper terms of each branching pair are indicated. The combination of source pressure, beam energy, and beam current resulting in optimum excitation of the emissions is indicated. Only integer values of the transition wavelength are shown. IOfl

Beam Current

Measured @&I)

Upper Term

Pressure Torr

Beam Energy

2s2ps ‘PO

2%10’4

150 eV

2mA

3.020.3

4x10’

150 eV

5mA

102217

4x1O-4

150 eV

5mA

6.1kl.l

4x1 W4

225 eV

3.8 mA

635A NII

@& 746

N II

747A 656A

A

2sz2p(2Pq3s ‘P“

N II 2s2ps ‘PO

17.3k2.4

FENG YANG and A. J. CUNNINGHAM

56

Table 3. Cl II, S II, and S III branching ratios determined in this study. The transition wavelengths and upper terms of each branching pair are indicated. The combination of source pressure, beam energy, and beam current resulting in optimum excitation of the emissions is indicated. Only integer values of the transition wavelengths are shown. Pressure Torr

Upper Term

Beam Energy

Beam Currant

Measured (AJ&)

19OeV

4 mA

5.3kO.58

Ion

4 h

CP II

mA

CP II

1 ,eA

)

3s23psFDO)3d‘PO

1

3x10’

1 190 eV 1 4 mA

1 3

I

2P

I

2~10~

I 180 eV I 3.5 mA I

s ll SII

gj&

SII

& 1031

9wA

s II

3Xl(r

2s%p3fP”)4s ‘P“

twA

A

1 ;mi

I

1

6.5k0.78 0.62-c0.03

1

2x10-4

180eV

3.5 mA

0.66+0.04

3s23p2(‘D)4s 2D

2x104

180 eV

3.5 mA

1.37kO.09

3s23p2(3~)4s2~

)

2x10’

1 180 eV I 3.5 mA ’ 1 2.72*0.17

sII’I Eli-iI

3d sDo

I

2x104

1 210 eV ( 3.5 mA I

S Ill

4s 5~~

1

2x10’

I 210 eV

3s3p3 w

I

2x10-4

I 210 eV ) 3.5 m~ (

(

3

I

slllI 3

20.9+3.3

I 3.5 mA ( 3.29*0.43 5.620.4

I 4s ‘PO

2X10’

210 eV

3.5 mA

5.7kO.53

3s3ps ‘PO

2x10.’

210 eV

3.5 mA

22920.27

calculated or measured transition probabilities were available for both branches. Each choice is discussed below. The ionized neon branching ratios listed in Table 1 were among those suggested by Beyer et al6 and Flaig et al7 for calibration of e.u.v. monochromators. Ryan et al8 also exploited these emissions

/

I

i/-

/

I

CoMWon Ch8mbw

-

Fig. 1. Schematic view of the experimental arrangement.

I

I

Ionic e.u.v. branching-ratio

measurements

57

in beam-foil investigations in this laboratory. The transition probabilities adopted to derive the theoretical (&/A,) ratio of column 6 in Table 1 were obtained using NCMEP”’ (non-closed-shellmany-electron-theory). In this study, the components of these emissions were found to be easily excited using the discharge lamp, and all were found to be free from blending problems. The measured (Z,,lZ,) ratio for each entry was found to remain constant over a range of source pressures (5-100 mtorr), indicating that the source was optically thin in this pressure domain. The ionized-oxygen emissions listed in Table 1 served to extend the system calibration to longer wavelengths. For the 0 III 526/598 A entry, Wiese and Martin” scaled NCMET transition probabilities with experimental lifetime data to obtain a value of 4.9 for the branching ratio. This result was substantiated by Bolotin et al’* using a simple configuration interaction treatment. While all of the 0 II emissions used for calibration purposes could be excited using lamp pressures between 5 and 10 mtorr and currents between 50 and 60 mA, it was necessary to operate the grating in higher orders to resolve blended features. Figures 2(a-e) illustrate how the improved resolution, attainable in higher order, allowed the 538.26 A branching emission to be isolated from adjacent features. In Fig. 2(a), the operating resolution of 0.5 8, in first order was clearly inadequate to resolve the branching emission from adjacent 0 II lines at 537.8 and 539.8 A. Some improvement was realized in second order [see Fig. 2(b)], and some of the underlying structure was revealed. In the third-order spectrum shown in Fig. 2(c), a clear identification was possible. The fourth-order 538 A spectrum shown in Fig. 2(d), revealed the presence of a new feature identified as third-order 0 II 718.5 A. This assignment was confirmed when the new feature could not be identified in the fifth-order spectrum shown in Fig. 2(e). Since the blended features were cleanly resolved in the higher-order spectra, the intensities of the branching emissions were directly obtained from the recorded spectra by integration over the profile. Each line emission was found to be of a Gaussian shape, and this allowed us to use a curve fitting routine, consisting of a sum of Gaussians centered at the appropriate line center, to the blended first-order spectrum. In this manner, the contribution of the branching emission to the first-order spectrum could be isolated. Integration over the Gaussian profile obtained by fitting provided the intensity information needed for a branching-ratio determination from the first-order spectrum. The results obtained upon curve fitting the first-order lamp spectrum were found to be in excellent agreement with ratios obtained using the higher-order resolved spectra. This strategy was required for the unresolved 0 II 581, 616, and 673 A branching emissions only. The 0 II and N II transition probabilities adopted for calibration purposes were

1500

1200 % .fl : E I! 5 $ v)

900

600

300

0 536.00

537.44

538.88 Wavelength

540.32 (A)

Fig. 2(a). Caption on p. 59.

541.76

543.20

58

FENG YANG and A. J. CUNNINGHAM

3500

539.09

tb)

2800 2. .= t P)

E

3 z z v)

2100

1400

700

0 535.10

536.24

537.38

538.52

Wavelength

539.68

540.80

(A)

Fig. 2(b)

3000 539.09 1

2400

539.55 rr 539.85

538.26

600

0 535.60

536.86

537.72 Wavelength

538.78

539.84

540.90

(A)

Fig. 2(c). Caption opposite.

taken from Ref. 13 and were obtained by Cowan using a multiconfiguration-based calculation. The ionized sulfur emissions listed in Table 1 were excited in the discharge lamp using a mixture containing CS2 vapor diluted in neon in approximately a 1: 50 ratio. The optimum total pressure and discharge current for stable lamp operation were found to be approx. 50 mtorr and 50 mA, respectively. CS2 was selected as a source for sulfur emissions to avoid the significant blending of sulfur- and oxygen-ion emissions experienced using SO, in our earlier investigations.14 In lieu of any reported multiconfiguration-based calculated transition probabilities for the S II 800 A/866 8,

Ionic e.u.v. branching-ratio

measurements

59

h

2700

011 716.5A

3rd ORDER

2160

540

0 537.20

538.48

537.84

539.12

Wavelength

539076

540.40

(A)

Fig. 2(d)

h

539.09

(e)

1

0.06A

538.28

+t-

539.85

J 538.54

539.22

Wavelength

539.91

540.80

(A)

Fig. 2(e)

~vicinity of the 538.26 8,O II branching emission. The 26A emission. The emissions were excited with a kussed in Sec. 4.

‘--tedearlier in this laboratory by Morrison 14 A/l 124 A emissions, the transition ‘culation by Ho and HenryI and ‘or the S II 938 A/1031 8, entry, wlted in a physically unrealistic

FENGYANG and A. J. CUNNINGHAM

60

discontinuity in the calibration curve in the vicinity of 1000 A. The smaller (4 x) branching ratios measured in beam-foil studies by Ryan et al’ (1.38) and by Morrison et ali4 using a discharge lamp (1.5) yielded a more consistent (smoothly varying) pattern and were adopted for this reason. The procedure of repeated interpolation and extrapolation of the slopes of S(n) was used to translate these derived ratios of spectral sensitivities at individual wavelengths to a calibration curve of the system shown in Fig. 3. Although the S/N ratios of the signal intensities were consistently less than lo%, a larger overall uncertainty must be assigned since calculated transition probabilities had to be adopted to allow calibration of the optical system. The total uncertainty of the reported ratios was estimated to be &30%.

5. RESULTS

AND

DISCUSSION

The electron-beam apparatus described in Sec. 3 was used in this study to determine the ionic branching ratios summarized in Tables 2 and 3. Included in the tabulations is relevant information concerning the optimum combination of source pressure, beam energy and current for each measurement. With the exception of studies involving oxygen ions, a range of source pressures between 10e3 and lo-’ torr was used in the branching-ratio determinations. While the intensities of the branching emissions were found to vary in almost linear fashion with source pressure, the derived branching ratios remained unchanged, indicating that optically-thin source conditions prevailed. Using oxygen, the filament lifetime was found to be greatly reduced because of oxidation. However, with source pressures less than 4 x 10e5 torr, an acceptable compromise between filament lifetime and the intensities of the electron-impact excited emissions was obtained. The ratio of intensities of the fine-structure components of the 3S-3P0,,,20 I resonance transition (1302/1305/1306 A) was found to be 5 : 3 : 1, indicating that for these neutral emissions the source was optically thin. Since the electron-impact-excited ion densities were expected to be comparable to or lower than the neutral atom densities in the source, it was assumed that optically-thin conditions were present for the 0 II measurements listed in Table 2. The beam current used in each measurement was determined, based on considerations of compromising the lifetime of the filament and the intensity of spectral line. No dependence of the measured branching ratios on electronbeam energy was identified.

100

10

Wavelength Fig. 3. Relative sensitivity curve for the e.u.v. de

(A)

Ionic e.u.v. branching-ratio Table 4. Comparisons

Beam FoiP

Discharge Lamp’4.‘E

New Data

Nell1

1

379A/426A

1

17.3

1

0 II

1

462A/515A

~1

0.51

I

464A/516A

1 T-

1.1

l

1

61

of branching ratios determined in this study with other experimental determinations.

Ion

OH

measurements

0.56

Rocket Data13

I

I

l I

I I

26

1.55

>5.5

0 II

538A/561A

0 II

555&6OlA

2.76

1.65

1.0

0 II

617A/673A

5.3

4.1

3.6

0 II

719A1797A

5.1

6.6

N II

572Al635A

.019

<.004

N II

ssoAn4sA

3.0

1.7

N II

747AwaA

102

>50

N II

74t3A186oA

6.1

>12.5

s II

I

6OOA/866A

I

0.62

I

0.56

s II

644Al919A

0.66

s II

93W1031 A

1.37

1.5

s II

1014K/i 124A

2.72

2.88

s Ill

I

ssoAn33A

I

20.9

I

16.4

s Ill

662&736A

3.29

2.62

s Ill

724Al769A

5.6

6.34

s Ill

73OA/825A

s III

1

C# II

I

Ce II

5.7

732A/797A

I

777Af869A

I

961 A/l 139A

I

5.3

I

2.34

5.9

257

I

I 1.38

I

5.2

2.29

6.5

7.4

I

5.3

I

I I

I

I

I

Except for the 0 II 484/518 A, 538/581 A, and 616/673 8, branching emissions, all of the entries listed in Tables 2 and 3 could be resolved in the first-order electron-impact-excited spectra, and the integrated intensities required in the branching-ratio determinations were obtained directly. As discussed in Sec. 4, blending among 0 II emissions required operation of the grating in higher orders to isolate the branching emission contribution. Unfortunately this option could not be exploited with the electron-impact source because the signal strengths were much lower than those obtained using the discharge lamp. This problem was compounded when using oxygen since the source pressures had to be lowered as much as possible to reduce filament oxidation. The spectral identifications obtained using the higher order discharge lamp spectra of 0 II blended features were used, together with the Gaussian fitting routine discussed in Sec. 4, to isolate the contribution of branching emissions to the recorded (first order) electron impact excited spectra. The 0 II branching ratios listed in Table 2 were derived by using intensities obtained from the fitting routine. Table 4 facilitates a formal comparison of the results of this work with prior measurements conducted in this laboratory using both a discharge lamp14*i6and beam-foil excitation* of the branching emissions. Also included in the tabulation are four 0 II branching ratios derived from e.u.v. airglow spectra. I3 The comparison reveals excellent agreement among the different determinations of the S II and S III branching ratios but indicates an overall poorer agreement in the case of ionized oxygen entries. As regards the N II branching ratios listed, in our prior lamp

62

FENG YANG and A. J. CUNNINGHAM

measurements we were only able to establish upper or lower limits on three of the ratios. Including the N II entries, a total of seven new ionic e.u.v. branching ratios were measured. The requirement for adequate spectral resolution in branching-ratio determinations was highlighted in Sec. 4. For the three 0 II branching-ratio determinations discussed, it proved necessary to operate in higher-order modes of the grating to resolve overlapping features. On reflection, it is now clear that at least for these 0 II ratios, the operating resolution available in prior discharge lamp studies was inadequate, and the results obtained were influenced by blending from other 0 II emissions. Preference should be given to the results of this study since the contribution, if any, from blended features could be accounted for by using the fitting procedure discussed in Sec. 4. Among the different excitation sources used in branching-ratio determinations, the electron-impact and beam-foil sources operate at sufficiently low pressures that optically-thin conditions should prevail in all measurements, but this condition cannot be assumed for the l&100 mtorr pressures required for stable discharge-lamp operation. Of concern is that differential absorption of the components of the branching emissions could influence the measured ratio. In this work, no systematic dependence of the measured ratios was evident over the range of pressures examined. The problems with filament oxidation dictated the use of very low oxygen pressures, and it was not possible to examine a large pressure range for this gas. However, the 0 II 718/796 8, results provide caution against using optical depth as a universal selection criterion since the optically-thin electron-beam source and beam-foil results do not agree. The beam-foil determination could be suspect because emissions from this source are Doppler-broadened, which greatly complicates the identification of emissions blended with the branching emissions of interest. Limitations of the available resolution (7.5 A) in the rocket measurements of the e.u.v. airglow spectra and uncertainty regarding the optical depth of the airglow at the branching emissions are plausible reasons for the lack of agreement between the rocket data and the 0 II lines measured in this study. As indicated the N II branching-ratio determinations benefited from improvements in the resolution and data-acquisition capabilities available in this investigation. For the N II 660/745A ratio, the smaller ratio obtained using the discharge-lamp, compared to the ratios obtained with the electron-beam and beam-foil sources, could indicate an optical depth problem with the short-wavelength component, when using the lamp. Despite the weakness of the N II signals it was possible to obtain an S/N ratio larger than 8 in the nitrogen-ion studies. The agreement between the electron-beam, discharge-lamp and beam-foil results for the S II 938/103 1 A and S III 729/825 A ratios indicates that optically-thin source conditions were realized in all of these measurements. For the 0 II 484/518 A, S II 843/938 A, Cl II 777/889 A, and Cl II 969/l 139 A ratios, no previous determinations have been reported. Branching ratios derived using calculated oscillator strengths can be compared with the measurements reported here by using Table 5. For both oxygen and nitrogen, the calculated oscillator strengths used to derive the different entries in columns A-G were obtained using different treatments of the distortion of atomic wavefunctions resulting from configuration-mixing effects”,is in these multi-electron atoms. The simpler approaches, labelled as single-configuration treatments in Table 5, include the self-consistent field treatment (column A), the restricted Hartree Fock treatment (column B), and the Coulomb approximation (column C). All of these treatments are computationally less demanding than the more sophisticated multi-configuration based treatments used to obtain the results shown in columns D-G. FOTOS (first-order theory of oscillator strengths) results are listed in column D; the results of a limited configuration treatment (called SOC) are shown in column E; the results of a limited configuration mixing treatment are collected in column F, and NCMET results are collected in column G. Comparisons of measured and calculated ratios for the different ions do not indicate any preferred calculational scheme at this time. Upon examination, the measured 0 II ratios seem to be in somewhat better accord with the results of the SOC treatment than with those of the NCMET treatment but the single-configuration calculation results also bracket the measured ratios. For the N II branching ratios, better overall agreement exists with the results of multiconfiguration-based calculations but again the scatter is too large to indicate a preferred calculational scheme. In the absence of SOC results (column E) for this ion, it appears that the NCMET results are in closer harmony with the reported ratios. The absence of different calculated S II and S III oscillator strengths for many of e.u.v. emissions monitored does not allow any conclusions to be reached

Ionic e.u.v. branching-ratio

measurements

63

Table 5. Comparisons of branching ratios determined in this study with ratios derived from calculated oscillator strengths.

s III

730&825A

s III

mAmvA

C( II

mAltmA

C@II

961&1139A

5.7

4.5

229 5.3 8.5

regarding a preferred calculational scheme. For other e.u.v. transitions of these ions, the limited configuration interaction approach of Ho and Henry (column F) appears to be a successful approach in calculating oscillator strengths and line strengths. Unfortunately, this approach has only been ap lied in two of the sulfur-ion branching emissions and, of these, only the S II 1014 A/l 124 w ratio agrees with the measured result. It is important to point out that the measured ratio of 938 A/1031 8, is in excellent agreement with the discharge-lamp and beam-foil measurements also. With all three experimental results in agreement, it appears necessary to reexamine the couplings assumed in the multiconfiguration-based calculations. The single-configuration-based calculations (RHF) for the branching ratios S II 938 &lo31 8, and 1014&1124 8, and S III 729 A/825 8, are close to our measured ratios, which suggests that the configuration-mixing effects may not be important in these emissions. Continued experimental and theoretical study is indicated to confirm these findings. Acknowledgements-This work was supported by Organized Research Funds of the University of Texas at Dallas. The authors also acknowledge the assistance of L. Anschutz and L. Brooks in the performance of the experiments.

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64

2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22.

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