Time-resolved studies of ionized sputtered atoms in pulsed radio

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Anal. Chem. 1993, 65,3107-3193

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Time-Resolved Studies of Ionized Sputtered Atoms in Pulsed Radio Frequency Powered Glow Discharge Mass Spectrometry Changkang Pan and F.L. King' Department of Chemistry, West Virginia University, Morgantown, West Virginia 26506

The temporal evolution of the ionized sputtered atom population in a pulsed radio frequency (rf) powered argon glow discharge is investigated usingmass spectrometry. Ionized sputtered atoms are found to exhibit temporal signal profiles that differ from the temporal signal profiles of ionized discharge gas species. The signals for ionized sputtered atoms are observed to maximize approximately 1.6 ms after the termination of the discharge power. The temporal signal profiles of ionized sputtered atoms are influenced by the discharge working parameters, Le., discharge power, duty cycle, support gas composition, and sampling distance. In addition, the ion kinetic energy distributions depend on the time regime during which the ions were formed. Ionized sputtered atoms formed during the afterpeak time regime exhibit a higher average kinetic energy and narrower kinetic energy distribution than those ions formed during the plateau time regime. These results indicate that the Penning process dominates analyte ionization during the afterpeak time regime of the pulsed rf-powered glow discharge plasma. The time-dependent nature of the ion signals permits the use of time-gated detection to provide discrimination against certain spectral background species.

INTRODUCTION Glow dischargemass spectrometry (GDMS) is regarded as a powerful analytical technique for the direct determination of trace elementa in solid materials.' Most GDMS systems operate with continuousdirect current (dc) or radio frequency (rf) power to yield a steady-state glow discharge plasma. However, operation in the steady-state mode at high powers can lead to cathode sample overheating and plasma instability. The application of discharge power in low frequency pulses minimizes these limitations and concurrently introduces a means of improving analytical performance through timegated data acquisition.MJ1 In the pulsed operating mode,

* Author to whom correspondence should be addreseed.

(1)Harrison, W. W.; Hem, K. R.; Marcus, R. K.; King, F. L. Anal. Chem. 1986,68,341A-356A. (2) Klingler, J. A.; Savickas, P.J.; Harrison, W. W. J.Am. SOC. Mass Spectrom. 1990,1, 138-143. (3) Klingler, J. A.; Barshick, C. M.;Harrison, W. W. Anal. Chem. 1991, 63,2671-2676. 0003-2700/93/03653187$04.00/0

the glow discharge power cycles, providing a plasma-off time duringwhich the samplecathodecan cool. This cooling period permits the application of higher instantaneous powers to provide enhanced absolute ion signal intensities without sample overheating. Interestingly, the ionization processes in the discharge vary in significance throughout each on-off cycle of the plasma. This variation permits temporal separation of analyte signals from background signals and discrimination against certain spectral interferences.2J Investigators in Harrison's laboratory reported on the examination of a pulsed dc-powered glow discharge using mass spectrometry.23 Those investigations demonstrated the existence of differences in temporal signal profiles between ionized sputtered atoms and ionized discharge gas species. Soon after the termination of applied power, the argon ion signal decayed; whereas, the ionized sputtered atom signal increased sharply to a maximum, termed "the afterpeak". In contrast, the argon ion signal exhibitedan intensity maximum, termed "the prepeak", at the beginning of the applied power pulse when there was no ionized sputtered atom signal. These differences in behavior indicated that different ionization mechanismsdominated different time regimes in these pulsed power plasmas. In this laboratory, pulsed operation of dc-powered glow discharges has been employed in atomic absorption, atomic emission, and mass spectrometries.4~6 We found further evidence indicating that the formation of the afterpeak resulted from Penning ionization facilitated by the production of metastable argon atoms in the recombinationof argon ions with thermalized electrons during discharge decay. Realtime measurements of the metastable argon atom population obtainedfrom time-reaolved glow discharge atomicabsorption spectrometry showed that metastable argon atoms exhibited a similar afterpeak temporally coincident with the sputtered ion after peaks4The various operatingparametersthat control the plasma strongly influenced the afterpeak signal for both metastable argon atoms and ionized sputtered atoms. Radio frequency powered glow dischargesare employed as atomizationlexcitationlionizationsources for the direct determination of trace elements in both conductive and nonconductivesolid samp1es.B-11 In a rf-powered glow discharge, (4) King, F. L.; Pan,C. Anal. Chem. 1993,66, 735-739. (5)Pan,C.; King, F.L. J. Am. SOC. Mass Spectrom. 1993,4,727-732. (6) Eckatein, E. W.; Coburn, J. W.; Kay, E. Znt. J.Mass Spectrom. Ion Phys. 1975,17, 129-138. (7) Winchester, M. R.; Lazik, C.; Marcue, R. K. Spectrochim. Acta 1991,46B, 483-499. (8) Duckworth, D. C.; Marcus, R. K. Anal. Chem. 1989,61,187S1886. (9) Kliiger, J. A.; Harrison, W. W. Proceedings of the 39th ASMS Conference on Mass Spectrometry and Allied Topics; ASMS:Santa Fe, NM, 1990, pp 943-944. 0 1993 Amerkan Chemical Society

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Adjustable Bellows

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Figure 1. Experimental setup for pulsed radio frequency powered glow discharge mass spectrometry.

a dc bias voltage is established on the sample surface as a result of the difference in mobilities between electrons and ions in the oscillating electric field. As a result of this bias, direct sputter atomization of the nonconducting sample surface, followed by excitation and ionization of the sputtered atoms, is achieved through normal plasma operation. In a comparison of mass spectrometric signals from pulsed dcand rf-poweredglow discharges,Klingler and Harrison found that the pulsed rf-powered glow discharge consistently produced temporally broader afterpeaks than those produced by the pulsed dc-poweredglow discharge^.^ We reported that the use of a pulsed rf-powered glow discharge permitted the application of higher operating powers and the production of greater emission intensities than attainable from the use of nonpulsed rf-powered glow discharges.1° In a recent study of the temporal emission from a pulsed rf-powered glow discharge, Winchester and Marcus found that the emission intensity maximum appeared, depending on the particular transition involved, either near the time of power application or just after the time of power termination.ll The present study focuses on the temporal dependence of sputtered atom ionization in a pulsed rf-powered glow discharge operating at a frequency of 50 Hz with a duty cycle of 25 % . The temporal profiles of selected mass-to-charge ratio ions are examined along with the influence of various plasma operating parameters, including rf peak power, pulse duty cycle, plasma sampling distance, and discharge gas composition, upon these temporal profiles. Relative measurements of ion kinetic energy distributions are presented for the ions produced in the plateau time regime and in the afterpeak time regime. The use of a narrow data collection gate positioned during the afterpeak regime is examined as a means of improving analytical selectivity and sensitivity.

EXPERIMENTAL SECTION Pulsed Radio Frequency Powered Glow Discharge. The discharge sputtering chamber consists of a six-way,stainlesssteel, high vacuum cross (MDC, Hayward, CA) equipped with 70-mm conflat-typeflanges. Fused-silicawindows,gas inlet attachments, a pressure monitoringthermocouple (HastingsTeledyne-Raydist, Hampton, VA), ion exit orifice, and sample holder are mounted (10) Pan, C.; King, F. L. Appl. Spectrosc., in press. (11)Winchester, M. R.; Marcus,R.K.Anal. Chem. 1992,64,2067-78.

onto the ports of the cross. A NIST Standard Reference Material 1267 ingot is machined into sample cathode disks, 5 mm in diameter and 4.5 mm in height, and mounted in a cathode holder. The sample cathode holder is attached via a 5-mm-0.d. solid copper rod to a rf power vacuum feedthroughequipped with an HN-type connecter for rf power connection on the ambient pressure side. The copper rod is insulated from a ground shield, which limits sputter atomization to the sample face, by two concentric quartz tubes of 10 and 14 mm diameter,respectively. The sample holder assembly is mounted to the cross via an adjustable bellows assembly (MDC, Hayward, CA) to permit optimization of the sample cathode position and of the resultant analytical signal. The sample holder assembly is connected to the rf power by a l-m RG-213coaxially shielded cable. Operating power is obtained from the output of a 13.56-MHzrf power supply (Model RFlOS, RF Plasma Products, Inc., Marlton, NJ) that is coupled through an automatic matching network (Model AM10, RF Plasma Products, Inc., Marlton, NJ). In the pulsed operation mode, the high and low rf power levels, duty cycle, and pulse frequencyare adjusted through front panel controls without extinguishing the plasma. In these studies, the applied power waveform is a square wave having a frequency of 50 Hz and a duty cycle of 25 % , unless otherwise indicated. Reagent-grade argon is used as the discharge support gas for all studies unless otherwise indicated. Mass Spectrometer. A schematic representation of the pulsed rf-powered glow discharge mass spectrometer employed in this study is shown in Figure 1. A triple-quadrupole mass spectrometer (ELQ-400,Extrel, Pittsburgh, PA) is operated with the first and second quadrupoles serving as rf-only ion guides and with the third quadrupole serving as a scanning mass filter. The mass spectrometer is modified to facilitate glow discharge mass spectrometryby replacingthe conventionalEI/CI ion source with a Bessel-box energy analyzer (Extrel, Pittsburgh, PA) and installing a set of Einzel lenses to transfer ions from the glow discharge source to the Bessel-box entrance aperture. Data acquisition can be performed in three ways as shown in the figure. In one mode, the amplified signal can be sent directly to the computer data system,yielding an output signal that is averaged over the entire pulse period. Alternatively, the temporal profile for ion signal at a single mass-to-charge ratio can be acquired in real time by taking the output of the continuous dynode electron multiplier through a preamplifier (Combo-100, MIT, Boulder, CO) and into a 100-MHz bandpass digital oscilloscope (2232, Tektronix, Beaverton, OR). Time-gated acquisition of a mass spectral range can be accomplished using a boxcar integrator (4152,EG&GPAR, Princeton, NJ) collecting ion signals through a data gate. A fixed gate window of 0.15 ms with 30 sample

ANALYTICAL CHEMISTRY, VOL. 65, NO. 22, NOVEMBER 15, 1993 5 ms -:

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averaging is employed in this work. The data gate can be positioned in any portion of the pulse cycle by the use of a digital delay generator (4144,EG&G PAR, Princeton, NJ) triggered by a synchronization pulse from the power supply. The temporal position of the data gate is readily observed on the oscilloscope through the gate monitor output of the boxcar. The boxcar output is then sent to the mass spectrometer data system to provide the mass spectral signal.

RESULTS AND DISCUSSION Temporal Ion Signal Profiles. Modulation of the discharge power generates time-dependent ion signals. Typical ion signal profiles obtained from the pulsed rf-powered glow discharge ion source are shown in Figure 2. Ion signals from both sputtered atoms and support gas atoms exhibit unique reproducible temporal profiles that do not match the applied rf power step function. As could be seen in Figure 2A, the aAr+ signal exhibits a surge in intensity upon the application of rf power. Similar to the prepeak surge reported for ah+in pulsed dc powered glow discharges,'J*4 this prepeak arises from the electric breakdown of the argon support gas upon the application of discharge power. In contrast to the behavior of argon ions, the uCu+ signal, shown in Figure 2C, does not respond immediately to the initiation of rf power. There is a delay time of about 1 ms before the signal starts to increase gradually. This delay time arises because Cu atoms must first be sputtered off the sample surface by argon ion bombardment and then diffuse to the negative glow region where they are subject to collisional ionization. Comparison of Figure 2A and C demonstrates this temporal correlation between the appearance of argon ion signal and the appearance of copper ion signal. The Cu ion signal appears only after argon ions have been formed by the electrical breakdown of the argon gas. Following the application of discharge power, it takes about 2 ms for the Cu ion signal to reach a steadystate level. During this plateau time regime, the plasma is in a quasi-equilibrium condition that is characterized by constant atomization and ionization. After the termination of applied power, the sputtered ion signal displays a marked increase in intensity, followed by a

Flgure9. Temporal ion signalprofiles of %u+ at peak applled discharge powers from 75 to 115 W. Duty cycle 25%, pressure 0.6 Torr, and sampling distance 10 mm.

gradual decay to the baseline value. The first derivative of the temporal profile of aAr+is shown in Figure 2B. The second negative peak indicates the temporal position of the maximum decay rate for argon ions followingdischarge power termination. It is of particular interest to note that the position of the afterpeak for ionized sputtered atoms, Figure 2C, is temporally coincident with the maximum for argon ion decay, Figure 2B. This indicates that afterpeak formation correlates closely with the extinction of argon ions. Argon ion depopulation likely arises from the recombinationof argon ions with thermalized electrons. Upon termination of the applied power, the energetic electrons in the glow discharge quickly lose kinetic energy through collisions with other plasma species. The rapid collisional coolingof the electrons and the consequent increase in thermalized electron density favor the ion-electron recombination. As a result, recombination becomes the dominant electron removal process in the afterglow. Biondi reports that ion-electron recombination processes result in the formation of metastable argon atoms.13914 Metastable argon atom formation is the result of a radiative recombination process. Recombination yields excited argon atoms in the 5p-level excited state. These excited atoms rapidly decay to the 4 s-level through radiative relaxation yielding argon atoms trapped in the metastable 3P0 and 3Pz states, Ar(,)*.13J5 Biondi also reports an initial increase in electron density following the removal of the applied electric field. This increase in electron density is the result of an enhancement in ionization processes involving metastable argon atoms such as Penning ionization. Consideration of the temporal profiles of the ionized sputtered species suggests that the Penning process is responsible for most sputtered atom ionization during the afterpeak time regime. Investigations into the influence of experimental parameters on the temporal ion signal profile can provide additional information regarding anal* ion formation during the afterpeak time regime. Radio Frequency Peak Power Level Influence. Compared to the steady-state glow discharge, pulsed operation permits the application of high instantaneous powers to the sample in a short time. Temporal ion signal profiles for sputtered copper atoms obtained from plasmas operated at different peak rf power levels appear in Figure 3. At a peak applied power of 75 W, only a slight afterpeak contribution appears in the ion signal profile. As the power increases, the (12)Chapman, B. Glow Discharge Process; John Wiley & Sons: New York, 1980. (13)Biondi, M.A. Phys. Rev. 1963,129,1181-1188. (14)Biondi, M.A. Phys. Reu. 1952,88,660-665. (15)Strauss, J. A.;Ferreira, N. P.; Human, H. G.C. Spectrochim. Acta 1982,37B,947-955.

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Time Flgure 4. Influence of duty cycle and peak power on ion signal profiles of s3Cu+:(A) 25% duty cycle, peak power 100 W, and 5 ms pulse-on tlme; (B) 30% duty cycle, peak power 83 W, and 6 ms pulsesn time; (C) 35% duty cycle, peak power 71 W, and 7 ms pulsesn time; (D) 40 % duty cycle, peak power 63 W, and 8 ms pulse-on tlme; (E) 45 % duty cycle, peak power 56 W, and 9 ms pulse-on time. Pressure 0.6 Torr, sampling distance 10 mm, average power 20 W, and pulse frquency 50 Hz for A-E.

afterpeak portions of the ion signal profiles increase, whereas the plateau portions remain nearly constant. This situation results in a net increase in the afterpeak signal to plateau signal ratio at greater peak applied powers. Interestingly, the afterpeak broadens at the higher peak powers. Such broadening is expected if the rate of ion-electron recombination decreases with increasing electron kinetic energy. Studies of the total collision cross section as a function of energy indicate that recombination rates decrease as electron energies increase.12 A decrease in the rate of argon ionelectron recombination translates into a decrease in the rate of metastable argon atom production. Application of higher powers increases the average kinetic energies of argon ions and electrons in the glow discharge. The greater the electron kinetic energy, the longer it takes for the electrons to thermalize sufficiently to undergo recombination with Ar+ ions. Because the number of both argon ions and electrons increases with rf power, more species in the plasma are available for the recombination process. Consequently, the intensity and temporal width of the afterpeak increase with the application of greater rf powers. Duty Cycle Influence. The average power applied to the glow discharge depends on both applied peak power and duty cycle. Although a high peak power is applied, the average power over the whole pulse period may not be high if a low duty cycle is used. A comparison of the effects of peak power and average power on ion signal intensity is of particular interest because this is the major difference between pulsed and nonpulsed glow discharge plasmas. Shown in Figure 4 are 63Cu+ ion signal profiles that illustrate the influences of peak power and duty cycle. Average power and pulse frequency are kept constant at 20 W and 50 Hz, respectively. Peak powers and pulse widths are changed simultaneously to achieve the duty cycle of interest at constant average powers. It is found that the ion signal intensities in plateau and afterpeak time regimes decrease with increasing duty cycle, even though the average power is not changed and the

pulse-on-time is continually increased. This indicates that the average power is not the major factor controlling the atomization and ionization processes. Instead, high instantaneous power with a short duty cycle increases the atomization rate because the resulting energetic argon ions increase the cathodic sputtering yield. In addition, high peak powers increase the ionization capability of the plasma to yield greater ion signal intensities. During the time period when no sustaining power is applied to the plasma, there is no potential present to accelerate argon ions toward the cathode; thus, no sputter atomization can occur. The shorter duty cycle results in a longer time period between sputtering periods. This allows the sample cathode to cool off sufficiently before the next pulse period so that the deleterious effects due to excessive sample heating are voided. These factors provide an enhancement of ion signals on both plateau and afterpeak, as evident in the figure. For analytical purposes, a high instantaneous power and a short duty cycle are optimal. The effect of pulse frequency on the ion profile is examined by simultaneously changing the pulse width and pulse period under conditions of constant peak power and duty cycle. As the pulse frequency (or repetititon rate) varies from 12 to 250 Hz, the afterpeak and plateau exhibit different responses. The afterpeak is found to be independent of pulse frequency because the intensity and temporal position do not change over the tested frequencies. In contrast, the plateau region starts to disappear as the frequency increases beyond 100 Hz. The disappearance of the plateau indicates the absence of any steady-state conditions at high repetition rates. Influence of a Quenching Reagent. In previous investigations, low levels of methane were introduced into a steadystate dc-powered glow discharge to provide an assessment of the extent to which the Penning process contributed to sputtered atom ionization.20-22 Results from those studies demonstrated that Penning ionization accounts for 40-80 % of the total sputtered atom ionization occurring in the dcpowered glow discharge. Methane exhibits a large crosssection for the collisional relaxation (de-excitation) of metastable argon atoms via potential energy transfer resulting in chemi-excitation and chem-dissociation of the methane molec~les.~5~~3 If the formation of the afterpeak arises from Penning ionization, then the addition of methane, a metastable quenching reagent, should effectively eliminate the afterpeak. However, the influence on the plasma of other processes arising from the addition methane must also be considered . Because the ionization potential of methane, 12.70 eV, is lower than that of argon, 15.76 eV, methane addition can depopulate argon ions through charge exchange and quench electrons capable of ionizing argon. Both possibilities result in a reduction of the current carried by argon ions and in an increase in the current carried by methyl and methyl fragment ions. A larger fraction of discharge current carried by these hydrocarbon ions would reduce the rate of cathodic sputtering. (16) Massey, H. S. W.; Burhop, E. H. S.; Gilbody, H. B. Electron and Ionic Impact Phenomena, Oxford University Press: London and New York, 1969; Vol. 1. (17) Ferreira, N. P.;Strauss, J. A,;Human, H. G. C. Spectrochim. Acta 1982, 37E, 273-279. (18) McDaniel, E. W. Collision Phenomena in Ionized Gases; John Wiley and Sons: New York, 1964; Chapter 4. (19) Nasser, E. Fundamentals of Gaseous Ionization and Plasma Electronics; John Wiley and Sons: New York, 1971; Chapter 3. (20) Smith, R. L.; Serxner,D.; Hess, K. R. Anal. Chem. 1989,61,11031108. (21) Serxner, D.; Smith, R. L.; Hess, K. R. Appl. Spectrosc. 1991,45, 1656-1664. (22) Levy, M. K.; Serxner, D.; Angstadt, A. D.; Smith, R. L.; Hess, K. R. Spectrochim. Acta 1991,46E, 253-267. (23) Velazco, J. E.; Kolts, J. H.; Setser, D. W. J.Chem. Phys. 1978,69, 4357-4373.

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Time Temporal ion signal profiles of %u+ at various sampling distances. Pressure 0.6 Torr and peak power 90 W. Figure 6,

If the change of signal profiles was caused only by the decrease in the sputtering rate, the plateau and the afterpeak should exhibit the same magnitude of signal reduction upon methane addition. The effect of methane addition on the plateau and afterpeak signalsappears in the temporal signal profiies shown in Figure 5. Upon the addition of methane at the 5 5% level, the MCu+ion signal intensity in the afterpeak region virtually disappears, whereas the ion signal in the plateau region decreases slightly. This observation is not consistent with the sputter rate effect discussed above because the afterpeak time regime signal is influenced to a greater extent than the plateau time regime signal. The plateau and afterpeak time regimes differ in that metastable argon atoms are generated continuously during the plateau time regime but not during the afterpeak. As a result, those metastable argon atoms lost to quenching reactions with methane can be replaced by newly generated metastable argon atoms during the plateau time regime. Because there is no mechanism to replenish metastable argon atoms during the afterpeak time regime, the addition of methane would eliminate metastable argon atoms and completely quench Penning ionization. The observations for the influence of methane addition of the W u + ion signal during the plateau and afterpeak time regimes are consistent with

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such an effect. The dramatic decrease in the afterpeak ion signal indicates that the metastable argon atoms are the dominant ionization agent during the afterpeak time regime. Sampling Distance Influence. The effect of the sampling distances, from 7 to 13 mm as measured between the cathode surface and the ion exit orifice, on the ionized sputtered atom temporal signal profiies is illustrated in Figure 6. The ratios of afterpeak to plateau decrease as the sampling distance increases. At a samplingdistance of 7 mm the plateau is rather weak, whereas its intensity is almost double at a distance of 13 mm. The ion signals in the plateau time regime increase with the sampling distance. This is consistent with observationsfor continuous dc- or rf-powered glow discharges in which the optimum sampling distance is 10-15 mm. The behavior of afterpeak at different sampling distances is of interest. Although the absolute afterpeak ion signal does not change appreciably in the distance range from 7 to 11 mm, the ratio of the afterpeak ion signal to the plateau ion signal decreases with increasing distance. At a distance of 13 mm the afterpeak becomes a small shoulder. This spatial behavior is due to the heterogeneous distributions of sputtered atoms and metastable argon atoms in the discharge plasma. Because of the Penning ionization mechanism, high ion signalsrequire high concentrations of both sputtered atoms and metastable argon atoms. Cathodic sputtering generates a high concentration of sputtered atoms in the cathode region. These atoms must travel toward the ion exit orifice prior to their ionization, resulting in decreasing concentration with increasing distance. The concentration of metastable argon atoms is also spatially dependent. Ferreira et al. made a study of the density of the AT(,)* in a Grimm-type discharge and found the maximum metastable argon atom population was in the cathode fall region.” The relative motion of argon ions and electronsin a collapsingplasma provides insight into the origin of this observation. When the discharge is on, the positive argon ions accelerate toward the cathode and electrons accelerate toward the anode because of the nature of electric force. This results in high concentrations of Ar+ ions nearby the cathode and high electron densities near the anode. Upon removal of the applied electric field, the electrons and Ar+ ions will move toward each other because of electrostatic attraction. The probability of ion-electron recombination will rise nearby the cathode since the electrons move faster toward Ar+ ions due to the much lighter mass. The relative motion of charged species in the plasma results in a relatively high concentration of metastable argon atoms nearby the cathode, resulting in strong Penning ionization of sputtered atoms. This influence declines at distances further away from the cathode.

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Influence of data acquisition gate position on the mass spectrum of a brass sample from a pulsed rf-powered glow discharge. (A) Mass spectrum obtained over the whole pulse period without using data gate. Mass spectra obtained with 0.15-ms data acquisitiongate positioned during (B)the plateau time regime, 4 ms after discharge power application;(C) the afterpeak time regime, 6.3 ms after discharge power application; and (D) the afterpeak time regime, 7 m s after discharge power application. Flgure 8.

Ion Kinetic Energy Distributions. Ion kinetic energy distributions, shown in Figure 7, are obtained by monitoring the time-gated ion signal as a function of bias voltage applied to the Bessel-box kinetic energy analyzer. In agreement with previous observations for pulsed dc-powered glow discharges,3,5 the mean value and the width of the kinetic energy distributions of ionized sputtered atoms differ between the plateau and afterpeak time regimes. These differences likely arise from changes that occur in the net plasma potential and ionization processes as a result of the transition from a steadystate plasma to a decaying plasma. Because the mobility of electrons is greater than that of ions,12 electrons are depopulated through collisions with chamber walls at a rate considerably greater than that for ions during plasma decay. The resulting change in plasma space charge likely results in a positive increase in the net plasma potential during the afterpeak regime. A result of this shift would be an increase in the kinetic energy with which positive ions exit the more positive plasma region that exists during the afterpeak time regime.5 From Figure 7 it is seen that the most probable kinetic energy observed for ions during the afterpeak time regime is indeed higher than that observed for the same ions during the plateau time regime. The ion kinetic energy distribution in the afterpeak is found to be narrower than that in the plateau consistent with the observationsl3J for pulsed dc glow discharges. This narrowing arises from the differences between the two time regimes in the relative contribution of various types of collisional ionization processes. These collisional ionization processes fall into two categories: collisions of the first kind, in which kinetic energy is transferred from one collision partner to the other partner,lsJg and collisions of the second kind, in which potential energy is transferred from one collision partner to another.20-22In the plateau region, both types of processes contribute to the ionization of sputtered analyte atoms via electron, charge transfer, and Penning ionization processes. In electron ionization, any electron with a kinetic energy greater than the ionization potential of an

atom could collisionally ionize that atom with any excess energy being partitioned between the kinetic energies of the resulting ion and electrons. Electrons in the steady-state glow discharge possess a broad range of kinetic energies, with sufficient numbers in the range where electron ionization of analytes is, although not dominant, important. Ionization by this process results in the production of ions with lower kinetic energies contributing to the low energy tail of the observed kinetic energy distributions. Another ionization process to be considered in the steady-state glow discharge is charge transfer from argon ions to lower ionization potential analyte atoms. Unlike electrons,argon ions are able to transfer both kinetic and potential energy to analyte atoms. A significant amount of the argon ion's kinetic energy transfers to the kinetic energy of the analyte atom during a collision. Thus, the resultant analyte ions possess the highest ion kinetic energies observed in the kinetic energy distributions. The remaining principal mechanism is Penning ionization. Like the argon ions, metastable argon atoms possess sufficient potentialenergy, 11.5or 11.7 eV, to bring about the ionization of analyte atoms during collisions. Because metastable argon atoms are charge neutral, they possess less kinetic energy than argon ions. Subsequently,they impart less kinetic energy to analyte atoms in ionizing collisions than do argon ions, but more than do electrons. Because the Penning process dominates sputtered atom ionization in the steady-state rf glow discharges: it would be expected that Penning ionization would dominate the kinetic energy distribution as well. The kinetic energy distribution narrows for analyte ions in the afterpeak because the principal contributors to broadening, electron and charge transfer ionization, contribute significantly less to the total ionization of analyte atoms than they did in the steady-state plateau region. This is consistent with the domination of analyte ionization in the afterpeak by a Penning process to an even greater extent than in the plateau. Because the ion kinetic energy distributions differ between the plateau and afterpeak time regimes, adjustment of the bias of the kinetic energy analyzer permits selective analysis

ANALYTICAL CHEMISTRY, VOL. 65, NO. 22, NOVEMBER 15, 1993

of those ions formed in the afterpeak with little interference from the ions in the plateau. Time-Gated Data Acquisition. As reported previously, the ionized sputtered atoms in pulsed glow discharges exhibit characteristictemporal profiles that are completely different from argon ion temporal profiles. Consequently, the use of the pulsed rf-powered glow discharge devices in conjunction with gated detection provides the analytical advantages of improved sensitivity and selectivity. Mass spectra collected through a 0.15-ms data gate positioned in different temporal regimes of the pulse are shown in Figure 8B-D. For all spectra, the glow discharge was operated at a peak power of 90 W, a source pressure of 0.6 Torr, and a sampling distance of 10 mm. The position of the data acquisition gate within the pulsed period was adjusted using a digital delay generator. Figure 8A shows the time-averaged mass spectrum obtained without gated detection. The signal actually reflects the average signal over the whole pulse period, including pulseon and pulse-off time. The support gas species dominate the mass spectrum; 40Ar+ and MArlH+ signals are very large compared to the signals of sputtered species W u + and W u + . Even mArz+ is visible in the spectrum. Shown in Figure 8B is the mass spectrum measured with the data gate placed in the plateau regime of the ion signal profile, 4 ms after the initiation of discharge power. There is no significant difference from the spectrum shown in Figure 8A in terms of the relative ion intensity of support gas versus sputtered species. This would be expected since the plateau region dominates the pulse profile. It is noted that the ion intensity is increased by a factor of approximately 400 because of the use of boxcar signal integration. When the data collection gate is placed on the afterpeak region, 6.3 ms after discharge power application, ion intensities for sputtered atoms are observed to be greater than those for support argon gas. The 63Cu+ signal intensity increases by a factor of approximately 6; whereas, the argon ion signal decreases by a factor of approximately 5, Figure 8C. If the data gate opens 7 ms after the start of the applied power pulse, the argon ion signals

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become very weak and the sputtered species signals become the dominant feature of the mass spectrum, Figure 8D. It is obvious from these spectra that data acquisition time gated during the afterpeak yields the optimal analytical response.

CONCLUSIONS The temporal dependence of ionization mechanisms responsible for the ionization of sputtered atoms in a pulsed rf-powered glow discharge has been investigated. The data presented here illustrate differences in the temporal ion signal profiles for discharge gas and sputtered atoms. Ionized sputtered atoms exhibit an afterpeak signal maximum similar to that previously observed in pulsed dc-powered glow discharges. These afterpeaks are influenced by various parameters, such as rf peak power, sampling distance, duty cycle, and gas composition. The kinetic energy distribution of ions is found to vary over the pulse cycle. Sputtered ions exhibit a greater average kinetic energy and narrower kinetic energy spread in the afterpeak than in the plateau. The domination of Penning ionization during the afterglow time regime is demonstrated by the drastic reduction in afterpeak ion signal compared to plateau ion signal occurring upon the addition of a metastable quenching reagent, methane, to the discharge. The differences between analyte and discharge gas ion signal temporal profiles permit the use of time-gated data acquisition to improve detection selectivity and sensitivity.

ACKNOWLEDGMENT We are grateful to the U S . Department of Energy and the Office of Naval Research for their support of this research. C.K.P. acknowledgespartial support from the West Virginia Coal and Energy Research Bureau.

RECEIVEDfor review June 4, 1993. Accepted August 5, 1993." .s Abstract published in Advance

ACS Abstracts, September 16,1993.