Anal. Chem. 1989, 67, 1103-1108
Aggarwal, S. K.; Duggal, R. K.; Rao, R.; Jain, H. C. Int. J . Mass Spectrom. Ion Processes 1988, 7 7 , 221-231. DeBievre, P.; Gallet, M.; Holden, M. E.; Barnes, I. L. J . Phys. Chem. Ref. Data 1984, 13, 809-891.
(7) Sunderman, F. W., Jr. I n Handbook on Toxlclty of Inorganic Compounds; Seiler, H. G., Sigel, H., Eds.; Marcel Dekker:
New York,
1988;p 460. (8) Brown, S.S.;Nomoto. S.;Stoeppler, M.; Sundermann, F. W., Jr. Pure Appl. Chem. 1981, 53, 773-781. (9) Anderson, I.; Torjussen, W.: Zachariasen, H. Clin. Chem. 1978. 24,
1198-1202. (10) Versieck, J. Crlt. Rev. Clln. Lab. Sci. 1985, 22, 97-184. (11) Flora, C. J.; Nieboer, E. Anal. Chem. 1980, 52, 1013-1019. (12) Kinter, M.; Aggarwal, S. K.; Wills, M. R.; Savory, J.; Herold, D. A. Abstr. No. 744,40th National AACC Meeting, July 24-28, 1988,New Orleans; Clin. Chem. 1988, 34, 1305. (13) Schaller, H.; Neeb, R . Fresenius 2. Anal. Chem. 1986, 323, 473-476. . . - .. . . (14) Veillon, C.; Wolf, W. R.; Guthrie, B. E. Anal. Chem. 1979, 57, 1022- 1024. (15) Hachey. 6. L.; Biais, J. C.; Klein, P. D. Anal. Chem. 1980, 52, 1131-1 135. (16) Buckley, W. T.; Huckin, S. N.; Budac, J. J.; Elgendorf, G. K. Anal. Chem. 1982, 5 4 , 504-510. (17) Reamer, D. C.; Veillon, C. Anal. Chem. 1981, 53, 2166-2169. (18) Siu, K. W. M.; Fraser, M. E.; Berman, S. R. J . Chromatogr. 1983, 256, 455-459. (19) Sucre, L.;Jennings, W. Anal. Lett. 1980, 13,497-501.
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RECEIVED for review October 17, 1988. Accepted February 21,1989. Funding for the purchase of the high-resolution mass spectrometer was obtained from the National Institute of Health, Division of Research Resources Shared Instrumentation Grant Program, Grant No. 1-S10-RRO-2418-01. Additional funding from the John Lee Pratt Fund of the University of Virginia and Grant ESO 4464 of the National Institute of Environmental Health Sciences is also gratefully acknowledged. S.K.A. thanks the Division of Experimental Pathology, Department of Pathology, University of Virginia Health Sciences Center, for a postdoctoral fellowship and the authorities at Bhabha Atomic Research Center, Trombay, Bombay-400 085, India, for granting leave.
Assessment of the Relative Role of Penning Ionization in Low-Pressure Glow Discharges Rebecca L. Smith, David Serxner, and Kenneth R. Hess* Department of Chemistry, Franklin and Marshall College, Lancaster, Pennsylvania 17604
Optical investigations into the relative role of Penning ionization as a mechanism for the ionization of sputtered species in low-pressure argon glow discharges were carried out. These studies employed methane as a quenching agent to effectively reduce the argon metastable population. This reduction of metastable atoms significantly limits the extent of Penning ionization present in the discharge, which wlll decrease ion emission signals from the sputtered species. The magnitude of the decrease in these ion emission signals may then be related to the relative importance of the Penning Ionization process In overall discharge ionization of the sputtered species. These studies show Penning ionization to account for approximately 40-80% of the ionlzation of sputtered species, depending upon dlscharge conditions of current and pressure.
impact, where there is a kinetic energy transfer from an energetic electron to the atom resulting in ionization, and Penning ionization in which ionization results from a transfer of potential energy from a metastable state of the discharge gas. electron impact:
-
Mo + ePenning ionization:
Mo + Ar*
M+ + 2e-
M+ + ArO + e-
For argon, these metastable levels are the 3P2at 11.55 eV and the 3P0at 11.72 eV. If the atom or molecule of interest has an ionization potential lower than these metastable levels, then ionization may occur with generally uniform cross sections (19, 20).
INTRODUCTION Glow discharge devices (1) are finding broad analytical use as sources of atoms and ions for atomic absorption (2-4), atomic emission (5-7), atomic fluorescence (8-10, resonance ionization (12), and mass spectrometry (13-16). With continuing research and development into quadrupole based instruments for glow discharge mass spectrometry, along with the installation of a commercial magnetic sector system (17) in an estimated 20 user laboratories (18),applications of glow discharge mass spectrometry appear to be on the verge of a significant expansion. In order to develop adequately the technique of glow discharge mass spectrometry, information on the fundamental processes that result in the formation of sampled ions is of interest. The two major mechanisms of ionization believed to be present in low-pressure glow discharge devices are electron
A quantitative assessment of the relative roles of Penning ionization vs electron impact ionization has not been demonstrated, although Penning ionization is believed to play an important role in the ionization of discharge species for a variety of reasons. First, mass spectra from discharge sources similar to that used in this work exhibit ion signals from the sputtered cathode material generally larger than ion signals from the discharge gas, even though the discharge gas is present in a much greater concentration. This would imply preferential ionization of the sputtered species, possibly through Penning ionization, which will ionize the sputtered species but not the discharge gas. In addition, relative sensitivity coefficients for ion signals of the sputtered species are generally within a factor of 3-5 in glow discharge sources. Sputter yields are known to vary by this amount (21, 22), indicating a uniform method of ionization for the sputtered species. Laser ablation mass spectra from redeposited sputtered material have shown elemental ion ratios for copper
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and zinc similar to those observed from the glow discharge (23, 24), which also approximate the fundamental sputter yields (21),providing a further indication that differences in sputter yields are responsible for the relative differences in elemental ion signals. The most conclusive evidence for Penning ionization arises from previous studies involving laser depopulation of the metastable levels (25). In these investigations, depopulation of the argon metastable states was observed to lower ion signals from species with ionization potentials less than the metastable energy of 11.72 eV, while ion signals from species with ionization potentials higher than these levels were not affected. When the discharge gas was switched to neon, all species with ionization potentials less than the neon metastable level at 16.71 eV exhibited decreased ion signals. The dependence of the signal reduction caused by laser depopulation of metastable states on ionization potential clearly indicates the existence of Penning ionization in the discharge. Another indication of the role of Penning ionization in the glow discharge source can be obtained by varying discharge conditions and monitoring atomic population profiles spectroscopically while simultaneously measuring ion signals mass spectrometrically (26, 27). If Penning ionization plays a substantial role in the ionization of sputtered discharge species, then the ion signal generated by the source should be proportional to the product of the sputtered atom population and the metastable population. Previous studies have shown this proportionality to exist (23),without quantifying the role of the metastable atoms in discharge ionization. If the metastable atoms are significantly reduced in the discharge region by some method, then this proportionality should change and this change will reflect the relative importance of the Penning ionization process in overall discharge ionization. We have designed and carried out optical experiments to compare the relationships that exist between sputtered ion emission signals and the product of the sputtered atom population and metastable atom population in a normal discharge to those which exist in a "quenched" discharge, employing methane as the quenching agent.
EXPERIMENTAL SECTION Ion population profiles by mass spectrometry were obtained at the University of Virginia by using a quadrupole system previously described (28,29). For both the mass spectrometric and optical work, the discharge source consisted of a 2.75-in. six-way cross fitted with quartz windows. A 1.5 mm diameter brass pin with 5 mm exposed to the discharge served as the cathode and was inserted into the discharge with a probe and inlet system described elsewhere (30). The discharge voltage was supplied by a Kepco OPS-3500 power supply. Discharge pressure ranged from 0.4 t o 1.2 Torr, discharge currents ranged from 1 to 5 mA, and discharge voltages ranged from 500 t o 3500 V. The optical system employed for these experiments is diagrammed in Figure 1. The wavelengths were selected by a Instruments SA 0.32-m monochromator fitted with 55.4 optics and a microprocessor-controlled scan drive unit. Two gratings were available, a 2400 grooves/mm for wavelengths 170-500 nm and a 1800 grooves/mm for wavelengths 450-850 nm. Entrance and exit slits were adjustable and normally set at 20 km. A Hamamatsu 752 photomultiplier tube (PMT) with coverage from 160 to 900 nm served as the detector. The high voltage to the PMT was supplied by a Pacific Model 124 photometer, which also served as the readout device for atomic emission studies. For atomic absorption measurements, a Heath pulse generator supplied a square wave of 50% duty cycle to the OPS-BOOB power supply which in turn pulsed a commercial argon-filled copper hollow cathode lamp at 120 Hz. The hollow cathode beam, 5 mm in diameter, traversed the negative glow region of the discharge. A portion of the output from the pulse generator was converted into TTL logic and used as the reference input to a EGG PAR Model 5209 lock-in amplifier, which served as the readout device for the atomic absorption studies.
A * c n i c A b s o r p t 3~
Figure 1. Diagrammatic representation of the instrumental system employed for optical measurements of glow discharge species.
RESULTS AND DISCUSSION ArgonIMethane Mixture. Recent studies have indicated that the introduction of organic gases into inductively coupled plasmas may increase the observed analyte emission. These increases were attributed to a mechanism whereby carbon from the organic gas reduced metal oxides, freeing metal atoms (31). Further studies on the addition of propane to a direct current plasma showed the analyte emission to decrease in the region normally viewed for analytical work (32). The argon metastable population was also monitored in this region and observed to decrease upon the addition of propane, indicating quenching of the metastable atoms by the propane. Metastable argon atoms have also been quenched by halogens in proportional counters (33)and by trace amounts of methane in studies of ion-electron recombination in afterglow plasmas (34). The metastable levels are quenched through efficient energy transfer from the metastable atom to the organic molecule, resulting in various excitation and dissociation reactions, which are discussed in more detail (35)and tabulated elsewhere (36,37). The effects of methane on the glow discharge are of interest as well, possibly providing a method for the reduction of the argon metastables and the Pencing ionization process while retaining discharge sputtering and electron impact ionization. Initial investigations probed the effects of methane on an argon glow discharge to determine the extent of metastable quenching and the impact of methane on discharge sputtering. The argon 3P2metastable population was monitored by atomic absorption (A = 811.5 nm) as the methane/argon mixture was varied by changing the partial pressures of argon and a 10% mix of methane in argon. As methane is added to the discharge, the metastable level is quickly quenched, illustrated in Figure 2. A discharge operating on the 10% methane mixture showed a metastable absorption (*) near the level of detection, indicating a reduction in the metastable population by more than a factor of 100. The 3P0argon metastable level exhibited the same behavior. The copper atomic absorption signal (+) (A = 327.4 nm), also illustrated in Figure 2, shows the ground-state copper population initially remains fairly constant and then decreases about 17% with a 10% methane in argon mixture. This may be explained through competing processes occurring in the discharge. As methane is added to the discharge, the voltage
ANALYTICAL CHEMISTRY, VOL. 61, NO. 10, MAY 15, 1989
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increases slightly (5-lo%), which results in increased sputtering by the argon. In addition, the carbon may be reducing copper oxides, increasing the free metal population. As more methane is added, more and more of the discharge current will be carried by highly mobile H+ ions, methane ions, and other fragment ions originating from the methane. As more of the discharge current is carried by the H+ and the molecular fragment ions, less sputtering will occur due to the very poor sputtering capabilities of the molecular ions and low mass H+. This will result in lower sputtered atom populations. In any event, with the addition of 10% methane, the atomic population decreases only slightly, and the discharge remains in operation, continuing to create a substantial sputtered atomic population. In an effort to determine the extent to which methane lowers electronic discharge excitation, excitation temperatures were monitored by using the two-line method with zinc emission lines at 307.6 and 328.2 nm as the thermometric species (38). The results, illustrated in Figure 3, show the T,, value vs pressure for the methane/argon mixture (+) to be nearly the same as when solely using argon (0). The discharge appears to continue to sputter atomize the cathode material and retain constant excitation temperature regardless of the discharge gas. In addition, by definition of constant current discharge operation, the number of electrons through the discharge will remain essentially constant with either discharge gas mixture. Only changes in the discharge current should impact the electron densities. This impact is linear and was experimentally verified by monitoring the width of a H, line a t 486.1 nm as discharge current was increased. The width of the line may be related to electron densities (39),the results of which did show a linear increase in electron density with increasing discharge current, with the same values for each discharge gas. Since there is no substantial change in electron excitation temperature or electron density between the two discharge gases, any impact on the ion signals with the changing of discharge gases would appear to reflect changes in the discharge arising from factors other than effects involving electrons. The impact on the copper ion emission ( 0 )(A = 627.3 nm) signal as methane is added to the discharge is shown in Figure 2. The ion emission signal is observed to decrease with the initial addition of methane, following the decrease in argon metastable population. This indicates the metastables do play
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Figure 3. Discharge excitation temperature vs pressure for a pure argon discharge (0)and a 10% methane in argon discharge (+): brass cathode, 3 mA.
a role in discharge ionization and their removal will decrease the ion emission signals. From such a plot the percentage Penning ionization may be calculated for those particular discharge conditions. After it was determined that methane does quench the argon metastables with only minor effects on the sputtering process, experiments were carried out to investigate the proportionalities that exist between the atomic populations and the ion emission signals with changing discharge conditions. These experiments allow the copper ion emission population profiles to be compared to previously determined ion population profiles (23),ensuring that the ion emission data accurately reflect ion populations as determined by mass spectrometry. The proportionalities that exist for a pure argon discharge may then be compared to those that exist in a methane/argon mixture, further quantifying the role of Penning ionization under various discharge conditions. Variation of Current. The density of copper atoms in the ground state was monitored by atomic absorption (+) (A = 327.4 nm) as discharge current was varied. The absorbance value was observed to increase with increasing current, Figure 4A. This increase in copper atoms may be attributed to two factors. First, as the current is increased, the number of argon ions bombarding the cathode increases linearly, increasing the observed sputtering. In addition, as the current increases, the discharge voltage also increases. Due to inelastic collisions in the plasma, the impacting argon ions generally have kinetic energies corresponding to approximately 20% of the full discharge voltage (40). At 0.8 Torr and 1-mA current, the discharge voltage was 630 V,increasing to 1470 V at 4 mA. A t these discharge voltages, the sputtering ions will impact with kinetic energies of approximately 125 eV at 1 mA and 300 eV at 4 mA. Sputter yield vs impacting ion energy plots are available for copper bombarded with argon ions ( I ) , which allows one to approximate the sputter yield at different discharge voltages. By use of these values, an estimate of the relative sputtering increases that occur due to the higher impacting energies of the argon ions may be obtained. This relative sputtering enhancement due to dischsrge voltage increases may then be multiplied by the increased number of sputtering ions as current rises, providing a value for the anticipated net sputtering increase due to increasing current. are also plotted in Figure 4A and show a close These data (0) correlation to the experimentally obtained increases in copper atom population. These two factors, increased sputter yield due to increased discharge voltage and increased number of sputtering ions, appear to account for the observed increases
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ANALYTICAL CHEMISTRY, VOL. 61,NO. 10, MAY 15, 1989 123
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Flgure 4. (A) Effect of discharge current on copper atomic absorption (+) (327.4nm), argon metastable absorption ( * ) (811.5 nm), and a calculated value of the anticipated increase in copper atom population (0)(see text for details): brass cathode, argon discharge, 0.8 Torr. (8)Effect of discharge current on copper ion emission (+) (627.3nm) and copper mass spectrometric signal ( * ) (from ref 23) in an argon
discharge, along with the effect of discharge current on copper ion emission (0)(627.3nm) with a 10% methane in argon discharge: brass cathode. 0.8 Torr. in copper atomic population as the current is increased. The argon metastable population (*) is also observed to increase with increasing current, as shown in Figure 4A for absorption of the 811.5-nm transition of the 3P2argon level. Delcroix et al. have developed an expression for the population of argon metastables in a low current discharge (41)
where ne is the electron number density, no is the concentration of the ground-state atoms, C, is the overall metastable production rate coefficient, and V, is the loss frequency, which a t low currents and pressures is proportional to the diffusion coefficient since the major loss mechanism for metastable atoms is collision with the discharge chamber walls. According to this expression, a t constant pressure, the metastable population should be a linear function of ne. As previously discussed, current increases will linearly increase the number of electrons. As the number of electrons increases, the number of exciting collisions also increases, resulting in more electron impact excitation to the metastable level. Delcroix and others (42-46) have presented experimental results for various discharges which showed the linear increases in metastable
densities as discharge current was increased. In addition to the atomic signals, copper ion signals were monitored both through emission of the Cu(I1) line a t 627.4 nm (+) and, previously, by mass spectrometry (*) (23) as current was increased. The results are presented in Figure 4B. The relative plots for the mass spectrometric data match those obtained by optical emission, confirming the emission signal does reflect the signals obtained by mass spectrometry. The observed increases in ion signal correspond to increases in electron densities (enhanced electron impact), copper ground state densities, and metastable densities. Since all parameters increase with increasing current, no direct evidence of Penning ionization is available from the studies of discharge current. employing only argon as the discharge gas. When the 10% methane in argon mixture is employed as the discharge gas, a lower copper ion emission signal is obas shown in Figure served for the methane/argon mixture (O), 4B. This decrease illustrates the role of metastable atoms on observed ion signals. Taking into account the reduced copper atomic population due to the reduction in sputtering when employing methane, the relative percentage of Penning ionization can be calculated. The relative importance of Penning ionization is observed to decrease linearly with increasing current, falling from 78% at 1mA to 42% at 3.5 mA. Previous investigations of discharge processes employing optogalvanic spectroscopy and mass spectrometry have also noted the decreasing role of Penning ionization as current is increased. Smyth et al. (47) investigated the role of Penning ionization in a neon-filled copper hollow cathode lamp and found both the optogalvanic signals from the neon metastable transition at 594.48 nm and the effect of laser metastable depopulation on Cu+ ion signals to fall with increasing discharge current. These results illustrate the reduced role of Penning ionization at higher discharge currents. Ben-Amar and co-workers have also shown the optogalvanic signal arising from metastable depopulation in a neon hollow cathode lamp to decrease with increasing discharge currents (48). The effect of discharge current on the relative role of Penning ionization in the methane/argon discharge gas mixture appears to confirm the results obtained with laser depopulation and optogalvanic effect spectroscopy. Variation in Pressure. Discharge pressure is known to play a substantial role on the various competing collisional excitation and deexcitation processes that create metastables and sputtered atoms in the discharge. Changes in discharge pressure would be expected to affect the collision environment of the discharge and the equilibrium atomic populations. The effect of discharge pressure on the copper ground state popis illustrated ulation (+) and argon metastable population (0) in Figure 5A. As pressure increases, the discharge voltage decreases, resulting in lower energy bombarding argon ions and lower sputter yields. In addition, as pressure increases the amount of redeposition of the sputtered material also increases. It has been estimated that up to 90% of the sputtered material is redeposited at 1.0 Torr ( 4 9 , 50), with more diffusion and less redeposition a t lower pressures. Any increase in pressure will result in lower copper atomic population through both reduced sputtering and increased redeposition. The population of argon metastables vs pressure may again be analyzed through the Delcroix expression presented earlier. Figure 5A shows the metastable population ( 0 )to peak a t approximately 0.7 Torr. As pressure is decreased from this peak, the number of ground-state atoms will decrease linearly and, with a constant electron density (constant current), the number of metastables should fall linearly. Additionally, the diffusion coefficient will increase with lower pressures, increasing the diffusion loss of metastables undergoing collisions
ANALYTICAL CHEMISTRY, VOL. 61, NO. 10,MAY 15, 1989 120
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signal and the argon metastable absorption signal (0):brass cathode, argon discharge, 3 mA. with the chamber walls. The excitation temperature will also increase with lower pressures (higher discharge voltages and lower collision rates), which will result in greater losses of the metastable state through electron impact ionization. This is evidenced by the observed increase in argon ion signal with decreasing pressure (23). As discharge pressure is increased from 0.7 Torr, the metastable density would be expected to rise with the increasing ground-state argon population. A point will be reached, however, where the destructive collision rates of the metastables, such as Penning ionization, will exceed the creation rate and the metastable population will then fall with increasing pressures. Kagan and Gofmeister (45, 46) have found peak metastable densities at 1.0 Torr for neon and argon hollow cathode discharges, falling a t either higher or lower pressures. Other workers (43, 5 1 ) have also observed peak metastable densities in the 0.5-2 Torr range for a variety of discharge configurations, illustrating the competing metastable creation and destruction mechanisms present in the discharge. Plots of the copper ion signal by both ion emission (*) ( A = 627.3 nm) and mass spectrometric monitoring (+) are provided in Figure 5B, again illustrating that ion emission measurements will accurately reflect ion signals observed mass spectrometrically. A comparison of the ion plots with the product of the argon metastable populations and the copper also plotted in Figure ground state populations vs pressure (O),
5B, illustrates the role of the metastable atoms in discharge ionization. The sputtered ion signal is clearly related to the argon metastable population. However, the degree to which Penning ionization is important in glow discharge ionization cannot be ascertained from these plots. If the copper ion emission is monitored vs pressure for the methane/argon mixture, the results are significantly different than those observed in a pure argon discharge. Copper ion emission vs pressure is plotted for both cases in Figure 6. In the case of pure argon, the Cu' emission signal (+) reaches a peak at approximately 0.8 Torr and follows the product of the Cuo.Ar* (where Ar* represents the argon metastable) populations as previously discussed. With the methane/argon mixture, the Cu' signal (0)falls linearly with increasing pressure. When the Cu+ vs pressure is plotted as a relative signal, the slope of a linear least-squares fit through the points is 0.66. The slope of the least-squares fit of the Cuopopulation vs pressure, Figure 5A, is 0.65. It appears the Cu+ emission signal with the methane/argon mixture is dependent solely upon the copper ground state, having removed the metastable dependence by quenching the metastables with methane. With electron impact ionization remaining fairly constant (constant current and nearly constant excitation temperature), this result would be expected. The difference in ion signals for the two gases results from a contribution of the removal of the Penning ionization process and from a lower sputter yield with methane. By correction of the differences in Cu+ signals for the differences in sputter yield between the two gases, a relative measure of the degree of Penning ionization may be obtained. Figure 7 illustrates the calculated percent of Penning ionization vs pressure for this discharge configuration. Extrapolation of this plot shows that at approximately 0.3 Torr the percentage of Penning ionization should fall to zero. According to Figure 6, at about 0.3 Torr the Cu+ profiles for argon and the methane/argon mixture intersect one another and the contribution of Penning ionization to overall discharge ionization of the sputtered species should be zero. Looking back to Figure 5A, which shows the Ar* population with varying pressure, at approximately 0.3 Torr the Ar* population would be zero and therefore all Penning ionization should be removed. If the calculated role of Penning ionization was in error for some reason, such as an incorrect value for the de-
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creased sputter yield with the methane mixture, then the plot of Penning ionization vs pressure (Figure 7) would be shifted from its current position and would not intersect zero at 0.3 Torr. In this case the experimental results would not collaborate one another. The close correlation of experimental results that is observed clearly substantiates the calculated role of Penning ionization in the discharge. These studies show Penning ionization to account for approximately 4 0 4 0 % of the ionization of sputtered species observed in the discharge, depending upon discharge current and pressure. Extension of these experiments to other discharge gas mixtures and sputtered materials would be of interest, as would mass spectrometric and optogalvanic studies of the observed effects.
ACKNOWLEDGMENT Support of this research by a Whitaker Foundation Grant of Research Corporation, by a Merck Undergraduate Science Research Grant, by the Hackman Scholars Program of Franklin and Marshall College, and by the Committee on Grants of Franklin and Marshall College, is gratefully acknowledged.
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RECEIVED for review November 15, 1988. Accepted March 1, 1989.
