Anal. Chem. 199S, 65, 735-739
785
Temporal Signal Profiles of Analytical Species in Modulated Glow Discharge Plasmas F. L. King' and C. Pan Department of Chemistry, West Virginia University, Morgantown, West Virginia 26506-6045
ReaCthe optical emkrlon, atomic abrorptlon, and m a r spectrometrk mearurmentsyield temporalprofilesof atomk species present in puked glow discharge plasmas. Optical emkrlon and m a r spectrometry signak for anaiyte atoms and low as well as atomic absorption spectrometry signals for metastable argon atoms reach sharp maxima wtthin 2 ms following the termlnatkn of dkcharge power. The rapid recomblnatlonof argon ions with electrons brought about by the termination of discharge power rebutts in the productlon of metastable argon atoms. Evaluation of temporal atomic absorption profiles for metastable argon and ground-state sputtered atom population8 Indlcates the dominance of sputtered atom ionizatlon vla the Penning mechankm during the t h e following dkcharge power termination.
INTRODUCTION The utilization of modulated glow discharges in analytical spectrometry offers advantages for atomic fluorescence,l atomic absorption,Z and mass spectrometry.3.4 Modulated glow discharge plasmas operate with larger instantaneous currents and voltages resulting in increased atomization, excitation, and ionization with respect to steady-state glow discharge plasmas operated a t the same average power. Additional analytical advantages arise from the introduction of temporal regimes during which analytical signals reach maxima and background signals reach minima. The examination of modulated glow discharge plasmas with laser fluorescence demonstratesthe utility of modulated glow discharges as atom re~ervoirs.~95 The existence of asignificant population of ground-state sputtered atoms in a time period subsequent to the decay of atomic emission permits analysts to conduct atomic fluorescence or atomic absorption measurements with little interference from background emission. Mass spectrometry of modulated glow discharges exhibits an increase in analytical signal followingthe decay of background signals, thereby permitting temporal discrimination against certain interference^.^^^ This enhancement of sputtered ion signal during the afterpeak time regime appears to arise from an increase in Penning ionization involving metastable argon atoms. However, discrete measurements during the time period from 0.6 ms prior to discharge power termination through 3.6 ms subsequent to discharge power termination indicate that both metastable argon atom and sputtered atom populations decay rapidly and precede the surge in the sputtered ion ~ i g n a l . ~ ~~~
~~
(1)Glick, M.; Smith, B. W.; Winefordner, J. D. Anal. Chern. 1990,62, 157-161. (2)Chakrabarti, C. L.; Headrick, K. L.; Hutton, J. C.; Bicheng, Z.; Bertels, P. C.; Back, M. H. Anal. Chem. 1990,62,574-586. (3)Klinger, J. A,; Savickas, P. J.; Harrison, W. W. J. Am. SOC.Mass. Spectrorn. 1990,1, 138-143. (4)Klinger, J. A.;Barshick, C. M.; Harrison, W. W. Anal. Chem. 1991, 63. - -, -2571-2576. - . - -_. -. (5) van Dijk, C.; Smith, B. W.; Winefordner, J. D. Spectrochirn. Acta 1982,37B,759-768. 0003-2700/93/0365-0735$04.00/0
The present report focuses on the acquisition, in real time, of complete temporal profiles from both mass and optical spectrometric measurements of analytically important species in modulated glow discharge plasmas. The mass spectrometric profiles agree with those reported from other laboratories.3~4 The interpretation of the optical measurements provides insight into excitation and ionization processes involving sputtered atoms during the time regime following the termination of discharge power.
EXPERIMENTAL SECTION Modulated Glow Discharge Source. The sample cathode, a stock brass rod 7 mm in length and 2 mm in diameter, was mounted on a direct insertion probes for introduction into the discharge chamberthrough a ball-valve assembly. This chamber, a stainless steel six-way high-vacuum cross, was equipped with suprasil (Heraeus Quartz, Duluth, GA) optical view porta perpendicular to the direct insertion probe axis. A steel plate with a 0.5-mm aperture was employed for both the evacuation of the chamber and the sampling of the plasma for mass spectrometry measurements. The sampling aperture was mounted coaxial with and opposite to the sample cathode at a distance of 10 mm. The glow discharge was powered by a fast programmable voltage amplifier (OPS-3500, Kepco, Flushing, NY). A 0.7-ms delay in the output of the voltage amplifier with respect to the input driving signal was introduced by the intrinsic rise time associated with this voltage amplifier. The input to the voltage amplifier, a step function of variable voltage, frequency, and duty cycle, was produced by a pulse generator that was constructed in the electronics shop of this department. In these experiments the step function was fixed at a 50-Hz repetition rate with a 25% duty cycle. The discharge was operated at a peak current of 4 mA with an atmosphere of 1Torr of argon to yield an average current of 1 mA. Mass Spectrometry System. Maas spectrometricmeasurements were made using a triple-quadrupole mass spectrometer (ELQ-400,Extrel, Pittsburgh, PA) modified to sample the glow discharge source described above. For these experiments the first and second quadrupoles were operated in the rf-only mode acting as ion beam guides. Data were acquired in real time by setting the third quadrupole to transmit the specific mlz of interest and monitoring the output of the continuous dynode electron multiplier (Galileo Electro-optics, Sturbridge, MA) through a preamplifier (Combo-100,MIT, Boulder, CO) with a digital oscilloscope (2232, Tektronix, Beaverton, OR). Hard copies of oscilloscopetraces, the averageof 32 scans, were obtained from a digital plotter (HC-100, Tektronix, Beaverton, OR). Optical Spectrometry System. The optical spectrometry system, Figure 1,was based on a 0.64-m monochromator (HR640, ISA, Edison, NJ) that was equipped with an extended red sensitivity photomultiplier tube (R-928, Hamamatau, Bridgewater, NJ). Optical emission spectrometry ( O B )measurements were made by focusing the image of the glow discharge plasma onto the entrance slit of the monochromatorwith a 100-mmfocal length plan0 convex lens. The signal from the photomultiplier tube was conditioned by a preamplifier (Combo-100, MIT, Boulder, CO). The amplified signal was monitored with adigital (6)King, F.L.;Harrison, W. W. Int. J. Mass Spectrorn. ZonProcesses 1989,89,171-185. 0 1993 Amerlcan Chemical Society
736
ANALYTICAL CHEMISTRY, VOL. 65, NO. 8. MARCH 15, 1993
LOW DISCHARGE
POWER
MONOCHROMATOR
SUPPLY PMT
CH0PPE R LENS
ENS ...
...
o,::,:::
PLOlTER
PREAMP Flgure 1. Modulated glow discharge optical spectrometry system. A
I,
5 ms
*nu c (
OFF
ON
A
V Flgura 2. (A) Step function driving the glow discharge power supply. (E) Temporal profile of 811.5-nm Ar(1) radiation. from an argon-flied ho low cathode lamp. transmined through the glow discharge plasma. oscilloscope (2232,'I'eklrunix. Heaverton.OR1 t hat was triggered by the step function driving the voltage amplifier of the source. A temporal signal profile was ohtained from the averagr of 92 scans. Fur atomic absorption spectrometry ~ A A S Jmenwrements. radiation from a suitable holluw cathode lamp WHS modulated at WUU Hz by a chopper ( l Y 7 , F:G&G PAR, Princeton, NJ,. 'I'he chopped heam was transmitted through theglow dischargeplasma regionof interest and into theentrance d i t of the monwhromator. The signal from the photomultiplier tuhe wns processed hv n preamplifier and then by B lock-in amplifier (5210, EG&G PAR. Princetun. NJJ that was tuned to the chopper frequency. The smallest time constant. 1 ms. was employed a,irh the lock-in amplifier to maximize the temporal response of the output. The output signal from the lock-in amplifier wns monitored hy the orcilloscupe,yieldinga temporal profile oft hetransmitted hollow cathode lamp radiation such as that shown in Fiwre 2 fur the Rll..i-nm argon atom transition. The raw data, the avrrage uf 32 scans. were deronvoliited to rorrect for r c trme constant distortion as described in the next experimental section. After decunvolution, thedata were converted into absorbance urofilrs hy a spreadsheet program. Signal Processing Effects. In order to permit proper interpretation of IQmpOralsignnl profiles, the Q f f Q C l of sipnnl procesringdeviceson these pruiilerwaaexamined. 'l'hechopped emissim signal fur copper atmns at 324.d n m wad moniturtd directly from the photomultiplier tuheoutput hg the oscilloscupe. yielding the temporal profile shown in FigllrQ Rh. Hecause the
Flgure 3. (A) Step function driving the glow discharge power supply. (B) Tempcfal emission profile of chopped 324.8-nm radlatlon from cu(1) monitored at the photomultiplier output. (C)Temporal emission profile of chopped 324.8-nm radiation from Cu(1) monitored at the preamplifier output. (D)Temporal emission profile 01 chopped 324.s nm radiation from Cu(1) monnored at the lock-In amplifier output. (E) Temporal emission profile of chopped 324.8-nm radiation from CNI) obtained from deconvolutlon of lock-In amplifier output. electron transit time and rise time stated b y the photomultiplier tube manufacturer are less than 0.1 ps, this signal profile waa considered to he a faithful representation of the true emission profile in these experiments and was employed as a reference. The choppedemission signalwas monitored after passingthrough the preamplifier yielding the temporal profile shown in Figure
ANALYTICAL CHEMISTRY, VOL. 65, NO. 6, MARCH 15, 1993
797
2 ms
2 ms
H
H
B
J
L
n
Fburr 4. (A) Step function driving the glow discharge power supply. (B)Temporal profile of ‘OAr+ mass spectrometry signal. (C) Temporal profile of e3Cu+mass spectrometry signal.
3C. No significantshift or distortion of the temporal signal profile was observed to result from processing by the preamplifier. Chopping was employed in all of these measurements to permit ready determination of the effect of signal processing by the lock-in amplifier. The preamplified signal was processed by the lock-in amplifier to yield the temporal signal profile shown in Figure 3D. Distortion arising from the lock-in amplifier rc time constant was observed to shift the signal temporally by 0.5 ms and to increase the decay time. The temporal profile shown in Figure 3D was digitized using commercially available software (Un-Scan-It,Silk Scientific, Orem, UT). The digitized data was deconvoluted mathematically to remove the rc time constant distortion. The temporal profileshown in Figure 3E was obtained by plotting the deconvoluted data. The temporal information contained in the signal shown in Figure 3C was recovered through this process. All data employed to obtain atomic absorption spectrometry profiles were deconvoluted in this manner. It should be noted that the emission was chopped only for these investigations of the signal processing effects. All subsequent emission data were obtained without the use of the chopper/ lock-in amplifier combination and, therefore, required no deconvolution.
RESULTS AND DISCUSSION Mass Spectrometric Observations. Temporal mass spectrometric signal profiles for 40Ar+and 63Cu+ are shown in Figure 4. Each profile is the result of 32 scans averaged by the oscilloscope. As reported by previous investigators,%4 these profiles are fundamentally different. The 40Ar+ signal, Figure 4b, exhibits an intensity maximum immediately following the application of discharge power. This prepeak results from the surge in electron energy that precedes the electrical breakdown of the discharge support gas during discharge initiation. Because sputtering has not begun, analyte atoms are not available for ionization; hence, analyte ions such as 63Cu+,Figure 4C, do not exhibit a prepeak. Only discharge support gas atoms or impurity molecules are available for ionization in collisions with the energetic electrons. Ion signals from both discharge gas and sputtered species exhibit a plateau region corresponding to the attainment of a steady state of discharge operation. In this plateau region, analyte atoms are now present in the gas phase and are subject to ionization by collisionswith electrons, ionized argon atoms, and metastable argon atoms in the negative glow of the discharge. Upon the cessation of the applied power the 40Ar+signal rapidly decays whereas the s3Cu+signal rapidly increases to a maximum. It is this afterpeak time regime that is of analytical interest. During this time regime the analyte ion signals reach a maximum in the absence of
E Flgure 5. (A) Step function driving the glow discharge power supply. (B) Temporal proflle of optical emission at 324.8 nm for Cu(1). (C) Temporal proflle of optlcal emission at 224.7 nm for Cu(I1). (D) Temporal profileof optlcal emissionat 427.2 nmfor Ar(1). (E) Temporal profile of optical emission at 664.4 nm for Ar(I1).
any discharge support gas ion signals. Optical spectroscopic investigations provide insight into the origin of this afterpeak. Optical Emission Observations. Examination of temporal signal profiles from optical emission measurements, Figure 5, indicates that the signal afterpeak is not an artifact of the mass spectrometric measurement. The process that leads to the mass spectrometry signal afterpeak also results in the production of atomic and ionic emission signal afterpeaks. It should be recalled that the temporal response of the voltage amplifier powering the discharge lags the driving signal by 0.7 ms. Whereas the profiles shown in Figure 5 are uncorrected for this shift, the discussion of these results employs temporal values corrected for this temporal shift. The temporal emission profile a t 324.8 nm for copper atoms, Figure 5B, exhibits a maximum 1.5ms after the signal driving the power supply, Figure 5A, reaches zero. Figure 5C shows the temporal emission profile a t 224.7 nm for excited copper ions. There is no discernable difference between the temporal locations of the afterpeak maxima of these two species. The optical emission profile of the copper ion exhibits a larger afterpeak-to-plateau intensity ratio than does the optical emission profile of the copper atoms. This observation indicates an enhancement in the ionization of sputtered atoms compared to their excitation during the afterpeak time regime. The atomic emission afterpeak likely results from either the excitation of copper atoms by electrons generated through Penning ionization or the radiative decay of excited copper atoms formed by the recombination of copper ions with electrons. A comprehensive examination of excitation and ionization during the afterpeak is the subject of future investigations in this laboratory. The temporal profile of argon atom emission at 427.2 nm, Figure 5D, exhibits an afterpeak, whereas the temporal profiie of argon ion emission at 664.4 nm, Figure 5E, does not show an afterpeak. The lack of an afterpeak in the argon ion emission profile is consistent with the mass spectrometric
798
ANALYTICAL CHEMISTRY, VOL. 85, NO. 8, MARCH 15, 1983 1 ms
A
-
J
L
r
L
ON
OFF
0.4 0.2
"
id
:
:
:
Flgwo 8. (A) Step functlon drlvlng the glow discharge power supply. (6)Temporal profile of background continuum emlsslon at 747.8 nm, no methane added. (C) Temporal profile of background continuum emiselon at 747.8 nm, methane added at 10%.
observations. Argon ion signal decays rapidly upon the cessation of discharge power in both instances. The decay in argon ion population results from the radiative recombination of argon ions with slow electrons. The probability for such recombination increases as the electrons present in the plasma thermalize upon the termination of the discharge sustaining potential.' A result of this recombination is the formation of a highly excited argon atoms.8 The argon atomic emiseion at 427.2 nm arises from the radiative decay of the 5p level to the 4s level for these excited-state argon atoms. Such a radiative decay populates the metastable states, 3 P ~ and 3P0,of the argon atom at 11.55 and 11.72 eV. The recombination of argon ions with electrons results in the emission of a background continuum.7~8The temporal profile for background continuum emission at 747.6 nm, Figure 6b, exhibits an afterpeak signal maximum that is temporally coincident with the other afterpeak signalmaxima. The presence of this afterpeak, in a spectral region free from analyte or discharge gas emission, indicates an increase in the recombination of argon ions with electrons following the termination of the applied potential. The addition of a collisionalquenching reagent, methane, to the plasma at 10% (by pressure) results in the deexcitation of the excited argon atoms.8~9The observable consequence of this deexcitation is the elimination of the afterpeakin the temporal profile, Figure 6C. Atomic Absorption Observations. Deconvolution and spreadsheet processing of raw transmittance data, such as that shown in Figure 2, yield temporal profiles of atomic absorption by metastable argon atoms, X = 811.5 nm, and copper atoms, X = 324.8 nm,that appear in Figure 7A,B, respectively. These profiles incorporate correction for the 0.7-ms lag in the discharge power arising from the voltage amplifier response time. Whereas the metastable argon atom population maximizes at the same time as emission and mass spectrometry signals, the sputtered atom population maximizes after these signals. Upon discharge power termination, the metastable argon atom population surges to a maximum that appears 1.3 ma following this termination. This obser(7) Biondi, M.A. Phys. Reu. 1963,129,1181-1188. (8)Straws, J. A.;Ferreira, N. P.;Human, H. G. C. Spectrochirn. Acta 1982,37B,947-954. (9) Smith, R. L.; Serxner, D.; Hew, K. R. Anal. Chem. 1989,61,11031108.
I
0.4
0.20 0
2
4
6
8
l
0
l
2
l
4
TIME(rns) Flgurr 7. (A) Temporal profile of atomic absorptbn at 81 1.5 nm for metastable argon atoms. (6)Temporal profile of atomlc absorption at 324.8 nm for ground-state copper atoms. (C) Temporal proflle of theproduct C = AB, where A isthe metastable argon atom absorbance and B is the ground-state copper atom absorbance.
vation is consistent with increasing recombination of argon ions with thermalized electrons to yield metastable argon atoms as the plasma decays. Such metastable generation occurs in a modulated Grimm glow discharge examined by previous investigators.8 The temporal profile of the sputtered copper atom population reflecta the evolution of sputtering and the diffusion of atoms from the cathode. The copper atom population reaches a maximum 1.6 ms after termination of the discharge power. Evaluation of the product of copper atom and metastable argon atom absorption profiles, Figure 7C, in a manner similar to that employed by Eckstein, Coburn, and Kay,lo provides more information regarding the analyte signal afterpeaka. The temporal location of the profile maximum for this product overlaps the profile maxima for optical emission and mass spectrometry of the copper species. The similarity of these profiles within experimental limits indicates that the production of excited neutral and ionized copper atoms during the afterpeak arises from "collisions of the second kind" involving metastable argon atoms. The clear difference between the calculated product profile, Figure 7C, and the anal@ atom profile,Figure 5B, or analyte ion profies, Figures 4C and 5C, is the delay in the appearance of a steady-state plateau region. This is indicative of the evolution of Penning ionization during the equilibration of discharge excitation/ ionization processes. The contribution from Penning ionization steadily increases during the f i s t milliseconds of the discharge. Equilibriumoccursjust before the diecharge power terminates at the pulse widths employed in these investigations. Under conditione of lower discharge pressures of longer pulse widths, the Penning ionization contribution reaches a steady-state value. (10) Eckstein, E. W.;Coburn, J. W.; Kay, E. Znt. J. Mass Spectrorn. Zon Phys. 1976,17,129-138.
ANALYTICAL CHEMISTRY, VOL. 65, NO. 6,MARCH 15, 1993
CONCLUSIONS The simultaneous availability of data regarding the temporal behavior of excited, metastable, and ionized atoms provides insight into the plasma chemistry occurring in the afterpeak time regime. The optical emission profiles of excited neutral and ionized analyte atoms exhibit afterpeaks temporally coincident with their mass spectrometric afterpeaks. Optical emission profiles of argon neutrals and ions indicate that argon ions and electronsrecombine immediatelyfollowing the termination of the applied potential. This recombination of argon ions with electrons results in the generation of metastable argon atoms. Significantly,the atomic absorption temporal profiles reveal that this recombination leads to a surge in the metastable argon atom population during the afterpeak time regime. These metastable argon atoms dominate the excitation and ionization of sputtered atoms during the afterpeak. Further studies are planned in which these methods will be employed to characterize mechanisms
798
of excitation and ionization in modulated glow discharges operated with rf power. At the present time a time-of-flight mass spectrometry system permitting the acquisition of full spectra during the on-off cycle of a modulated glow discharge is under development in this laboratory.
ACKNOWLEDGMENT Financial support from the Office of Naval Research for the purchase of equipment used in these investigations is gratefully acknowledged. We thank Prof. H. 0. Finklea for the use of his BASIC program that removes rc time constant distortion. The many helpful comments of the reviewers are appreciated.
RECEIVED for review July 29,1992. Accepted December 4, 1992.
