2020
Anal. Chem. 1984, 56,2020-2024
Vacuum Ultraviolet Emission Line for Determination of Aluminum by Inductively Coupled Plasma Atomic Emission Spectrometry Takashi Uehiro,* Masatoshi Morita, and Keiichiro F u w a
National Institute for Environmental Studies, Yatabe, Tsukuba, Ibaraki 305, Japan
A vacuum ultravlolet emlsslon llne (167.1 nm) of alumlnum In ICP shows promise for determlnatlon of trace levels of aluminum. Under ordinary conditions for ICP-AES, this llne has very hlgh sensitivity-background equlvalent concentration of 10 ng/g and detecllon llmlt of below 1 ng/g were obtalnable wlth a normal vacuum type polychromator. Spectral interferences from other elements examlned are not more than 0.0005 g of AVg of element except for Iron (0.0023 g of Al/g of Fe). Water samples from an unpolluted lake (Lake Mashu) were analyzed wRhout preconcentrationto glve aluminum concentrations of 2-3 ng/g.
In inductively coupled plasma atomic emission spectroscopy (ICP-AES), emission lines in the vacuum ultraviolet region (VUV) have not been studied as extensively as those in the conventional ultraviolet and visible region (CUV), e.g., 185-700 nm. Recently several rep,orts of the determination of several elements by ICP-AES in VUV have appeared (1-3). ICP-AES in VUV has some merits. Some elements do not have line(s) sensitive and/or selective enough in CUV. Sulfur and phosphorus are examples. For sulfur, there are no practical analytical lines in CUV. For phosphorus, severe interferences is observed from iron and/or copper on the 214.9-nm or 213.6-nm line. For these elements, therefore, emission lines in VUV-S (180.7 nm) and P (178.3 nm)-have been introduced for the analysis of iron or alloy and now vacuum type polychromators are frequently equipped with these lines. Recently lines of iodine (178.3 nm) and bromine (163.3 nm) have drawn attention; these elements also do not have practical analytical lines in CUV. Other elements-As, B, Hg, Sn-have been examined for higher sensitivities. In a polychromator system, lines in W V , can take the place of lines in CUV. Many prominent lines fall into the wavelength range of 190-250 nm ( 4 , 5 ) ;therefore, some channels may compete with one another because their components (slits, mirrors, PMTs, and so on) take almost the same space. In such cases sensitive analytical lines in other spectral region could solve the problem. It is well-known that the relative emission line intensities in the ICP do not correspond to those in the spark or arc (4, 5 ) . When the ICP emission lines in VUV for the element analysis are used, the line intensity table based on the spark or arc emission is not sufficient; therefore, we decided to start making a line table for the ICP-AES in VUV. For about 60 elements, intensity measurements from 185 to 165 nm (for some elements down to 150 nm) were carried out. Among the many lines examined, the most interesting one is the aluminum 167.1 nm ionic emission line. This line has much higher sensitivity than the common 309.3, 308.2, and 396.2 nm lines. In this report, we will discuss the potential of this line (A1 167.1 nm) in analytical ICP-AES. Usually, aluminum detection limits of ICP-AES using common analytical lines are about 20 ng/g ( 4 , 5 )and determination limits are about 100 ng/g; therefore, aluminum
Table I. Instrument Used (a) Monochromator System
ICP rf generator ICP and matching
Plasma-Therm system HFP-5000D with APCS-3 ICP-5000 with AMN-PS-1
box
nebulizer optical interface mirror nozzle monochromator type, grating slit (width X height) PMT recorder
pneumatic glass concentric one concave and one flat quartz tube (20 mm 0.d.) JOBIN YVON, JY38P Czerny-Turner,f = 1 m, F = 5.6 3600 grooves/mm, Holographic Master entrance 20 pm X 6 mm exit 40 pm X 10 mm Hamamatsu TV, RlO6UH and R166UI Rika Denki, R16 (b) Polychromator System
ICP rf generator
ICP and matching box nebulizer thermal mass-flow controller polychromator tvpe
grating PMT
RF Plasma Products System HFS-2500F with APCS-3 ICP-2500 with AMN-PS-1 pneumatic glass concentric outer gas: Tyran, FC-261 nebulizer gas: Tyran, FC-260 Daini Seiko Sha, JY48PVH Paschen-Runge, Rowland circle = 1 m 2550 grooves/;nm, Holographic Master Hamamatsu TV, R300, R306, and R427
concentration cannot be determined in such samples as seawater (ca. 10 ng/g), healthy human plasma (below 20 ng/g) (6, 7), and water from Lake Mashu (ca. 2-3 ng/g), which is the demonstrating sample used in this paper, without preconcentration. When the AI 167.1 nm emission line was used, the detection limit became about 2 orders of magnitude lower than usual and direct determination of aluminum concentration in water from Lake Mashu was carried out successfully.
EXPERIMENTAL SECTION Instrumentation. Instruments used are shown in Table I. The monochromator (JY38P) was purged with 30 L/min nitrogen gas from a liquid nitr, en tank. The optical interface between the plasma and the mL.rochromator was sheathed with nitrogen (6 L/min) also. In the light path, there are five mirrors including one J Y master holographic grating and no lens or windows except for the quartz wall of PMT (R166UH or RlO6UH). The distance between the out hedge of the plasma torch and the interface nozzle head was about 2 mm, which did not result in overheating of the nozzle. A demagnified image (1/2) of the ICP was focused on the entrance slit (width, 20 pm; height, 5 mm) with one concave and one flat mirror in the optical interface;therefore, a zone of 10 mm along the plasma axis was observed. The center of the observation height was determined to 15 mm above the load coil top. The incident power of the rf generator was 1.1 kW with the reflected power below 5 W. The flow rates of outer gas (coolant gas) and intermediate gas (auxiliary gas) were 18 L/min and 0 L/min, respectively. Nebulizer gas pressure was 28 psi, which resulted
0003-2700/84/0356-2020$01.50/00 1984 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 56, NO. 12, OCTOBER 1984
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- _ _ _ - - ~
Table 11. Aluminum Emission Lines in Vacuum Ultraviolet termC peak wavelength, - BEC, no.a nm high low !&/g 0.0074 3s2 ’S 1 167.0787 (11) 3p ‘P 2 171.9440 (11) 3d 3D1 3p 3P, 3.7 172.1244 (11) 3d 3D1 3p 3P, 3{ 172.1271 (11) 3d 3D2 3p 3P, 172.4952 (11) 3d ’D, 3p 3P, }0.63 4 { 172.4984 (11) 3d 3D2 3p 3P2 5 176.0104 (11) 3p2 ’P, 3p ’P, 3.7 6 176.1975 (11) 3p2 3P, 3p 3P, 4.2 7 176.300 (I) 3pz ’P3i2 3p 1.7 176.3869 (11) 3p2 ’PI 3p 3P, 176.3952 (II){ }I”
1
li
I
1/50
165
t1.2
8i
1,
I
1
175
185
W a v e Iengt h/n
9 176.564 ( I ) 10 176.5815 (11) 11 176.638 ( I ) 12 176.7731 (11) 1 3 176.914(1) 185.5929(11) 185.8026 (11) 186.2311 (11)
m
Flgure 1. Aluminum emission spectra in vacuum ultraviolet region
(165-185 nm). Wide and selective regional scans were done for 100 pg/g and 20 hg/g AI, respectively. Scale attached to selected region is relative sensitivlty of recorder. Numer of each peak corresponds to peak number in Table 11.
in a sample uptake rate of 2 mL/min. All of these conditions were normal compromised conditions for simultaneous multielement analysis. The vacuum polychromator was a Daini Seikosha’s version of JY48PVH with a specially ordered aluminum 167.1 nm channel. Seath gas was nitrogen and the flow rate was 8 L/min. Pressure torr. The PMT for the in the polychromator was below 2 X 167.1-nm A1 channel was an R306. The plasma operating conditions were 1.25 kW incident and below 5 W reflected rf power, 18 L/min outer gas, 0 L/min intermediate gas, and 0.75 L/min (26 psi) nebulizer gas with a sample uptake rate of 1.1mL/min. The outer gas and the nebulizer gas flow rate were controlled with thermal mass flow controllers. The observation height was 14 mm above the load coil top. Chemicals and Samples. Subjected standard solutions were prepared from 1mg/g stock standard solutions (Kanto Kagaku, Standard for Atomic Absorption Spectrometry) by decadal dilution with direct weighing into polyethylene bottles and addition of 0.1 mol/L nitric acid solution. Water samples from Lake Mashu were sampled by a GO-FLO sampler (General Oceanics, 5 L) and acidified to pH 1.4 by subboiling distilled nitric acid on 9 September 1982, and stored at 5 “C. Measurement. Spectral profiles (165-185 nm) of standard solutions were taken by the JY38P with a strip chart recorder. Ratios between line and background intensities ( S I B ratio) were calculated from those profile data and converted to background equivalent concentrations (BEC’s). Aluminum determination of Lake Mashu samples were carried out on the JY48PVH in “profilemode” with the standard addition method. In “profile mode”, a wavelength scanning around the set wavelength could be made by moving the entrance slit along the Rowland circle under the computer control (up to ca. 0.85 nm, 0.17 pm/step). RESULTS A N D DISCUSSION Sensitivity. A sample VUV spectrum is shown in Figure 1. To obtain the spectrum, high concentrations of aluminum solution (100,20 pg/g) were used to distinguish emission lines from background noise. There are some lines which are not assigned to aluminum in the spectrum (B), for example, a strong 174 nm doublet due to nitrogen from the sheath gas and atmosphere, a 166 nm complex, and a weak 175.2 nm line due to carbon from impurity in argon and/or nitrogen as well as in the air.
gt tF:
F:
ZF;
3p2 3p2 3P, 3p2 ‘P3i2 3p2 3P, 3p2 4s )S 4s )S 4s 3S
3p 3p 3p 3p 3p 3p 3p 3p
’PI/, 3P, 2P3,z 3P, ( ? ) 2P3,2
3p 3p 3p 3p
2 P l / z 1.5d 2p3/2 0.77d 2P3,2 2P3iz 0.95d
308.2135 ( I ) 3d 309.2710 (I) 3d 309.2839 (I) 3d 396.1520 (I) 4s
‘D3i2 ZDi,2 ’D3/, ‘S
IP, 3P1 3P,
0.83 3.4 0.34 3.3 1.5 8.0
2.5 1.5
Number of peak corresponds to spectrum (Figure 1). Wavelength values (ref 8). Tentative assignment calculated from ref 8 and 9. Recalculated from ref 4. a
A gradual decrease of background emission toward the lower wavelength could be attributed to two sources, first, decrease in emission intensity of ICP of itself and, second, decrease in detecting power of our system-reflectance of mirrors and sensitivity of PMT. The lines assigned to aluminum are listed in Table I1 together with the background equivalent concentrations (BEC’s) and the transition terms assigned tentatively from calculation with energy level tables (8, 9). BEC’s were calculated by subtracting the dark current of PMT from the background intensity, because the PMT dark current is one of the instrumental parameters and can change from one PMT to another. Of 13 observed aluminum emission lines in VUV, the 167.1-nm line was uniquely strong, considering the lower detecting power of the system a t the lower wavelength. In order to obtain the accurate BEC of this line, intensity measurements of deionized water and aluminum solution of 10 ng/g (about BEC) were made at the peak wavelength. BEC was calculated as 7.4 ng/g or 11 ng/g, with or without subtracting the PMT dark current intensity from the background intensity, respectively. With a 1Hz low pass filter, detection limit ( S I N = 2) was 0.6 ng/g. To compare the results with the commonly used aluminum analytical lines, some lines from the CUV are also listed in Table 11. Data from Winge et al. (4) were converted to BEC’s. It is noteworthy that the 167.1-nm line is much more sensitive than commonly used lines (308.2 nm, 309.3 nm, and 396.2 nm) by about 2 orders of magnitude. Other lines such as 176.6 nm and 172.5 nm are almost as sensitive as conventional region lines. With the polychromator system (JY48PVH), the sensitivity of the 167.1-nm line was also checked. Under previously described conditions, BEC was found 10 ng/g and the relative standard deviation for intensity of deionized water sample (integration time = 10 s and n = 5 ) was below 1.5%; consequently, the detection limit (3u) was calculated below 0.5 ng/g,
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ANALYTICAL CHEMISTRY, VOL. 56, NO. 12, OCTOBER 1984 Fe 167.074 nm
~~
Table 111. Interference Factors of Several Elements on A1 (167.0787 nm)a backgroundC
lineb
Fe(167.074) P(167.107)
Ca Mg C
2.3 0.09 0.05
Ni(167.09?)
0.12
0.07 0.06
B, Cr, Cu I, Mn, Mo < 0.05 S, Si, Zn
“Values are in g of Al/g of element. bOverlappingof line Structureless,flat background increase.
center or foot.
Flgure 2. Spectral interference of Fe (167.074 nm) on AI (167.0787
nm). which agreed well with the result of JY38P. Allein and Mauras (7) reported that the 396.2-nm line had a BEC of 25 ng/g and the detection limit for water was 0.4 ng/g. Their experimental conditions, however, were quite different from the cornpromized conditions used for the simultaneous multielement analysis, high sample uptake (6 mL/min) and high observation height (25 nm above the load coil). On the other hand, the 167.1-nm line in this paper has higher sensitivity under conditions very much like common cornpromized ones. Selectivity. Among 60 elements examined, there were several elements that had emission lines near the aluminum 167.0787-nm line. Of those elements, iron has the strongest emission line (167.074 nm) (8). Figure 2 showed wavelength scan profiies of aluminum 100 ng/g and iron 100 pg/g solution around 167.1 nm (JY38P). The peaks of each element severely overlapped each other within a spectral bandwidth and could not be resolved. The interference factor based on weight was calculated as 0.0018 g of Al/g of Fe. A small peak observed a t a little longer wavelength could not be assigned till now but probably was due to weak emission from iron. Any other element studied has an interference factor of not more than 0.0005 g of Al/g of element. It was difficult to estimate weak interference factors for each element accurately because there were several possible sources for aluminum contamination, for example, the standard solution of each element and the instrument itself (desorption and/or dissolution from the sample flow line consisted of a Tygon tube, a Pyrex concentric nebulizer, a Pyrex chamber, and a quartz torch). Interference factors were also obtained with the JY48PVH and the results are summarized in Table 111. Among these elements, Fe, Ca, and Mg would be significant in usual samples, but their interferences could be corrected by the interference factors together with their concentrations, determined by careful measurements of their analytical lines. For qualitative analysis, the 167.1-nm line also has a great advantage over the 308.2 nm and 309.3 nm lines. The latter lines are observed in the OH molecular band region; therefore, a weak peak of a low concentration of aluminum is obscured by the strong OH band structure. Figure 3 shows, spectra of a 100 ng/g aluminum solution obtained with the JY48PVH in “profile mode”. Comparing those profiles, it is obvious that the 167.1-nm line shows existence of aluminum much more clearly than the 308.2-nm line. The aluminum peak at 308.2
0
20
0
-20
20
0
-20
Entrance slit position
Flgure 3. Spectra of 167.1 nm and 306.2 nm lines of AI obtained simultaneously with JY48PVH: (e)100 ng/g AI, (0)deionized water; (A) 167.1 nm, (B) 308.2 nm. Abscissa is entrance slit position (1.7 pm/division, negative to longer wavelength).
nm is overlapped with a small OH band and moreover a strong band is observed at a little shorter wavelength (ca. 0.06 nm). Stability. One of disadvantages of the 167.1-nm line is that the wavelength is quite short. In order to ensure high transmissivity of the spectrometer at such a short wavelength, high quality mirrors, gratings, and the lenses are required. Degradation of aging of these components would affect the stability of measurements, especially the long term stability. As to our instruments, the concave mirror in the optical interface of the monochromator system was damaged over a period of a month during summer vacation probably due to humidity or acid fumes, but collimating and camera mirrors and a grating in the monochromator have not been damaged for over 2 years. For the polychromator system, which has been running on for over 6 months, there have been no transmissivity troubles. Analysis of Lake Mashu Water. Analytical application of the aluminum 167.1-nmline is made to a lake water sample. Lake Mashu in the northern part of Japan (144.5’ E, 43.6’ W) is one of the most beautiful and cleanest lakes in the world having the visual transparency of 41.6 m (in 1931). Aluminum has never been determined by conventional region ICP-AES without preconcentration because the level was too low. For water samples from Lake Mashu, dissolving elements which should be considered for interferences are sodium (13 pg/g), potassium (0.9 pg/g), calcium (8 pg/g), magnesium (3.5 pg/g), silicon (5 pg/g), sulfur (4 pg/g) and chlorine (7 pg/g) (IO). The concentration of iron was about 1-3 ng/g (IO); therefore the iron interference on aluminum determination was negligible (interference factor = 0.0023 g of Al/g of Fe). Spectral profiles of these elements around 167.0787 nm were obtained with the JY48PVH in “profile mode”. There were no noticeable structural backgrounds at the aluminum line wavelength, and a small flat increase of background due to calcium and magnesium at Lake Mashu concentrations was
ANALYTICAL CHEMISTRY, VOL. 56, NO. 12, OCTOBER 1984
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Table IV. Aluminum Content in Lake Mashu A1 content, ng/g direct' preconcnb
depth, m 50
0 0
0
.. .
0
"
I
I
I
I
1.1 1.6 1.5 1.3
1000-fold concentration a Direct measurement in this work. using oxine (8-hydroxyquinoline)and ODs-gel column (10).
" e
30
1.7 1.8 2.8 2.6
100 150 200
c
I
0 Entrance slit position
I
I
-30
Flgure 4. Spectra of Lake Mashu sample (abscissa is same as Figure 3): (0, B) sample; (0, 0 ) sample spiked with AI (2.9 nglg); (V) deionized water blank; (A)add blank (0.04 mol/L "0,).
observed. Sodium, potassium, silicon, sulfur, and chlorine did not cause any spectral interferences. Figure 4 shows spectra of Lake Mashu samples. A couple of Mashu samples and spiked samples were measured twice over an interval of 2 h in order to check the reproducibility of the method. The observed increase of background from deionized water is consistent with calculated interference due to calcium and magnesium. With the one point standard addition method, concentration of aluminum was determined to 2.1 ng/g for the sample. For acidified water (0.04 mol/L nitric acid), there was a slight increase in intensity around the aluminum peak position, while no increase was observed for deionized water (Millipore, Milli-Q). This increase would suggest the contamination of aluminum, which corresponds to 0.4 ng/g. It is probable that main contamination did not come from nitric acid itself but came from dissolution of aluminum from the wall of the flow line. There are two reasons for this; first, an increase in acid concentration was not followed by the calculated increase in contamination level-0.4 ng/g at 0.04 mol/L HN03 and 1.0-1.2 ng/g at 1 mol/L "0,; second, an initial contamination level of 160 ng/g in concentrated nitric acid (dilution factor = ca. 400) in unthinkable since acid was purified by the fused synthesized quartz subboiling distiller. Assuming the situations were almost the same for other acidified samples, aluminum concentration of the water sample from Lake Mashu was determined to be 1.7 ng/g. Other samples from Lake Mashu were analyzed in the same way and results are tabulated in Table IV together with the results obtained by the preconcentration method-samples were concentrated 1000-fold and analyzed by ICP-AES at 308.2 nm (10). In order to get more accurate/precise data, we are now examining alternative flow line components to minimize instrumental contamination. Excitation of t h e Ionic State. A partial energy level diagram of the aluminum atom and ion (8, 9) is shown in Figure 5. A line of 167.1 nm corresponds to the transition from the lowest allowed excited state (3p-lP) to the ground
0
1
2$76-177 2s
,
2p 3 P . T q 7 5
AI I
l p
%---3SF3D AI I1
Figure 5. Partial energy level diagram for atom and ion of aluminum.
state (3s-lS) of aluminum ion (resonance line). Lines of 172 nm and some of 176-177 nm correspond to transitions between the excited and the lowest triplet of aluminum ion. Lines of 185-186 nm, which are not shown in Figure 1,are transitions between the excited and lowest triplet of the ion, also. The ionization potential of argon (12.7 and 12.9 X lo6 m-l) is greater than the excitation energy of the 167.1-nm line from the atom ground state (10.8 X lo6m-l) but smaller than those of upper triplet states ((13.9-14.4) X lo6 m-l); therefore, through ionlatom reaction argon ion could excite the ground state of the aluminum atom to ionic state 3p-lP but not to upper triplets. There must be other excitation process(es) for these triplets. Assuming typical data range for ICP, e.g., plasma temperature 6000-7000 K and electron number density (1-2) x 1015cm-3 (I1,12),the degree of ionization is calculated to be 0.64-0.96 from Saha's equation. It seems that this high degree of ionization plays a great role in excitation of aluminum in ICP. $ome of the 176-177 nm lines are assigned to the transitions between atomic ground states and the so-called abnormal term ( ~ P ~ - 5.7 ~ P X, lo6 m-l) which have higher excitation energy than ionization potential (4.83 X IO6 m-l for aluminum, 207.3 nm). Atomic lines from another abnormal term have been observed in CUV (193.2 and 193.6 nm, 3p2-%3, 5.2 x IO6 m-')-the 193.6-nm line is one of the main causes of spectral interference of aluminum on arsenic determination at 193.7 nm. Emission lines of the same nature in W V reveal the high A1 population in the higher excitation states in the plasma. Registry No. AI, 7429-90-5;water, 7732-18-5. LITERATURE CITED (1) Heine, D. R.; Babls, J. S.; Denton, M. B. A@. Spectrosc. 1980, 3 4 , 595-598. (2) Miles, D. L.; Cook, J. M. Anal. Chim. Acta 1982, 141, 207-212. (3) Mayakawa, T.; Kikui, F.; Ikeda, S . Spectrochim. Acta, Pari 8 1982, 378, 1069-1073. (4) Winae. R. K.: Peterson. V. J.: Fassel, V. A. ADD/. 1979, . . Smctrosc. . 33,-206-219. (5) Boumans, P. W. J. M. Spectrochim. Acta, Part 8 1981, 368, 169-203.
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Anal. Chem. 1904, 56,2024-2028
(6) Kostyniak, P. J. J. Anal. Toxicol. 1983, 7 , 20-23. (7) Aiiain, P.; Mauras. Y. Anal. Chem. 1979, 51, 2089-2091. (8) Reader, J., Coriiss, C., Eds. "CRC Handbook of Chemistry and Physics", 59th ed.; CRC Press: Boca Ratan, FL, 1978; Line Spectra of the Elements, pp E-216-E-348. (9) Moore, C. E. "Atomic Energy Levels" Natl. Stand. Ref. Data Ser. ( U . S . , NaN. Bur. Stand.) 1971, NSRDS-NBS 35, 124-143. (10) Nojiri, Y., National Institute for Environmental Studies, Tsukuba, Ibaraki, Japan, unpublished work, Nov-Dec 1982.
(11) Aider, J. F.; Bombeika, R. M.; Kirkbright, G. F. Spectrochim. Acta, Part B 1980, 358, 163-175. (12) Nojiri, Y.; Tanabe, K.; Uchida, H.; Haraguchi, H.; Fuwa, K.; Winefordner. J. D. Spectrochim. Acta, Part 8 1983, 388, 61-74.
for review December 2, l98.3. Resubmitted April 30, 1984. Accepted May 1, 1984.
Emission Spectroscopic Studies of Sputtering in a Low-Power Glow Discharge Kazuaki Wagatsuma and Kichinosuke Hirokawa*
The Research Institute for Iron, Steel, and Other Metals, Tohoku University, Sendai, Japan
The Ion sputtering In a glow discharge plasma Is lnvestlgated by an emlsslon spectroscopic method. Prlmary Ions (Ar' ions In our work), which colilde wlth the cathode In a discharge tube, emit their characterlstlc radlation. The intenslties of Ar I I emhlon lines, whlch are asslgned to the optical transltions related to varlous exclted levels, reflect the relatlve populations among dlfferent energy states and, therefore, the energy dlstrlbution among the projectlle Ions. I t is useful to study how the emlsslon lntensitles of different kinds of Ar I I lines depend on Input power In order to compare each plasma condltlon created for some cathode materlais (Ag, Cu, NI, Co, and Fe). By use of the Ar+ ion line analysis, ll was found that when a glow lamp was operated at the same power level, slmllar sputterlng condltlons occurred for these elements. The emission lntensitles of some llnes of sputtered elements were recorded as a function of Input power, and the sputterlng yleid ratios of one element to the others were estlmated from the intenslty ratios for two related emission Ilnes.
Sputtering phenomena have been widely investigated for various reasons, for example, the application for producing thin films, wall attacks by high energy particles in a nuclear reactor or discharge tube, and analytical method for surface analysis. Different light sources are employed in optical emission spectroscopy. While the sample introduction in the usual light source such as an arc or spark discharge results from melting or vaporization, this process is based upon cathode sputtering in a glow discharge source. The glow discharge lamp suggested by Grimm ( I , 2) has recently been used for the study of surfaces (3-6). The emission lines are sharp (7) and self-absorption is small compared to the other light sources (8) due to the discharge under reduced pressures. Knowledge on glow discharge sputtering can be obtained from the various spectral lines emitted by the glow discharge plasma. In the case of a high power glow discharge, as employed in the earlier works (9, IO), the sputtering rate is rather high and enough intensity of the emission lines can be observed due to the high ion density of projectiles. However, the resolving power in depth is insufficient and, because much deposit is heaped around the anode, it is difficult to control constant plasma conditions for a long time. On the other hand, mild sputtering conditions, realized in a low-power discharge, are suitable for the study of surfaces.
We have reported the results on several alloy systems using this low power glow discharge spectrometry (11, 12). The variation of emission intensities, which probably depends on the sputtering yields of constituent elements in an alloy, has been observed especially in very low wattage regions. Furthermore, the emission intensities give quantitative information on ejected atoms in the plasma. Emission lines of Ar+ ions (Ar 11) provide available knowledge on the populated argon ion states. In this paper, the power dependence of the emission intensities was observed and the sputtering conditions in the glow discharge could be deduced from the analysis of Ar I1 lines. Emission intensities from target materials were also monitored as a function of input power. Sputtering yield ratios were estimated from intensity ratio of some spectral lines pairs for Ag, Cu, Ni, Co, and Fe and compared with the publised values in earlier works. EXPERIMENTAL SECTION The equipment for our measurements was described elsewhere (12). Our glow discharge lamp was made according to the original model reported by Grimm (1). The inner diameter of the hollow anode was 8.0 mm and the distance between the anode and cathode was adjusted to be 0.4-0.6 mm and kept constant for each measurement. The lamp was evacuated to 4.0-1.3 Pa and then argon gas was introduced (99.9995%,purity) until a pressure of 9.3 X lo2 Pa was reached. Plates of pure Ag, Cu, Ni, Co, and Fe, with the purity of 98-99.99%, and their alloys were used as the cathode (target material). The surfaces were mechanically polished with water-proof emery papers (no. 600 to no. 1500) and then finished to mirror faces with emery cloth. THEORETICAL SECTION As already discussed elsewhere (12), the intensity ratio ( I a / I b ) of resonance emission lines for two different elements a and b is approximately given by
R = constant
(1)
where N,/Nb is the atomic ratio for two elements in the plasma if the following major assumptions are satisfied. The first is that the energy level of one excited state should be nearly equal to that of the other state. If these two excited levels have the same energy levels, exp[(Eb - E,)/kT] (the Boltzmann distribution) becomes unity (where E is the energy level of an excited state and Tis the excitation temperature in the plasma); that is, the measured intensity ratio would be
0003-2700/84/0356-2024$01.50/00 1984 American Chemical Society
