Observation of singly-ionized copper emission lines from a Grimm-type glow discharge plasma with argon—helium gas mixtures in a visible wavelength region

Specrrochimica Acta, Vol. 48B, No. 8, pp. 1039-1044, 1993

0584--854793 $6.00 + .00 © 1993 Pergamon Press Ltd

Printed in Great Britain.

RESEARCH NOTE

Observation of singly-ionized copper emission lines from a Grimm-type glow discharge plasma with argon-helium gas mixtures in a visible wavelength region (Received 12 October 1992; accepted 20 January 1993)

INTRODUCTION Ir~ OUR previous paper [1], we reported the classification of singly-ionized copper lines (Cu II) emitted from a Grimm-type glow discharge lamp. It has been indicated that the ionization and excitation mechanism of the Cu II emission lines could be explained from the comparison of their intensities for different plasma gases, for instance by observing the resonance charge transfer reaction [2] between copper atoms and plasma gas species. Emission lines derived from the following optical transitions can be observed only in the plasmas containing helium [1, 3]: 3d96s (16.56-16.83 eV) --->3d94p (8.23-9.12 eV),

3dS4s4p (16.5%16.84 eV) --->3d84s2 (8.64-9.02 eV) [4]. These lines, concerned with higher excited states of singly-ionized copper, appear primarily in the vacuum ultraviolet wavelength region. However, Cu II emission lines that can be excited with the helium mixture plasmas are also observed in the wavelength region of more than 400 nm. The Cu II lines in the visible wavelength region result from the following transitions: 3d94f (16.85-17.14 eV) --->3d94d (14.20-14.69 eV) 3d95d (16.90-17.25 eV) --> 3d95p (14.88-15.32 eV) 3d96s

--> 3d94p [4].

Our previous report lacked these results [1]. In this paper, the emission lines deriving from these transitions are compiled and the excitation mechanisms are discussed.

EXPERIMENTAL Our instrumentation and the operating procedure have been described in detail elsewhere [1, 5]. In order to regulate the partial pressure of a constituent more accurately, the Pirani gauge, which was employed in the experimental system of the previous study [1], was replaced with a diaphragm vacuum gauge (WD-1 model, ULVAC Corp., Japan). Two different methods for mixing argon with helium gas were performed: (1) the partial pressure of argon was fixed and subsequently appropriate amounts of helium gas were introduced into the discharge cell; and (2) the partial pressures of both argon and helium gas were controlled individually so that the total pressure of the plasma gas could be kept constant. The erosion rate per minute was estimated from specimen weighing after discharge of 10 rain duration. The area of the sputtered crater was approximately 49 mm 2.

RESULTS AND DISCUSSION Table 1 summarizes the wavelength of observed emission lines assigned to the copper ion 3d94f-3d94d, 3d95d-3d95p, or 3d96s-3d94p transitions together with their relative intensities. It is found that the Cu II 490.97 and 493.18 nm lines are the most intense in the emission lines listed in Table 1. Figure 1 shows spectral scans recorded with (a) an argon-helium mixed gas 1039

1040

Research note Table 1. Emission lines of singly-ionized copper observed only from the argon-helium mixture glow discharge plasma only over the wavelength range of 300--650 nm Assignment (eV) Wavelength (nm) in air

Upper

384.15 384.95 387.32 448.55 467.16 467.36 468.19 485.14 485.49 490.13

4p 4p 4p 4p 4f 4f 4f 4f 4]" 4f

3PI, 3D3, 3P2, 3D2, 3P2, 3Po, 3p|, 3G4, 3G5, 3F3,

490.66 490.90 490.97 491.24 491.29 491.82 492.64 493.18 493.79 494.01 494.30 "494.94 495.16 495.38 495.60 498.54 500.96 501.26 502.12 504.12

4f 4/" 4f 4f 4f 4]" 4f 4f 4f 4f 4f 4]"

3D2, 3H5, 3/-/6, 3D3, 3D2, 3D3, tPt, 3Hs, 3P2, 3Po, 3el, 3Pi,

505.17 505.88 506.55 507.22 508.38 508.82 509.39 512.07 512.44 518.34 535.67 536.56 580.59 589.81 594.12 600.00 607.22 611.45 615.06 615.40 618.87 621.72 627.34 627.69 630.62 631.14 641.47 644.19 647.03 648.46

4f 4f

4p 4f 4f 4]" 4f 4f 4]" 4f 4f 4f 4f 4]" 4f 4/" 4p 4f 4p 4f 6s 6s 5d 5d 6s 5d 5d 5d 5d 5d 5d 6s 5d 5d 6s 5d 6s 5d

16.658 16.612 16.592 16.961 16.850 16.849 16.845 16.895 16.881 16.866

5s 5s 5s 4d 4d 4d 4d 4d 4d 4d

16.863 16.853 16.853 16.860 16.863 16.860 16.856 16.853 16.850 16.849 16.845 16.845 3G4, 16.895 tHs, 17.117 3G4, 16.841 3F4, 16.878 3F3, 16.866 3G4, 16.895 3D3, 16.860 3P2, 16.850

4d 4d 4d 4d 4d 4d 4d 4d 4d 4d 4d 4d 4d 4d 4d 4d 4d 4d 4d 4d

3P2, 3G5, 3G5, 3P2, 3P1, 3G4, aPt, 3G4, 3PI, 3Pi, 3P2, 3Pl, 3D3, IG4, 3G4, 303,

3G5, 3F4, 1G4, 3F3, 3/73, 3F3, 3D2, 3P2,

4d 4d 4d 4d 4d 4d 4d 4d 4d 4d 4d 4d 4p 4p 5p 5p 4p 5p 5p 5p 5p 5p 5p 4p 5p 4p 4p 5p 4p 5p

3F4, 3F4, 1F3, 3F3, 3F4, 3D2, 3D2, 3D 2,

3G4, 3P1, 3F4, 3PI, 3D3, 3D3, 393, 3P2, 3D3, 3F3, 3F4, 3SI, 302,

3G4, 3G5, 303, 3PI, 303, 3D2, 3F4, 3D3, 3SI,

Relative intensity*

Lower

16.881 16.878 17.137 16.866 16.866 16.866 16.863 16.850 16.841 16.845 16.706 16.845 16.563 16.563 16.975 16.955 16.563 16.990 16.991 16.903 16.995 16.956 16.952 16.563 16.957 16.975 16.580 16.991 16.563 16.903

3D2, 3D3, 303, 3S1, 3Sl, 3S1, 3St,

3G4, 3G5, 3P2,

13.432 13.392 13.392 14.197 14.197 14.197 14.197 14.340 14.328 14.337

14.337 14.328 14.328 14.337 14.340 14.340 14.340 14.340 14.340 14.340 14.337 14.340 14.392 14.615 14.340 14.392 3D3, 14.392 3F3, 14.422 3D3, 14.392 3D3, 14.392

3F3, 3Po, 303, IPt, 3D3, 3F3, 3P2, 3P2, 3D 2, 3F3, 3F4, 3P2, 3PI, 3F3, 3F4,

3F4, 3Pt, IF3, 3G3,

3D3, 3G3, 3PI,

14.428 14.428 14.690 14.422 14.428 14.430 14.430 14.430 14.422 14.453 14.392 14.535 14.428 14.462 14.889 14.889 14.522 14.963 14.976 14.889 14.992 14.963 14.976 14.589 14.992 15.012 14.648 15.067 14.648 14.992

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* The emission intensity of each Cu II line is classified when that of the Cu I 510.56 nm is estimated as 100 in an argon-50% helium mixed-gas plasma.

Research note

1041

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Fig. 1. Spectral scans of Cu II lines in the wavelength range 490-494 nm with (a) argon at 6.7 × 102 Pa and helium at 4.0 x 102 Pa, and (b) pure argon at 6.7 x 102 Pa, when the discharge voltage is maintained at 380 V.

and (b) a pure argon gas in the wavelength range from 490 to 494 nm. Whereas no Cu II emission lines are observed in the pure argon plasma, several copper lines belonging to the 3d94f-3d94d transition can be detected in the a r g o n - h e l i u m mixture plasma. The H e 1 492.18 nm line is also found in the plasma mixture. Figure 2 indicates a relation between the net emission intensities of some Cu II lines and the helium line and the partial pressure of helium gas added when the argon partial pressure is fixed at 4.7 x 102 Pa and then the helium pressure varied from 0 to 5.3 x 102 Pa. It is found

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Fig. 2. Variation of the net emission intensities of Cu II 490.97 nm (O), Cu II 493.18 nm (&), Cu II 491.29 nm ( I ) , and He I 492.18 nm (O) as a function of the partial pressure of helium gas added (partial pressure of argon was maintained at 4.7 x 102 Pa and the discharge voltage was 600 V (fixed)).

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Fig. 3. (a) Plots of the net emission intensities of Cu II 490.97 nm (squares) and Cu I 510.56 nm (circles) against the discharge current, using pure argon plasma at 4.7 x 102 Pa (open symbols) and a plasma mixture of argon at 4.7 x 102 Pa and helium at 4.7 x 102 Pa (closed symbols). (b) Plots of the net emission intensities of Cu II 429.25 nm (&) observed in the plasma mixture.

that the Cu II intensities are raised with an increase in the helium pressure and that helium principally contributes to excitation of the corresponding upper energy states. In the absence of helium plasma gas, such excited states having higher excitation energies are less populated in the Grimm-type glow discharge plasma. Plots of the net emission intensities of the Cu II 490.97 and the Cu I 510.56 nm lines against the discharge current are shown in Fig. 3(a) when the pure argon plasma (open symbols) as well as the argon-helium mixture plasma (closed symbols) are employed. The intensity of the atomic line (circles in Fig. 3(a)) is similarly elevated along with increasing the discharge current for these plasmas. However, the ionic line cannot be observed in the pure argon plasma independent of the discharge currents, which indicates that the discharge conditions are less dominant for the excitation of the ionic line. Figure 4(a) shows dependence of the net emission intensities of the Cu II and the He I emission lines on the helium composition in the plasma. In this case, the total pressure of the plasma gas in the discharge cell is kept at 9.3 x 102 Pa and the partial pressures of argon and helium are controlled, respectively. The net intensity of the Cu II line (closed circles in Fig. 4(a)) reaches a maximum when the helium content in the plasma gas is round 45%. However, this result strongly depends on changes in the sputtered amount of copper species in the discharge cell. As illustrated in Fig. 4(b), the discharge current of the lamp drastically decreases with an increment of the helium content, indicating that the population of copper species supplied by the cathode sputtering is lowered when a part of the plasma gas is replaced with helium gas. A variation in the erosion rate measured is shown in Fig. 5(b). Furthermore, the excitation reactions in the mixed-gas plasma also occur less actively compared to those in the pure argon plasma. In fact, the intensity of the Cu I 510.56 nm line monotonously decreases along with an increase in the helium content (open squares in Fig. 4(a)). A decline in the excitations can also be used to interpret the situation where the net emission intensities of the He I line (closed triangles in Fig. 3(b)) decrease in the case where the helium composition exceeds 70%. In order to correct the change in the sputter rate for each helium content, we computed the intensities of the Cu II lines normalized per unit amount of the sputtered sample, as indicated in Fig. 5(a). It is found that the normalized intensities increase when the helium content is elevated to c. 80%. The intensity decrease at 87% He, probably due to the abrupt changes in the excitation conditions in the plasma. It seems that, in this case, helium offers a strong effect on the excitation of these Cu II lines. It should be noted that the Cu II emission lines whose excitation energies are round 16.8 eV can be observed in the helium mixture plasma, as summarized in Table 1. Because the first ionization potential of copper is 7.72 eV [4], the total energy to obtain these copper ion excited

Research note

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Fig. 4. Plots of (a) the net emission intensities of Cu I 510.56 nm (lq), Cu II 490.97 nm (O) and He I 492.18 nm (A), and (b) the discharge current (O) against the helium composition in the plasma gas (total pressure of the plasma gas was maintained at 9.3 x 102 Pa and the discharge voltage was 600 V (fixed)).

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Fig. 5. Variation of (a) the normalized intensities of Cu II 490.97 nm (O), Cu II 493.18 nm (A), and Cu II 491.29 nm (11) corrected per unit amount of sputtered specimen, and (b) the erosion rate (O) as a function of the helium content in the plasma gas (total pressure of the plasma gas was maintained at 9.3 x 102 Pa and the discharge voltage was 600 V (fixed)).

states from the ground state of the copper atom is calculated to be (16.8 + 7.7) 24.5 eV. This value is very similar to the ionization potential of helium (24.58 eV) [4]. Therefore, it is considered that, in copper-helium collisions, the excited states of the copper ion can be produced through the resonance charge transfer reaction [2] between ground state copper atoms and helium ions [6, 7]. It is known that copper ionic lines identified to arise from the transitions between much higher excited levels appear in a visible wavelength region:

Research note

1044

3d97s (18.22-17.95 eV) ~ 3d95p (15.32-14.88 eV) [8]. Unfortunately, oscillator strengths of the Cu II lines, which are discussed in this study, are not available in the literature. However, it is assumed from the intensity data in Ref. [8] that the oscillator strengths are not so different among the Cu II lines. Figure 4(b) indicates a change in the intensities of the Cu II 429.25 nm line [7s 3D 3 (17.95 eV) - 5p 3D 3 (15.07 eV)] as a function of the discharge current when the argon-helium mixture plasma is examined. It should be noted that this line cannot be detected independently of the discharge conditions. In fact, no emission lines assigned to the 3dO7s-3d95p transition are observed, not only in the argon plasma, but also the argon-helium plasma. This result may be explained from an insufficient supply of energy from the helium ion to the copper atom to produce the 3d97s excited states.

SUMMARY In the argon-helium plasmas, the emission lines of singly-ionized copper, which cannot be excited in the pure argon plasma, can be observed in the visible wavelength region. These lines are identified to the 3d94f-3d94d, 3d95d-3d95p, or 3d96s-3d94p transitions. The eollisional reactions occurring and the resonance energy transfer between copper atoms and helium ions are a feasible mechanism for the observed excitations.

Institute for Materials Research Tohoku University Katahira 2-1-1 Sendai 980 Japan

KAZUArd WAGATSUMA and KICHINOSUKE HIROKAWA

REFERENCES [1] [2] [3] [4] [5] [6] [7] [8]

K. Wagatsuma and K. Hirokawa, Spectrochim. Acta 46B, 269 (1991). O. S. Duffendach and J. G. Black, Phys. Rev. 34, 35 (1929). K. Wagatsuma and K. Hirokawa, Anal. Chem. 60, 702 (1988). C. E. Moore, Atomic Energy Levels. National Bureau of Standards, Circular No. 467, U.S. Government Printing Office, Washington D.C. (1949). K. Wagatsuma and K. Hirokawa, Surf. Interface Anal. 6, 167 (1984). D. W. Ernie and H. J. Oskam, Phys. Rev. A23, 325 (1981). P. Mezei, K. Rozsa, M. Janossy and P. Apai, Appl. Phys. B. 44, 71 (1987). A. G. Shenstone, Proc. R. Soc. London A235, 196 (1936).