- Email: [email protected]
Electrical characteristics of a millisecond pulsed glow discharge
Spectrochimica Acta Part B 63 (2008) 630–637
Contents lists available at ScienceDirect
Spectrochimica Acta Part B j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / s a b
Electrical characteristics of a millisecond pulsed glow discharge Daniel Fliegel, Detlef Günther ⁎ Laboratory of Inorganic Chemistry, ETH Zurich, Zurich, Switzerland
A R T I C L E
I N F O
Article history: Received 24 September 2007 Accepted 14 March 2008 Available online 31 March 2008 Keywords: Pulsed glow discharge Paschen curve Breakdown Discharge frequency
A B S T R A C T A μs and ms pulsed argon glow discharge was investigated with respect to the breakdown condition (Paschen curve). Moreover, current–voltage profiles were acquired for different discharge frequencies, pulse durations, cathode–anode spacing and discharge pressures. The breakdown voltage was dependent on the cathode material (Cu, steel, Ti and Al). No severe change in the breakdown voltage was observed for a 1 ms pulse at different frequencies. However, the theoretical breakdown curve, calculated based on the Paschen equation did not fit the experimental data. The current plots for different cathode–anode spacing showed a maximum at intermediate distance (8–10 mm). These data were consistent with mass spectrometric data acquired using the same instrument in a GC-GD-TOFMS chemical speciation study. A higher discharge frequency with constant pulse duration leads to an increase in discharge current, probably due to a more dense plasma during the pulse. This is supported by the total emission of the plasma, which was not reaching the baseline for short delays in-between the plasma pulses. In addition, the total emission of the plasma shows that at least 2 ms is needed for the plasma to reach a stable quasi steady-state. The current–voltage (traces) for different discharge pressures show a constant relationship for different pulse durations. The current–voltage relation is pressure dependent and similar to dc-glow discharges. A fast photodiode was used for precise observation of the entire Penning ionization in the afterpeak at different pressure settings. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Glow discharges operated in noble gases like argon or helium are used as excitation sources in analytical spectroscopy. These plasma sources have been applied for direct solid analysis together with optical emission spectroscopy or mass spectrometry [1,2]. Since these ion and excitation sources have been used for a long time, the working properties of steady-state glow discharges are well known and have been described in several studies [3,4]. However, the use of none steady-state glow discharges as ion sources is a comparably new feature when coupled to secondary sample introduction systems such as gas chromatography. For the majority of applications in analytical chemistry the glow discharge has been used in steady-state-dc or -rf mode. During the operation of glow discharges as pulsed plasma source, the benefit of soft Penning ionization, after the plasma is extinguished, has been used to discriminate certain analyte species from discharge species [5–8]. Even more interesting has been the use of a time gated ionization and detection for chemical speciation studies [9–12]. The knowledge of the speciation of a target analyte is one of the most ⁎ Corresponding author. Tel.: +41 44 6324687; fax: +41 44 6331071 E-mail address: [email protected] (D. Günther). 0584-8547/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.sab.2008.03.009
important and challenging information in biology and environmental sciences. As important as the total amount of a target analyte is the binding form, oxidation state or complexation, often defining properties such as toxicity, bioavailability and other parameters [13,14]. Unfortunately, the simultaneous detection of the elemental composition at low analyte concentrations, together with structural and molecular information of the analyte rule out each other due to different ionization regimes. Pulsed glow discharges provide transient ionization from a hard electron ionization plasma to a very soft Penning ionization recombination plasma within a few ms [15,16]. Therefore, the elemental, structural and molecular information of a target analyte is accessible in quasi real time, when pulsed glow discharges are coupled to a fast or simultaneous mass analyzer such as a time of flight mass spectrometer [12]. Hence, there is an emerging interest to study the fundamental processes of pulsed glow discharges with respect to the optimization of such devices as an ion source for chemical speciation analysis. This becomes especially evident when considering recent research demonstrating the dependence of a wide variety of tuning parameters such as sampling distance, time gating and pressure on the gained information [10,17]. The different ionization pathways (electron impact, charge exchange and Penning ionization), electron temperature, density
D. Fliegel, D. Günther / Spectrochimica Acta Part B 63 (2008) 630–637
and energy distribution or the population of certain species like Ar+ or the argon metastables (Ar⁎) have been shown to be dependent on the glow discharge source parameters such as applied voltage, current and pressure [18–22]. Moreover, the distance between cathode and anode alters the plasma structure significantly. At higher pressures the glow is getting compressed, whereas at lower pressures the regions of the glow extend. This influences significantly the obtainable information [22,10,17]. For high voltage hydrogen and nitrogen pulsed glow discharge plasmas, Tian and Chu have shown the dependence of the breakdown voltage on the applied pulse frequency and pulse duration [23]. Most of the studies available for pulsed glow discharges, in a working range useful as an excitation source, were conducted based on steady-state dc plasmas. However, little has been reported on the electrical properties. However, a detailed description of the electrical properties of the pulsed glow discharge would help to select experimental conditions allowing to take full advantage of the transient ionization process. In addition, insights into the pulse structure of the current and voltage trace may help to understand further the ionization pathways in the pulsed glow discharge. For a μs pulsed Ar glow discharge Bogaerts and Gijbels have demonstrated that theoretical modeling of the argon ion distribution, the ionization and recombination processes could be linked with good agreement to measured data of the current and voltage characteristics of the glow discharge [24]. Unfortunately, the investigation of transient ion distributions within the plasma of pulsed glow discharges coupled to mass spectrometry or other spectroscopic methods involves the acquisition of extensive amounts of data. Therefore, an additional monitoring and description of the pulsed glow discharge by recording the electrical properties, or total emission, could be beneficial for the optimization of such devices. Consequently, this study was carried out, to gain more detailed insights into the processes of ms and μs dc-pulsed Ar plasmas. The experiments were focused on current–voltage profiles and breakdown conditions of the discharge. 2. Theoretical-breakdown condition
I¼
I0 eaNd 1 geaNd1
field strength of the applied electrical field [27,28]. Below a critical kinetic energy (500 eV [29]) of the bombarding ions γ is constant and independent of the kinetic energy due to the potential ejection. This is valid for clean cathode surfaces, however in analytical glow discharges, the surface is often not entirely clean which effects on the secondary emission of electrons from the cathode surface. Bogaerts and Gijbels have shown that γ differs up to a factor of 1.6 between a clean and a dirty cathode [30]. Additionally, it has been shown, that the absolute number calculation of γ is difficult due the fact that the cathode surface state is difficult to estimate [31]. Furthermore, the gas temperature has been determined in a range of 500–1000 K, and the pressure measurements show always an uncertainty [32,22,33]. The energy of Ar ions bombarding the cathode in glow discharges, with planar and parallel electrodes, has been determined in the order of several tens up to several hundred eV depending on the pulse time and the applied electrical field [19,34]. The breakdown voltage is governed by Paschen's law (Eq. (3)) [35,25]. At voltages higher than the breakdown voltage, the discharge moves from non-self-sustained to self-sustained. 0 1 pd A A VB ¼ B ; C ¼ ln@ ð3Þ C þ ln ðpdÞ ln 1 þ 1 g
where A and B are gas dependent constants, and pd the product of pressure and gap between cathode and anode (expressed traditionally in Torr cm). The breakdown also depends on factors such as charged and non-charged particles in the gas, electrode configuration [36,37] and the surface properties of the electrodes [29]. However, these factors are not considered in the Paschen equation and might therefore be responsible for the deviation between experimental and theoretical results. Furthermore, the secondary electron emission due to surface modification, e.g. the implementation of sputtering atoms into the top most cathode layer [38], plays an important role in the left branch of the Paschen curve [39]. The minimum voltage (VBmin) for a breakdown in a certain gas is defined by the following equation [25]. VBmin ¼
To generate a self-sustaining discharge an electron avalanche in the gap of the electrodes has to be generated. Only if a sufficient amount of free electrons are available the electron avalanche will appear and the discharge will move from non-self-sustaining to selfsustaining [25]. When a sufficient number of gas molecules or atoms are excited by the electrons crossing the gap, the discharge will start to emit light and the characteristic zones of a normal dc discharge can be observed. The emission of electrons (the current) of a steady-state discharge is given by Eq. (1) [25,26]. ð1Þ
Where I is the current leaving the cathode, I0 the current generated by ionization and attracted by the cathode, α the Townsend ionization coefficient, γ the secondary electron emission coefficient of the cathode, d the distance between cathode and anode and N the concentration of particles. The discharge is self-sustained when current flows, even in absence of outside-source electrons. In this case the denominator of Eq. (1) is N0. The transition condition for generating a self-sustaining discharge is given by the Townsend's law (Eq. (2)) [25]. 1 ad ¼ ln þ1 : ð2Þ g The secondary electron emission (γ) depends on the cathode material, on the discharge species impacting the cathode and on the
631
expðBÞ 1 ln 1 þ A g
ð4Þ
These breakdown conditions define the lowest working range for a glow discharge. Without fulfilling these conditions, the plasma cannot be used for excitation and ionization of the analyte. The other end of the working region represents the transition of the plasma from conventional glow discharge to an arcing discharge. Since the arc is not forming uniform plasma over the entire plasma cavity and the flowing current is much higher than in a conventional glow discharge, the arc is not a desired ion source for analytical spectrometry with secondary sample introduction. Therefore, the knowledge of the working range of a pulsed glow discharge is important for the use of pulsed glow discharges as ion sources in mass spectrometry. The aim of this study was therefore to evaluate a pulsed glow discharge in respect to the breakdown behavior. Additionally, current–voltage relations and the total emission of the plasma were recorded and compared to dc discharges. A further goal was to determine whether these recorded parameters can be used for optimizing glow discharge plasmas as ion source for mass spectrometric speciation analysis. 3. Experimental The experiments were carried out using an in-house built GD chamber made of stainless steel. The chamber provides 4 viewing ports and an orthogonal insertion port for a direct insertion probe (cathode). The design of the glow discharge chamber has been described elsewhere [17]. The cathode consists of a flat, round copper target with a diameter of 5 mm (Good Fellow, 99.99+%). Other cathode
632
D. Fliegel, D. Günther / Spectrochimica Acta Part B 63 (2008) 630–637
Fig. 1. Experimental setup for the evaluation of the transient discharge power and the breakdown conditions.
configurations used for experiments will be stated in the manuscript. The voltage was supplied by a pulsed power supply (GBS, RUP 3-3A, Grosserkmannsdorf, Germany) triggered by the timer card of the TOFMS (Tofwerk, Thun, Switzerland) which was coupled to the glow discharge source. The voltage was measured using a voltage divider (1:1000) which was included in the pulsed power supply. The current trace was recorded by a current probe (Tektronix Inc., AM 503). Current and voltage were recorded by 4 channel digital phosphorous oscilloscope (Tektronix Inc., TDS 5140B). The emission of the plasma was measured using a high speed silicon detector (photodiode) with a spectral response of 185–1100 nm and a rise/fall time shorter than 1 ns (Thorlabs Inc., Newton, NJ, DET210) which was attached to an oscilloscope. The photodiode installed on our glow discharge chamber also served as an indicator for the ignition and as soon as an emission signal was detected, the plasma was supposed to be ignited. Further evidence of the suitability of emission measurements using a photodiode is its correlation to the simultaneously measured current. The pressure was adjusted by applying different gas flows via a needle valve. Ar purified by oxygen and water traps, was used as discharge gas. A schematic drawing of the experimental setup is shown in Fig. 1. As shown in Fig. 2, the cathode and anode were two parallel plates. As long as distance between sampler and skimmer is below 22.5 mm, the discharge configuration is a flat and parallel configuration. Therefore, the maximum distance of cathode and anode was kept shorter than 20 mm, which ensures that the electrical field is within our experimental setup as homogeneous as possible.
Fig. 2. Dimensions of the glow discharge chamber used in this section.
4. Results 4.1. Breakdown voltage in dependence on cathode material The experiments were carried out using in-house machined cathodes from standards of Al, Ti and stainless steel. These cathodes were produced using the same dimension as the Cu cathode described above and Paschen curves were measured. The curves support that the breakdown voltage (Vb) in ms pulsed-discharges is dependent on the cathode material. For example, Fig. 3 shows a 5 ms pulsed discharge with a duty cycle of 50%. As it can be seen, the breakdown for Cu appears at a slightly lower voltage when compared to Al. The breakdown for Ti and for steel appears at even higher voltages. The breakdown for the different cathode materials is basically defined by γ. Due to the fact that Ar was used throughout the experiments, the gas constants A and B (Eq. (3)) have no influence. According to the γ values reported in Guillot et al., the breakdown voltage should follow Vb(Cu) N Vb (Fe) N Vb (Ti) [40]. However, our experiments show in contrast to these data, that the breakdown for Cu is at the lowest
Fig. 3. Paschen curve in Ar (ms discharge) for different cathode materials. A significant contamination of the cathode itself, leading to a modification of the breakdown, can be excluded since shapes of the Paschen curve are identical for different materials. The determination of the “on” state of the glow contains some measurement uncertainties. The breakdown was defined as the voltage where an emission, measured by the photodiode, was observed. Thus, there can be an error of few tens of volts for the breakdown condition based on the sensitivity of the photodiode used for this study, given by the dark current of the photodiode (2.5 nA) which shifts the Paschen curve to higher voltages.
D. Fliegel, D. Günther / Spectrochimica Acta Part B 63 (2008) 630–637
633
voltage. This might be attributed to effects which have been already described by Bogaerts and Gijbels [30]. In contrast to previously reported data, no minimum in the Paschen curve was observed. However, it needs to be mentioned that our measurements represent only data points up to a pd value of 1.6 Torr cm. Higher pd values require either higher pressure or a wider distance between cathode and anode, which were not applicable in our glow discharge geometry. Furthermore, as illustrated in Fig. 4, the left branch of the Paschen curve is not in agreement with the theoretical data, which were calculated based on A = 12 cm− 1 Torr− 1, B 180 V/(cm Torr) [25] and γCu = 0.01 [25,41]. Note that γ is not only dependent on Ar cations impacting the cathode, but also on other species and the state of the cathode [29], especially for lower energetic (up to 1 keV) Ar ions. The environment of the discharge determines a proportion of γ [27]. Theoretical models have shown that sputtering of the cathode in ms pulsed glow discharges is not only caused by Ar+ but also at least to the same extent by Cu+ (self-sputtering) and Ar0fast [16]. Therefore, the theoretical curve obtained by the Paschen law is only an approximation and must not represent experimental results. Deviations between measured Paschen curves to theoretical calculated values have already been reported by Garamoon et al. and the citations therein [42]. Furthermore, an influence of the gap distance on the Paschen curve has been reported [27] and was explained by the deviations of the electrical field around the electrodes at different distances. In our study we recorded the Paschen curves by changing both, gap length and pressure (Figs. 3 and 4), which partially explains the scattering of the measured data points. 4.2. Breakdown voltage in dependence of pulse duration and pulse frequency Fig. 4 summarizes the dependence of the breakdown voltage on the pulse frequency. As shown, the breakdown voltage of a 1 ms pulsed discharge remains the same at different delays (0.1–9 ms) inbetween two plasma pulses. This indicates that the breakdown voltage is not governed by the frequency of the discharge. However, it has been reported that an easier breakdown occurs when the delay in-between two pulses is significantly small, or a memory effect of a previous pulse exists [35,23]. This has been
Fig. 5. Power in dependence on voltage (a, b) and current in dependence on voltage. The data in figure (b) were normalized to a 2 ms discharge. Cu was used as cathode material.
Fig. 4. Experimental and modeled Paschen curves for different glow discharge frequencies and Cu as cathode material.
explained by the existence of remaining charged gas species such as + Arm+ n or Cu , or excited species, like Ar⁎m, in-between the electrodes or on the cathode. This helps to generate the electron avalanche by impacting the cathode, as soon as the electrical field is applied. The electron avalanche is moving between the gap of the anode and cathode and has been visually shown by high speed imagining of the glow at the point of the breakdown [43].
634
D. Fliegel, D. Günther / Spectrochimica Acta Part B 63 (2008) 630–637
Pejovic et al. and Stepanović et al. have pointed out that the delay time (τ) between the onset of the voltage pulse and the breakdown depend on the delay time in-between two pulses or the pulse duration [35,44]. However, this was not observed in the experiments at the timescale measurable on our system. At the breakdown voltage we investigated, the plasma is not sufficiently dense and contains most likely not enough charge carriers to ignite an earlier breakdown of the plasma. When comparing the breakdown voltage of a 1 ms to a 5 ms pulsed glow discharge (Fig. 4) we found that the 5 ms pulsed glow discharge ignites at a lower voltage. This might be attributed to an increased cathode heating during a 5 ms glow discharge pulse due to the fact that the cathode was not cooled by a cooling fluid or gas. Increased cathode temperature affects the emission of secondary electrons and alters the breakdown conditions. The dependence of the cathode temperature on the current–voltage relation has been described by Popović et al. [45]. 4.3. Current and power in dependence on pressure and pulse duration Fig. 5 shows the power in dependence on voltage and current at different pressures and pulse duration. At a given time, as soon as the pulsed discharge is reaching its plateau current and voltage, the peak power is the same for μs and ms pulse discharges. This is shown in Fig. 5b where the power is normalized to a 2 ms discharge. Moreover, the recorded current–voltage relationship (see Fig. 5c) is similar to data which have been reported for steady-state dc discharges, e.g. by Tong and Harrison [46], Fang and Marcus [47] and others [48,23]. With increasing voltages, the response of the current to the increased voltage was more pronounced at higher pressures. This behavior can be
explained by a more efficient collision process at higher pressures. This implies that at elevated pressures lower initial secondary electron number density is required for a self-sustaining discharge. In principle, the charges at higher current are concentrated at the same area (cathode surface). Thus, the electrical resistance is higher and causes an increase in voltage. In all measurements a current spike was observed at the beginning of the pulse which is a displacement-current caused by building up a capacity load during fast current and voltage ramping. Similar current traces have been described by several authors [22,49,45] and underlines the comparability of the system used in this study in comparison to other setups. 4.4. Current in dependence of sampling distance The pulse power of a 5 ms pulse discharge in dependence on the cathode–anode distance is shown in Fig. 6a. Due to the fact that the pulse generator was operated in voltage limited mode at constant discharge pressure, the potential change in the discharge power (U I) is based on a current change. It was found that the current for distances in the range of 4–8 mm increases towards a maximum value, and remains constant within the uncertainty of the measurement. The observed increase in the discharge current is based on the fact that with increasing distance, electrons ionize more gas molecules by electron impact ionization. As a result, more (secondary) electrons are generated. At the turnover point, the electrons are thermalized and do not have sufficient energy to generate secondary electrons by electron impact anymore. Therefore, the number of electrons that might hit the anode and transport the current reaches a constant value or even decreases slightly due to ion–electron recombination. The emission intensity of the plasma was depending on the distance between the electrodes, which is shown in Fig. 6b. This could be explained by an increased possibility of excitation of gas molecules by electrons at longer distances between the electrodes. Secondary electrons formed by electron ionization can be accelerated again and excite more neutrals. Furthermore, it was found that the maximum power of the plasma in space (assuming symmetry of the plasma) is the same as the secondary maximum of the ion population when introducing an organic analyte via gas chromatography. These GC data have been previously reported using the same glow discharge source configuration and identical cathode material [17]. The observation, that the maximum power of a glow discharge is correlated with the maximum signal intensity during secondary analyte introduction emphasizes that the monitoring of source parameters (e.g. voltage and current) are suitable for optimizing a glow discharge as ion source for mass spectrometry. This improves the optimization procedure of such plasma source significantly and is of particular interest in hyphenated glow discharge techniques. 4.5. Voltage and current in dependence of pulse frequency and duration
Fig. 6. Influence of the cathode–anode spacing on the glow discharge power (a) and the emission (b) for an ms glow discharge in Ar using a Cu cathode.
Fig. 7 shows the current, voltage and total emission of a 2 ms discharge in dependence on frequency. It can be seen, that the voltage was not significantly influenced by the glow discharge frequency. This is due to the fact that the pulse generator was operated in a voltage limited mode. Therefore, only the current responds on the frequency. Tian and Chu have pointed out that the steady-state current of a pulsed high voltage glow discharge is not significantly influenced by the time gap between two pulses [23]. The voltage–current–pressure relationship governs the power of the discharge. Since the discharge is still in recombination between two plasma pulses when the gap is short enough, the current can be transported via electrons in the recombining plasma more easily at the beginning of the following plasma pulse. Therefore, higher frequency of the discharge supports a slightly higher current, which is shown in Fig. 7. The difference to the observation described by Tian and Chu [23] might be attributed to the fact that they observed the steady-state
D. Fliegel, D. Günther / Spectrochimica Acta Part B 63 (2008) 630–637
635
with recombination and Penning ionization in the afterpeak, as described in mass spectrometry [51,8,52,50,53]. The recombination processes in the afterpeak can be either reaction between a cation and one electron, two electrons, or an electron and a neutral, or even of argon dimers with an electron [54]. For all of these processes thermalized electrons are necessary. In this context, Bogaerts has shown the electron number density peaks slightly at the end of a plasma pulse [54]. The data shown in Fig. 8 is the total emission of the glow discharge and not selective ionic or exited lines of an analyte. The fact that the peak is more pronounced for low pressures than for high pressures can be related to the fact that the Penning ionization (and
Fig. 7. Mean voltage and current for different glow discharge frequencies with constant pulse length. The curves represent the transient behavior of the total glow discharge emission at different glow discharge frequencies.
current. Here, a higher current at the beginning of a pulse (prepeak) might influence the average current, which is also supported by MS measurements. The prepeak signal is more intense using higher discharge frequencies. The total emission of the glow discharge was also higher in a high frequency plasma when compared to a low frequency plasma. This observation is either an indication for denser plasma leading to a higher number of excited gas particles or an indication that more cathode material is sputtered, excited/ionized and recombines with thermalized electrons, leading to an increase in the emission. It was shown that the existence of precursors (excited gas species) before the ignition of the plasma was not influencing the breakdown behavior of the discharge. The total power, however, was influenced by the discharge frequency (see Fig. 7). 4.6. Spectral equilibrium of a pulsed discharge The total emission (passing a fused silica observation window above ~ 230 nm) of the glow discharge was considered as an indicator for the total amount of excited species in the discharge. Fig. 8 demonstrates that a considerable amount of time is required to reach a quasi equilibrium of the discharge. The time to reach stable emission was determined to be at least 2–3 ms, which is in good agreement with previously reported data [44,50]. The time until the discharge reaches the maximum emission is longer at lower pressure (see Fig. 8). Furthermore, the recombination behavior of the plasma is indicated by a sharp increase in emission after voltage is terminated. At lower pressures this spike is more pronounced, which is in good agreement
Fig. 8. Total glow discharge emission for different glow discharge pulse durations and different glow discharge pressure settings.
636
D. Fliegel, D. Günther / Spectrochimica Acta Part B 63 (2008) 630–637
therefore the argon metastable population) is strongly depending on the pressure applied [18,20,55,56]. At low pressures the Ar+ or Ar+2 abundance is lower in comparison to elevated pressures and therefore, recombination with electrons does not lead to a high metastable density. At high pressures, the Ar+ and Ar+2 may get neutralized not only by electron recombination, but also by two or three body recombinations with neutral gas molecules (charge exchange reaction). Therefore, a pressure dependence of the emission in the afterpeak was expected and demonstrated in the experiment summarized in Fig. 8. The data show a clear difference of the ratio of the afterpeak to the plateau. It seems that the afterpeak becomes more dominant at low pressure conditions. During GC-GDMS coupling it can be observed that there is a pressure optimum for molecular information. Normally the maximum of molecular information in a comparable GD source configuration was observed at pressures between 0.2 and 1 mbar. Therefore, the data shown in Fig. 8 are consistent to data observed in mass spectrometry. The data shown here are not in contradiction to models described in GD literature. Fig. 8 shows apparently only the optimum pressure condition for the afterpeak for this GD source configuration. The decrease of the metastable population in the afterpeak at high pressures can be expected due to an increased possibility of the precursor recombination with neutrals (data for lower pressure are not shown). Additionally, during the interpretation and comparison of absolute pressure values one has to keep in mind that the pressure measurement is not directly in the glow discharge chamber. This and the use of different pressure gauges in different studies can cause a difference in the absolute pressure values. A possible solution would be the measurement of the gas density by means of spectroscopic methods in the glow discharge chamber. This indicates, that the information about the Penning ionization (important for obtaining the molecular information in chemical speciation analysis [11,12]) can be evaluated without a mass spectrometer by using a more “simple” photodiode. 5. Conclusion Low pressure pulsed glow discharges in Ar were investigated in terms of breakdown conditions for different cathode materials and discharge frequencies. The breakdown behavior (Paschen plots) showed no significant dependence on the frequency. However such dependence was found on the cathode material, which is in agreement to steady-state glow discharge. Furthermore, the current–voltage profiles of different pressures, cathode–anode distances and pulse frequencies were studied. In addition, the investigation of power in dependence on the cathode–anode spacing showed a maximum at intermediate distances (~ 8–10 mm). These data are in agreement with ion intensity data obtained by GC-GDMS measurements reported elsewhere [17]. This indicates that ion intensities depend on the power of a glow discharge and is a critical parameter to be optimized in glow discharge mass spectrometry to determine the optimum sampling point for gaseous analytes. The evaluation of the current and voltage profiles showed that the normalized power of a pulsed glow discharge is not dependent on the applied frequency. This, in combination with the Paschen plots, is a strong indication that the ms glow discharge follows the principle equations of dc steady-state glow discharges. This leads to the conclusion that the operating parameters of the pulsed glow discharge can be optimized based on already established knowledge of dc steady-state glow discharges. For the use of the pulsed glow discharge as an ion source for mass spectrometry together with secondary analyte introductions this is an important conclusion since all previously described reports can be included in designing and optimizing such a system. However, these data represent pure Ar discharges only. Therefore, the extensively described quenching of the
plasma [9,57,58], will still effect the ionization and excitation properties of the discharge. That implies that the influence of impurities within the plasma gas on the discharge properties needs further fundamental studies. Acknowledgment This work was financially supported by ETH Zurich (research grant TH-16/04-3). The contributions to the system by the machine shop of DCHAB (ETH Zurich), in particular P. Trüssel, are greatly acknowledged. Furthermore, the authors like to thank J. Pisonero for fruitful discussions. C. Lewis and V. Majidi from Los Alamos National Laboratory, (NM, USA) are acknowledged for providing support during the design of the pulsed glow discharge source. K. Fuhrer and M. Gonin from Tofwerk (Thun, Switzerland) are acknowledged for continuous software support. The current probe was provided by the Laboratory of Physical Chemistry, ETH Zurich. References [1] J. Angeli, A. Bengtson, A. Bogaerts, V. Hoffmann, V.D. Hodoroaba, E. Steers, Glow discharge optical emission spectrometry: moving towards reliable thin film analysis — a short review, J. Anal. At. Spectrom. 6 (2003) 670–679. [2] A. Bogaerts, R. Gijbels, Fundamental aspects and applications of glow discharge spectrometric techniques, Spectrochim. Acta Part B 1 (1998) 1–42. [3] J.D. Craggs, J.M. Meek, The initiation of low pressure glow discharges, Proc. Phys. Soc. London 346 (1948) 327–340. [4] M.J. Druyvesteyn, F.M. Penning, The mechanism of electrical discharges in gases of low pressure, Rev. Mod. Phys. 2 (1940) 0087–0174. [5] C.L. Yang, M. Mohill, W.W. Harrison, Microsecond-pulsed Grimm glow discharge as a source for time-of-flight mass spectrometry, J. Anal. At. Spectrom. 9 (2000) 1255–1260. [6] C.L. Yang, E. Oxley, W.W. Harrison, A double pulse method for glow discharge enhancement, J. Anal. At. Spectrom. 10 (2001) 1131–1134. [7] C.L. Yang, K. Ingeneri, W.W. Harrison, A pulsed Grimm glow discharge as an atomic emission source, J. Anal. At. Spectrom. 4 (1999) 693–698. [8] C.L. Lewis, E.S. Oxley, C.K. Ban, R.E. Steiner, F.L. King, Determination of Ca-40(+) in the presence of Ar-40(+): an illustration of the utility of time-gated detection in pulsed glow discharge mass spectrometry, Anal. Chem. 1 (1999) 230–234. [9] D. Fliegel, R. Waddell, V. Majidi, D. Günther, C.L. Lewis, Quantification of aromatic and halogenated hydrocarbons and alcohol mixtures at the elemental, structural, and parent molecular ion level, Anal. Chem. 6 (2005) 1847–1852. [10] C.L. Lewis, M.A. Moser, W. Hang, D.E. Dale, D.C. Hassell, V. Majidi, Influence of discharge parameters on real-time chemical speciation for gas chromatography pulsed glow discharge plasma time-of-flight mass spectrometry, J. Anal. At. Spectrom. 6 (2003) 629–636. [11] C.L. Lewis, M.A. Moser, D.E. Dale, W. Hang, C. Hassell, F.L. King, V. Majidi, Timegated pulsed glow discharge: real-time chemical speciation at the elemental, structural, and molecular level for gas chromatography time-of-flight mass spectrometry, Anal. Chem. 9 (2003) 1983–1996. [12] V. Majidi, M. Moser, C. Lewis, W. Hang, F.L. King, Explicit chemical speciation by microsecond pulsed glow discharge time-of-flight mass spectrometry: concurrent acquisition of structural, molecular and elemental information, J. Anal. At. Spectrom. 1 (1999) 19–25. [13] M. Bernhard, F.E. Brinckman, P.J. Sadler, The Importance of Chemical “Speciation” in Environmental Processes, 1986. [14] J.A.C. Broekaert, S. Gücer, F. Adams, Metal Speciation in the Environment, , 1990. [15] G.P. Jackson, F.L. King, Bulk plasma properties in the pulsed glow discharge, Spectrochim. Acta Part B 8 (2003) 1417–1433. [16] A. Bogaerts, R. Gijbels, G.P. Jackson, Modeling of a millisecond pulsed glow discharge: investigation of the afterpeak, J. Anal. At. Spectrom. 6 (2003) 533–548. [17] D. Fliegel, K. Fuhrer, M. Gonin, D. Günther, Evaluation of a pulsed glow discharge time-of-flight mass spectrometer as a detector for gas chromatography and the influence of the glow discharge source parameters on the information volume in chemical speciation analysis, Anal. Bioanal Chem. 1 (2006) 169–179. [18] R.K. Marcus, J.A.C. Broekaert, Glow Discharge Plasmas in Analytical Spectroscopy, 2003. [19] R.K. Marcus, Glow Discharge Spectroscopy, Plenum press, New York, 1993. [20] B. Chapmann, Glow Discharge Processes, Wiley-Interscience, New York, 1980. [21] L. Li, J.T. Millay, J.P. Turner, F.L. King, Millisecond pulsed radio frequency glow discharge time of flight mass spectrometry: temporal and spatial variations in molecular energetics, J. Am. Soc. Mass. Spectrom. 1 (2004) 87–102. [22] G. Gamez, A. Bogaerts, G.M. Hieftje, Temporal and spatially resolved laserscattering plasma diagnostics for the characterization of a ms-pulsed glow discharge, J. Anal. At. Spectrom. 3 (2006) 350–359. [23] X.B. Tian, P.K. Chu, Experimental investigation of the electrical characteristics and initiation dynamics of pulsed high-voltage glow discharge, J. Phys. D: Appl. Phys. 3 (2001) 354–359. [24] A. Bogaerts, R. Gijbels, Hybrid Monte Carlo — fluid model for a microsecond pulsed glow discharge, J. Anal. At. Spectrom. 8 (2000) 895–905.
D. Fliegel, D. Günther / Spectrochimica Acta Part B 63 (2008) 630–637 [25] Y.P. Raizer, Gas Discharge Physics, Springer, Berlin, 1991. [26] A.P. Bokhan, P.A. Bokhan, D.E. Zakrevsky, Peculiarities of electron emission from the cathode in an abnormal glow discharge, Appl. Phys. Lett. 15 (2005). [27] G. Auday, P. Guillot, J. Galy, H. Brunet, Experimental study of the effective secondary emission coefficient for rare gases and copper electrodes, J. Appl. Phys. 11 (1998) 5917–5921. [28] Y. Chutopa, B. Yotsombat, I.G. Brown, Measurement of secondary electron emission yields, IEEE Trans. Plasma Sci. 5 (2003) 1095–1099. [29] A.V. Phelps, Z.L. Petrovic, Cold-cathode discharges and breakdown in argon: surface and gas phase production of secondary electrons, Plasma Sources Sci. Technol. 3 (1999) R21–R44. [30] A. Bogaerts, R. Gijbels, The ion- and atom-induced secondary electron emission yield: numerical study for the effect of clean and dirty cathode surfaces, Plasma Sources Sci. Technol. 1 (2002) 27–36. [31] A.P. Bokhan, P.A. Bokhan, D.E. Zakrevsky, Electron emission from a cathode doped by fast particles of a working gas, Tech. Phys. 9 (2005) 1233–1235. [32] A. Bogaerts, R. Gijbels, G. Gamez, G.M. Hieftje, Fundamental studies on a planarcathode direct current glow discharge. Part II: numerical modeling and comparison with laser scattering experiments, Spectrochim. Acta Part B 4 (2004) 449–460. [33] G. Gamez, A. Bogaerts, F. Andrade, G.M. Hieftje, Fundamental studies on a planarcathode direct current glow discharge. Part I: characterization via laser scattering techniques, Spectrochim. Acta Part B 4 (2004) 435–447. [34] C.V. Budtz-Jorgensen, J. Bottiger, P. Kringhoj, Energetic ion bombardment of the grounded anode in pulsed DC-glow discharges, Surf. Coat. Technol. 1 (2001) 104–110. [35] M.M. Pejovic, G.S. Ristic, J.P. Karamarkovic, Electrical breakdown in low pressure gases, J. Phys. D: Appl. Phys. 10 (2002) R91–R103. [36] H. Eichhorn, K.H. Schoenbach, T. Tessnow, Paschen Law for a hollow-cathode discharge, Appl. Phys. Lett. 18 (1993) 2481–2483. [37] P.D. Mixon, S.T. Griffin, J.C. Williams, Breakdown voltage scaling characteristics of an Argon hollow-cathode discharge, J. Appl. Spectrosc. 10 (1993) 1567–1570. [38] P.A. Bokhan, D.E. Zakrevsky, Surface condition and electron emission from cold cathodes in vacuum and in noble gas glow discharge, Tech. Phys. 1 (2007) 104–112. [39] H.B. Smith, C. Charles, R.W. Boswell, Breakdown behavior in radio-frequency argon discharges, Phys. Plasmas 3 (2003) 875–881. [40] P. Guillot, P. Belenguer, L. Therese, V. Lavoine, H. Chollet, Secondary electron emission coefficients of standard samples for GDOES, Surf. Interface Anal. 7 (2003) 590–592. [41] S.C. Brown, W.P. Allis, Basic Data of Electrical Discharges, MIT, Cambridge, 1956. [42] A.A. Garamoon, A. Samir, F.F. Elakshar, E.F. Kotp, Electrical characteristics of a DC glow discharge Plasma Sources, Sci. Technol. 3 (2003) 417–420. [43] M.D. Wagenaars, M.D. Bowden, G.M.W. Kroesen, Plasma emission imaging of a low-pressure argon breakdown, Plasma Sources, Sci. Technol. (2005) 342–350.
637
[44] O.M. Stepanovic, M.K. Radovic, C.A. Maluckov, The glow duration time influence on the ionization rate detected in the diodes filled with noble gases on mbar pressures, Facta Universitatis, Phys. Chem. Technol. 5 (2003) 259–267. [45] I. Popovic, V. Rajovic, M. Zlatanovic, Dynamic Voltage–Current Characteristics of Unipolar Pulse Glow Discharge, 2005, pp. 315–320. [46] S.L. Tong, W.W. Harrison, Glow-discharge sputtering of chromium and niobium disk cathodes in Argon, Anal. Chem. 12 (1984) 2028–2033. [47] D. Fang, R.K. Marcus, Parametric evaluation of sputtering in a planar, diode glowdischarge.1. Sputtering of oxygen-free hard copper (Ofhc), Spectrochim. Acta Part B 12 (1988) 1451–1460. [48] M. Dogan, K. Laqua, H. Massmann, Spectrochemical analysis with glow discharge lamp as light source.1. Electric properties, sample decomposition and spectral characteristic, Spectrochim. Acta Part B 10 (1971) 631–&. [49] X.B. Tian, B.Y. Tang, P.K. Chu, Accurate determination of pulsed current waveform in plasma immersion ion implantation processes, J. Appl. Phys. 7 (1999) 3567–3570. [50] G.P. Jackson, C.L. Lewis, S.K. Doorn, V. Majidi, F.L. King, Spectral, spatial and temporal characteristics of a millisecond pulsed glow discharge: metastable argon atom production, Spectrochim. Acta Part B (2001) 2449–2464. [51] C.L. Lewis, G.P. Jackson, S.K. Doorn, V. Majidi, F.L. King, Spectral, spatial and temporal characterization of a millisecond pulsed glow discharge: copper analyte emission and ionization, Spectrochim. Acta Part B 5 (2001) 487–501. [52] A. Bogaerts, R. Gijbels, Two-dimensional model of a direct current glow discharge: description of the argon metastable atoms, sputtered atoms, and ions, Anal. Chem. 15 (1996) 2676–2685. [53] C.K. Pan, F.L. King, Ion formation processes in the afterpeak time regime of pulsed glow-discharge plasmas, J. Am. Soc. Mass Spectrom. 9 (1993) 727–732. [54] A. Bogaerts, The afterglow mystery of pulsed glow discharges and the role of dissociative electron–ion recombination, J. Anal. At. Spectrom. (2007) 502–512. [55] M.K. Levy, D. Serxner, A.D. Angstadt, R.L. Smith, K.R. Hess, Optical investigations of excitation processes responsible for ionized sputtered species in a low-pressure, low current, coaxial geometry glow-discharge, Spectrochim. Acta Part B 2 (1991) 253–267. [56] K.A. Hardy, J.W. Sheldon, Metastable atom density in helium, neon, and argon glow-discharges, J. Appl. Phys. 12 (1982) 8532–8536. [57] G.P. Jackson, F.L. King, Probing excitation/ionization processes in millisecondpulsed glow discharges in argon through the addition of nitrogen, Spectrochim. Acta Part B 2–3 (2003) 185–209. [58] A. Bogaerts, R. Gijbels, Effects of adding hydrogen to an argon glow discharge: overview of relevant processes and some qualitative explanations, J. Anal. At. Spectrom. 4 (2000) 441–449.
Contents lists available at ScienceDirect
Spectrochimica Acta Part B j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / s a b
Electrical characteristics of a millisecond pulsed glow discharge Daniel Fliegel, Detlef Günther ⁎ Laboratory of Inorganic Chemistry, ETH Zurich, Zurich, Switzerland
A R T I C L E
I N F O
Article history: Received 24 September 2007 Accepted 14 March 2008 Available online 31 March 2008 Keywords: Pulsed glow discharge Paschen curve Breakdown Discharge frequency
A B S T R A C T A μs and ms pulsed argon glow discharge was investigated with respect to the breakdown condition (Paschen curve). Moreover, current–voltage profiles were acquired for different discharge frequencies, pulse durations, cathode–anode spacing and discharge pressures. The breakdown voltage was dependent on the cathode material (Cu, steel, Ti and Al). No severe change in the breakdown voltage was observed for a 1 ms pulse at different frequencies. However, the theoretical breakdown curve, calculated based on the Paschen equation did not fit the experimental data. The current plots for different cathode–anode spacing showed a maximum at intermediate distance (8–10 mm). These data were consistent with mass spectrometric data acquired using the same instrument in a GC-GD-TOFMS chemical speciation study. A higher discharge frequency with constant pulse duration leads to an increase in discharge current, probably due to a more dense plasma during the pulse. This is supported by the total emission of the plasma, which was not reaching the baseline for short delays in-between the plasma pulses. In addition, the total emission of the plasma shows that at least 2 ms is needed for the plasma to reach a stable quasi steady-state. The current–voltage (traces) for different discharge pressures show a constant relationship for different pulse durations. The current–voltage relation is pressure dependent and similar to dc-glow discharges. A fast photodiode was used for precise observation of the entire Penning ionization in the afterpeak at different pressure settings. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Glow discharges operated in noble gases like argon or helium are used as excitation sources in analytical spectroscopy. These plasma sources have been applied for direct solid analysis together with optical emission spectroscopy or mass spectrometry [1,2]. Since these ion and excitation sources have been used for a long time, the working properties of steady-state glow discharges are well known and have been described in several studies [3,4]. However, the use of none steady-state glow discharges as ion sources is a comparably new feature when coupled to secondary sample introduction systems such as gas chromatography. For the majority of applications in analytical chemistry the glow discharge has been used in steady-state-dc or -rf mode. During the operation of glow discharges as pulsed plasma source, the benefit of soft Penning ionization, after the plasma is extinguished, has been used to discriminate certain analyte species from discharge species [5–8]. Even more interesting has been the use of a time gated ionization and detection for chemical speciation studies [9–12]. The knowledge of the speciation of a target analyte is one of the most ⁎ Corresponding author. Tel.: +41 44 6324687; fax: +41 44 6331071 E-mail address: [email protected] (D. Günther). 0584-8547/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.sab.2008.03.009
important and challenging information in biology and environmental sciences. As important as the total amount of a target analyte is the binding form, oxidation state or complexation, often defining properties such as toxicity, bioavailability and other parameters [13,14]. Unfortunately, the simultaneous detection of the elemental composition at low analyte concentrations, together with structural and molecular information of the analyte rule out each other due to different ionization regimes. Pulsed glow discharges provide transient ionization from a hard electron ionization plasma to a very soft Penning ionization recombination plasma within a few ms [15,16]. Therefore, the elemental, structural and molecular information of a target analyte is accessible in quasi real time, when pulsed glow discharges are coupled to a fast or simultaneous mass analyzer such as a time of flight mass spectrometer [12]. Hence, there is an emerging interest to study the fundamental processes of pulsed glow discharges with respect to the optimization of such devices as an ion source for chemical speciation analysis. This becomes especially evident when considering recent research demonstrating the dependence of a wide variety of tuning parameters such as sampling distance, time gating and pressure on the gained information [10,17]. The different ionization pathways (electron impact, charge exchange and Penning ionization), electron temperature, density
D. Fliegel, D. Günther / Spectrochimica Acta Part B 63 (2008) 630–637
and energy distribution or the population of certain species like Ar+ or the argon metastables (Ar⁎) have been shown to be dependent on the glow discharge source parameters such as applied voltage, current and pressure [18–22]. Moreover, the distance between cathode and anode alters the plasma structure significantly. At higher pressures the glow is getting compressed, whereas at lower pressures the regions of the glow extend. This influences significantly the obtainable information [22,10,17]. For high voltage hydrogen and nitrogen pulsed glow discharge plasmas, Tian and Chu have shown the dependence of the breakdown voltage on the applied pulse frequency and pulse duration [23]. Most of the studies available for pulsed glow discharges, in a working range useful as an excitation source, were conducted based on steady-state dc plasmas. However, little has been reported on the electrical properties. However, a detailed description of the electrical properties of the pulsed glow discharge would help to select experimental conditions allowing to take full advantage of the transient ionization process. In addition, insights into the pulse structure of the current and voltage trace may help to understand further the ionization pathways in the pulsed glow discharge. For a μs pulsed Ar glow discharge Bogaerts and Gijbels have demonstrated that theoretical modeling of the argon ion distribution, the ionization and recombination processes could be linked with good agreement to measured data of the current and voltage characteristics of the glow discharge [24]. Unfortunately, the investigation of transient ion distributions within the plasma of pulsed glow discharges coupled to mass spectrometry or other spectroscopic methods involves the acquisition of extensive amounts of data. Therefore, an additional monitoring and description of the pulsed glow discharge by recording the electrical properties, or total emission, could be beneficial for the optimization of such devices. Consequently, this study was carried out, to gain more detailed insights into the processes of ms and μs dc-pulsed Ar plasmas. The experiments were focused on current–voltage profiles and breakdown conditions of the discharge. 2. Theoretical-breakdown condition
I¼
I0 eaNd 1 geaNd1
field strength of the applied electrical field [27,28]. Below a critical kinetic energy (500 eV [29]) of the bombarding ions γ is constant and independent of the kinetic energy due to the potential ejection. This is valid for clean cathode surfaces, however in analytical glow discharges, the surface is often not entirely clean which effects on the secondary emission of electrons from the cathode surface. Bogaerts and Gijbels have shown that γ differs up to a factor of 1.6 between a clean and a dirty cathode [30]. Additionally, it has been shown, that the absolute number calculation of γ is difficult due the fact that the cathode surface state is difficult to estimate [31]. Furthermore, the gas temperature has been determined in a range of 500–1000 K, and the pressure measurements show always an uncertainty [32,22,33]. The energy of Ar ions bombarding the cathode in glow discharges, with planar and parallel electrodes, has been determined in the order of several tens up to several hundred eV depending on the pulse time and the applied electrical field [19,34]. The breakdown voltage is governed by Paschen's law (Eq. (3)) [35,25]. At voltages higher than the breakdown voltage, the discharge moves from non-self-sustained to self-sustained. 0 1 pd A A VB ¼ B ; C ¼ ln@ ð3Þ C þ ln ðpdÞ ln 1 þ 1 g
where A and B are gas dependent constants, and pd the product of pressure and gap between cathode and anode (expressed traditionally in Torr cm). The breakdown also depends on factors such as charged and non-charged particles in the gas, electrode configuration [36,37] and the surface properties of the electrodes [29]. However, these factors are not considered in the Paschen equation and might therefore be responsible for the deviation between experimental and theoretical results. Furthermore, the secondary electron emission due to surface modification, e.g. the implementation of sputtering atoms into the top most cathode layer [38], plays an important role in the left branch of the Paschen curve [39]. The minimum voltage (VBmin) for a breakdown in a certain gas is defined by the following equation [25]. VBmin ¼
To generate a self-sustaining discharge an electron avalanche in the gap of the electrodes has to be generated. Only if a sufficient amount of free electrons are available the electron avalanche will appear and the discharge will move from non-self-sustaining to selfsustaining [25]. When a sufficient number of gas molecules or atoms are excited by the electrons crossing the gap, the discharge will start to emit light and the characteristic zones of a normal dc discharge can be observed. The emission of electrons (the current) of a steady-state discharge is given by Eq. (1) [25,26]. ð1Þ
Where I is the current leaving the cathode, I0 the current generated by ionization and attracted by the cathode, α the Townsend ionization coefficient, γ the secondary electron emission coefficient of the cathode, d the distance between cathode and anode and N the concentration of particles. The discharge is self-sustained when current flows, even in absence of outside-source electrons. In this case the denominator of Eq. (1) is N0. The transition condition for generating a self-sustaining discharge is given by the Townsend's law (Eq. (2)) [25]. 1 ad ¼ ln þ1 : ð2Þ g The secondary electron emission (γ) depends on the cathode material, on the discharge species impacting the cathode and on the
631
expðBÞ 1 ln 1 þ A g
ð4Þ
These breakdown conditions define the lowest working range for a glow discharge. Without fulfilling these conditions, the plasma cannot be used for excitation and ionization of the analyte. The other end of the working region represents the transition of the plasma from conventional glow discharge to an arcing discharge. Since the arc is not forming uniform plasma over the entire plasma cavity and the flowing current is much higher than in a conventional glow discharge, the arc is not a desired ion source for analytical spectrometry with secondary sample introduction. Therefore, the knowledge of the working range of a pulsed glow discharge is important for the use of pulsed glow discharges as ion sources in mass spectrometry. The aim of this study was therefore to evaluate a pulsed glow discharge in respect to the breakdown behavior. Additionally, current–voltage relations and the total emission of the plasma were recorded and compared to dc discharges. A further goal was to determine whether these recorded parameters can be used for optimizing glow discharge plasmas as ion source for mass spectrometric speciation analysis. 3. Experimental The experiments were carried out using an in-house built GD chamber made of stainless steel. The chamber provides 4 viewing ports and an orthogonal insertion port for a direct insertion probe (cathode). The design of the glow discharge chamber has been described elsewhere [17]. The cathode consists of a flat, round copper target with a diameter of 5 mm (Good Fellow, 99.99+%). Other cathode
632
D. Fliegel, D. Günther / Spectrochimica Acta Part B 63 (2008) 630–637
Fig. 1. Experimental setup for the evaluation of the transient discharge power and the breakdown conditions.
configurations used for experiments will be stated in the manuscript. The voltage was supplied by a pulsed power supply (GBS, RUP 3-3A, Grosserkmannsdorf, Germany) triggered by the timer card of the TOFMS (Tofwerk, Thun, Switzerland) which was coupled to the glow discharge source. The voltage was measured using a voltage divider (1:1000) which was included in the pulsed power supply. The current trace was recorded by a current probe (Tektronix Inc., AM 503). Current and voltage were recorded by 4 channel digital phosphorous oscilloscope (Tektronix Inc., TDS 5140B). The emission of the plasma was measured using a high speed silicon detector (photodiode) with a spectral response of 185–1100 nm and a rise/fall time shorter than 1 ns (Thorlabs Inc., Newton, NJ, DET210) which was attached to an oscilloscope. The photodiode installed on our glow discharge chamber also served as an indicator for the ignition and as soon as an emission signal was detected, the plasma was supposed to be ignited. Further evidence of the suitability of emission measurements using a photodiode is its correlation to the simultaneously measured current. The pressure was adjusted by applying different gas flows via a needle valve. Ar purified by oxygen and water traps, was used as discharge gas. A schematic drawing of the experimental setup is shown in Fig. 1. As shown in Fig. 2, the cathode and anode were two parallel plates. As long as distance between sampler and skimmer is below 22.5 mm, the discharge configuration is a flat and parallel configuration. Therefore, the maximum distance of cathode and anode was kept shorter than 20 mm, which ensures that the electrical field is within our experimental setup as homogeneous as possible.
Fig. 2. Dimensions of the glow discharge chamber used in this section.
4. Results 4.1. Breakdown voltage in dependence on cathode material The experiments were carried out using in-house machined cathodes from standards of Al, Ti and stainless steel. These cathodes were produced using the same dimension as the Cu cathode described above and Paschen curves were measured. The curves support that the breakdown voltage (Vb) in ms pulsed-discharges is dependent on the cathode material. For example, Fig. 3 shows a 5 ms pulsed discharge with a duty cycle of 50%. As it can be seen, the breakdown for Cu appears at a slightly lower voltage when compared to Al. The breakdown for Ti and for steel appears at even higher voltages. The breakdown for the different cathode materials is basically defined by γ. Due to the fact that Ar was used throughout the experiments, the gas constants A and B (Eq. (3)) have no influence. According to the γ values reported in Guillot et al., the breakdown voltage should follow Vb(Cu) N Vb (Fe) N Vb (Ti) [40]. However, our experiments show in contrast to these data, that the breakdown for Cu is at the lowest
Fig. 3. Paschen curve in Ar (ms discharge) for different cathode materials. A significant contamination of the cathode itself, leading to a modification of the breakdown, can be excluded since shapes of the Paschen curve are identical for different materials. The determination of the “on” state of the glow contains some measurement uncertainties. The breakdown was defined as the voltage where an emission, measured by the photodiode, was observed. Thus, there can be an error of few tens of volts for the breakdown condition based on the sensitivity of the photodiode used for this study, given by the dark current of the photodiode (2.5 nA) which shifts the Paschen curve to higher voltages.
D. Fliegel, D. Günther / Spectrochimica Acta Part B 63 (2008) 630–637
633
voltage. This might be attributed to effects which have been already described by Bogaerts and Gijbels [30]. In contrast to previously reported data, no minimum in the Paschen curve was observed. However, it needs to be mentioned that our measurements represent only data points up to a pd value of 1.6 Torr cm. Higher pd values require either higher pressure or a wider distance between cathode and anode, which were not applicable in our glow discharge geometry. Furthermore, as illustrated in Fig. 4, the left branch of the Paschen curve is not in agreement with the theoretical data, which were calculated based on A = 12 cm− 1 Torr− 1, B 180 V/(cm Torr) [25] and γCu = 0.01 [25,41]. Note that γ is not only dependent on Ar cations impacting the cathode, but also on other species and the state of the cathode [29], especially for lower energetic (up to 1 keV) Ar ions. The environment of the discharge determines a proportion of γ [27]. Theoretical models have shown that sputtering of the cathode in ms pulsed glow discharges is not only caused by Ar+ but also at least to the same extent by Cu+ (self-sputtering) and Ar0fast [16]. Therefore, the theoretical curve obtained by the Paschen law is only an approximation and must not represent experimental results. Deviations between measured Paschen curves to theoretical calculated values have already been reported by Garamoon et al. and the citations therein [42]. Furthermore, an influence of the gap distance on the Paschen curve has been reported [27] and was explained by the deviations of the electrical field around the electrodes at different distances. In our study we recorded the Paschen curves by changing both, gap length and pressure (Figs. 3 and 4), which partially explains the scattering of the measured data points. 4.2. Breakdown voltage in dependence of pulse duration and pulse frequency Fig. 4 summarizes the dependence of the breakdown voltage on the pulse frequency. As shown, the breakdown voltage of a 1 ms pulsed discharge remains the same at different delays (0.1–9 ms) inbetween two plasma pulses. This indicates that the breakdown voltage is not governed by the frequency of the discharge. However, it has been reported that an easier breakdown occurs when the delay in-between two pulses is significantly small, or a memory effect of a previous pulse exists [35,23]. This has been
Fig. 5. Power in dependence on voltage (a, b) and current in dependence on voltage. The data in figure (b) were normalized to a 2 ms discharge. Cu was used as cathode material.
Fig. 4. Experimental and modeled Paschen curves for different glow discharge frequencies and Cu as cathode material.
explained by the existence of remaining charged gas species such as + Arm+ n or Cu , or excited species, like Ar⁎m, in-between the electrodes or on the cathode. This helps to generate the electron avalanche by impacting the cathode, as soon as the electrical field is applied. The electron avalanche is moving between the gap of the anode and cathode and has been visually shown by high speed imagining of the glow at the point of the breakdown [43].
634
D. Fliegel, D. Günther / Spectrochimica Acta Part B 63 (2008) 630–637
Pejovic et al. and Stepanović et al. have pointed out that the delay time (τ) between the onset of the voltage pulse and the breakdown depend on the delay time in-between two pulses or the pulse duration [35,44]. However, this was not observed in the experiments at the timescale measurable on our system. At the breakdown voltage we investigated, the plasma is not sufficiently dense and contains most likely not enough charge carriers to ignite an earlier breakdown of the plasma. When comparing the breakdown voltage of a 1 ms to a 5 ms pulsed glow discharge (Fig. 4) we found that the 5 ms pulsed glow discharge ignites at a lower voltage. This might be attributed to an increased cathode heating during a 5 ms glow discharge pulse due to the fact that the cathode was not cooled by a cooling fluid or gas. Increased cathode temperature affects the emission of secondary electrons and alters the breakdown conditions. The dependence of the cathode temperature on the current–voltage relation has been described by Popović et al. [45]. 4.3. Current and power in dependence on pressure and pulse duration Fig. 5 shows the power in dependence on voltage and current at different pressures and pulse duration. At a given time, as soon as the pulsed discharge is reaching its plateau current and voltage, the peak power is the same for μs and ms pulse discharges. This is shown in Fig. 5b where the power is normalized to a 2 ms discharge. Moreover, the recorded current–voltage relationship (see Fig. 5c) is similar to data which have been reported for steady-state dc discharges, e.g. by Tong and Harrison [46], Fang and Marcus [47] and others [48,23]. With increasing voltages, the response of the current to the increased voltage was more pronounced at higher pressures. This behavior can be
explained by a more efficient collision process at higher pressures. This implies that at elevated pressures lower initial secondary electron number density is required for a self-sustaining discharge. In principle, the charges at higher current are concentrated at the same area (cathode surface). Thus, the electrical resistance is higher and causes an increase in voltage. In all measurements a current spike was observed at the beginning of the pulse which is a displacement-current caused by building up a capacity load during fast current and voltage ramping. Similar current traces have been described by several authors [22,49,45] and underlines the comparability of the system used in this study in comparison to other setups. 4.4. Current in dependence of sampling distance The pulse power of a 5 ms pulse discharge in dependence on the cathode–anode distance is shown in Fig. 6a. Due to the fact that the pulse generator was operated in voltage limited mode at constant discharge pressure, the potential change in the discharge power (U I) is based on a current change. It was found that the current for distances in the range of 4–8 mm increases towards a maximum value, and remains constant within the uncertainty of the measurement. The observed increase in the discharge current is based on the fact that with increasing distance, electrons ionize more gas molecules by electron impact ionization. As a result, more (secondary) electrons are generated. At the turnover point, the electrons are thermalized and do not have sufficient energy to generate secondary electrons by electron impact anymore. Therefore, the number of electrons that might hit the anode and transport the current reaches a constant value or even decreases slightly due to ion–electron recombination. The emission intensity of the plasma was depending on the distance between the electrodes, which is shown in Fig. 6b. This could be explained by an increased possibility of excitation of gas molecules by electrons at longer distances between the electrodes. Secondary electrons formed by electron ionization can be accelerated again and excite more neutrals. Furthermore, it was found that the maximum power of the plasma in space (assuming symmetry of the plasma) is the same as the secondary maximum of the ion population when introducing an organic analyte via gas chromatography. These GC data have been previously reported using the same glow discharge source configuration and identical cathode material [17]. The observation, that the maximum power of a glow discharge is correlated with the maximum signal intensity during secondary analyte introduction emphasizes that the monitoring of source parameters (e.g. voltage and current) are suitable for optimizing a glow discharge as ion source for mass spectrometry. This improves the optimization procedure of such plasma source significantly and is of particular interest in hyphenated glow discharge techniques. 4.5. Voltage and current in dependence of pulse frequency and duration
Fig. 6. Influence of the cathode–anode spacing on the glow discharge power (a) and the emission (b) for an ms glow discharge in Ar using a Cu cathode.
Fig. 7 shows the current, voltage and total emission of a 2 ms discharge in dependence on frequency. It can be seen, that the voltage was not significantly influenced by the glow discharge frequency. This is due to the fact that the pulse generator was operated in a voltage limited mode. Therefore, only the current responds on the frequency. Tian and Chu have pointed out that the steady-state current of a pulsed high voltage glow discharge is not significantly influenced by the time gap between two pulses [23]. The voltage–current–pressure relationship governs the power of the discharge. Since the discharge is still in recombination between two plasma pulses when the gap is short enough, the current can be transported via electrons in the recombining plasma more easily at the beginning of the following plasma pulse. Therefore, higher frequency of the discharge supports a slightly higher current, which is shown in Fig. 7. The difference to the observation described by Tian and Chu [23] might be attributed to the fact that they observed the steady-state
D. Fliegel, D. Günther / Spectrochimica Acta Part B 63 (2008) 630–637
635
with recombination and Penning ionization in the afterpeak, as described in mass spectrometry [51,8,52,50,53]. The recombination processes in the afterpeak can be either reaction between a cation and one electron, two electrons, or an electron and a neutral, or even of argon dimers with an electron [54]. For all of these processes thermalized electrons are necessary. In this context, Bogaerts has shown the electron number density peaks slightly at the end of a plasma pulse [54]. The data shown in Fig. 8 is the total emission of the glow discharge and not selective ionic or exited lines of an analyte. The fact that the peak is more pronounced for low pressures than for high pressures can be related to the fact that the Penning ionization (and
Fig. 7. Mean voltage and current for different glow discharge frequencies with constant pulse length. The curves represent the transient behavior of the total glow discharge emission at different glow discharge frequencies.
current. Here, a higher current at the beginning of a pulse (prepeak) might influence the average current, which is also supported by MS measurements. The prepeak signal is more intense using higher discharge frequencies. The total emission of the glow discharge was also higher in a high frequency plasma when compared to a low frequency plasma. This observation is either an indication for denser plasma leading to a higher number of excited gas particles or an indication that more cathode material is sputtered, excited/ionized and recombines with thermalized electrons, leading to an increase in the emission. It was shown that the existence of precursors (excited gas species) before the ignition of the plasma was not influencing the breakdown behavior of the discharge. The total power, however, was influenced by the discharge frequency (see Fig. 7). 4.6. Spectral equilibrium of a pulsed discharge The total emission (passing a fused silica observation window above ~ 230 nm) of the glow discharge was considered as an indicator for the total amount of excited species in the discharge. Fig. 8 demonstrates that a considerable amount of time is required to reach a quasi equilibrium of the discharge. The time to reach stable emission was determined to be at least 2–3 ms, which is in good agreement with previously reported data [44,50]. The time until the discharge reaches the maximum emission is longer at lower pressure (see Fig. 8). Furthermore, the recombination behavior of the plasma is indicated by a sharp increase in emission after voltage is terminated. At lower pressures this spike is more pronounced, which is in good agreement
Fig. 8. Total glow discharge emission for different glow discharge pulse durations and different glow discharge pressure settings.
636
D. Fliegel, D. Günther / Spectrochimica Acta Part B 63 (2008) 630–637
therefore the argon metastable population) is strongly depending on the pressure applied [18,20,55,56]. At low pressures the Ar+ or Ar+2 abundance is lower in comparison to elevated pressures and therefore, recombination with electrons does not lead to a high metastable density. At high pressures, the Ar+ and Ar+2 may get neutralized not only by electron recombination, but also by two or three body recombinations with neutral gas molecules (charge exchange reaction). Therefore, a pressure dependence of the emission in the afterpeak was expected and demonstrated in the experiment summarized in Fig. 8. The data show a clear difference of the ratio of the afterpeak to the plateau. It seems that the afterpeak becomes more dominant at low pressure conditions. During GC-GDMS coupling it can be observed that there is a pressure optimum for molecular information. Normally the maximum of molecular information in a comparable GD source configuration was observed at pressures between 0.2 and 1 mbar. Therefore, the data shown in Fig. 8 are consistent to data observed in mass spectrometry. The data shown here are not in contradiction to models described in GD literature. Fig. 8 shows apparently only the optimum pressure condition for the afterpeak for this GD source configuration. The decrease of the metastable population in the afterpeak at high pressures can be expected due to an increased possibility of the precursor recombination with neutrals (data for lower pressure are not shown). Additionally, during the interpretation and comparison of absolute pressure values one has to keep in mind that the pressure measurement is not directly in the glow discharge chamber. This and the use of different pressure gauges in different studies can cause a difference in the absolute pressure values. A possible solution would be the measurement of the gas density by means of spectroscopic methods in the glow discharge chamber. This indicates, that the information about the Penning ionization (important for obtaining the molecular information in chemical speciation analysis [11,12]) can be evaluated without a mass spectrometer by using a more “simple” photodiode. 5. Conclusion Low pressure pulsed glow discharges in Ar were investigated in terms of breakdown conditions for different cathode materials and discharge frequencies. The breakdown behavior (Paschen plots) showed no significant dependence on the frequency. However such dependence was found on the cathode material, which is in agreement to steady-state glow discharge. Furthermore, the current–voltage profiles of different pressures, cathode–anode distances and pulse frequencies were studied. In addition, the investigation of power in dependence on the cathode–anode spacing showed a maximum at intermediate distances (~ 8–10 mm). These data are in agreement with ion intensity data obtained by GC-GDMS measurements reported elsewhere [17]. This indicates that ion intensities depend on the power of a glow discharge and is a critical parameter to be optimized in glow discharge mass spectrometry to determine the optimum sampling point for gaseous analytes. The evaluation of the current and voltage profiles showed that the normalized power of a pulsed glow discharge is not dependent on the applied frequency. This, in combination with the Paschen plots, is a strong indication that the ms glow discharge follows the principle equations of dc steady-state glow discharges. This leads to the conclusion that the operating parameters of the pulsed glow discharge can be optimized based on already established knowledge of dc steady-state glow discharges. For the use of the pulsed glow discharge as an ion source for mass spectrometry together with secondary analyte introductions this is an important conclusion since all previously described reports can be included in designing and optimizing such a system. However, these data represent pure Ar discharges only. Therefore, the extensively described quenching of the
plasma [9,57,58], will still effect the ionization and excitation properties of the discharge. That implies that the influence of impurities within the plasma gas on the discharge properties needs further fundamental studies. Acknowledgment This work was financially supported by ETH Zurich (research grant TH-16/04-3). The contributions to the system by the machine shop of DCHAB (ETH Zurich), in particular P. Trüssel, are greatly acknowledged. Furthermore, the authors like to thank J. Pisonero for fruitful discussions. C. Lewis and V. Majidi from Los Alamos National Laboratory, (NM, USA) are acknowledged for providing support during the design of the pulsed glow discharge source. K. Fuhrer and M. Gonin from Tofwerk (Thun, Switzerland) are acknowledged for continuous software support. The current probe was provided by the Laboratory of Physical Chemistry, ETH Zurich. References [1] J. Angeli, A. Bengtson, A. Bogaerts, V. Hoffmann, V.D. Hodoroaba, E. Steers, Glow discharge optical emission spectrometry: moving towards reliable thin film analysis — a short review, J. Anal. At. Spectrom. 6 (2003) 670–679. [2] A. Bogaerts, R. Gijbels, Fundamental aspects and applications of glow discharge spectrometric techniques, Spectrochim. Acta Part B 1 (1998) 1–42. [3] J.D. Craggs, J.M. Meek, The initiation of low pressure glow discharges, Proc. Phys. Soc. London 346 (1948) 327–340. [4] M.J. Druyvesteyn, F.M. Penning, The mechanism of electrical discharges in gases of low pressure, Rev. Mod. Phys. 2 (1940) 0087–0174. [5] C.L. Yang, M. Mohill, W.W. Harrison, Microsecond-pulsed Grimm glow discharge as a source for time-of-flight mass spectrometry, J. Anal. At. Spectrom. 9 (2000) 1255–1260. [6] C.L. Yang, E. Oxley, W.W. Harrison, A double pulse method for glow discharge enhancement, J. Anal. At. Spectrom. 10 (2001) 1131–1134. [7] C.L. Yang, K. Ingeneri, W.W. Harrison, A pulsed Grimm glow discharge as an atomic emission source, J. Anal. At. Spectrom. 4 (1999) 693–698. [8] C.L. Lewis, E.S. Oxley, C.K. Ban, R.E. Steiner, F.L. King, Determination of Ca-40(+) in the presence of Ar-40(+): an illustration of the utility of time-gated detection in pulsed glow discharge mass spectrometry, Anal. Chem. 1 (1999) 230–234. [9] D. Fliegel, R. Waddell, V. Majidi, D. Günther, C.L. Lewis, Quantification of aromatic and halogenated hydrocarbons and alcohol mixtures at the elemental, structural, and parent molecular ion level, Anal. Chem. 6 (2005) 1847–1852. [10] C.L. Lewis, M.A. Moser, W. Hang, D.E. Dale, D.C. Hassell, V. Majidi, Influence of discharge parameters on real-time chemical speciation for gas chromatography pulsed glow discharge plasma time-of-flight mass spectrometry, J. Anal. At. Spectrom. 6 (2003) 629–636. [11] C.L. Lewis, M.A. Moser, D.E. Dale, W. Hang, C. Hassell, F.L. King, V. Majidi, Timegated pulsed glow discharge: real-time chemical speciation at the elemental, structural, and molecular level for gas chromatography time-of-flight mass spectrometry, Anal. Chem. 9 (2003) 1983–1996. [12] V. Majidi, M. Moser, C. Lewis, W. Hang, F.L. King, Explicit chemical speciation by microsecond pulsed glow discharge time-of-flight mass spectrometry: concurrent acquisition of structural, molecular and elemental information, J. Anal. At. Spectrom. 1 (1999) 19–25. [13] M. Bernhard, F.E. Brinckman, P.J. Sadler, The Importance of Chemical “Speciation” in Environmental Processes, 1986. [14] J.A.C. Broekaert, S. Gücer, F. Adams, Metal Speciation in the Environment, , 1990. [15] G.P. Jackson, F.L. King, Bulk plasma properties in the pulsed glow discharge, Spectrochim. Acta Part B 8 (2003) 1417–1433. [16] A. Bogaerts, R. Gijbels, G.P. Jackson, Modeling of a millisecond pulsed glow discharge: investigation of the afterpeak, J. Anal. At. Spectrom. 6 (2003) 533–548. [17] D. Fliegel, K. Fuhrer, M. Gonin, D. Günther, Evaluation of a pulsed glow discharge time-of-flight mass spectrometer as a detector for gas chromatography and the influence of the glow discharge source parameters on the information volume in chemical speciation analysis, Anal. Bioanal Chem. 1 (2006) 169–179. [18] R.K. Marcus, J.A.C. Broekaert, Glow Discharge Plasmas in Analytical Spectroscopy, 2003. [19] R.K. Marcus, Glow Discharge Spectroscopy, Plenum press, New York, 1993. [20] B. Chapmann, Glow Discharge Processes, Wiley-Interscience, New York, 1980. [21] L. Li, J.T. Millay, J.P. Turner, F.L. King, Millisecond pulsed radio frequency glow discharge time of flight mass spectrometry: temporal and spatial variations in molecular energetics, J. Am. Soc. Mass. Spectrom. 1 (2004) 87–102. [22] G. Gamez, A. Bogaerts, G.M. Hieftje, Temporal and spatially resolved laserscattering plasma diagnostics for the characterization of a ms-pulsed glow discharge, J. Anal. At. Spectrom. 3 (2006) 350–359. [23] X.B. Tian, P.K. Chu, Experimental investigation of the electrical characteristics and initiation dynamics of pulsed high-voltage glow discharge, J. Phys. D: Appl. Phys. 3 (2001) 354–359. [24] A. Bogaerts, R. Gijbels, Hybrid Monte Carlo — fluid model for a microsecond pulsed glow discharge, J. Anal. At. Spectrom. 8 (2000) 895–905.
D. Fliegel, D. Günther / Spectrochimica Acta Part B 63 (2008) 630–637 [25] Y.P. Raizer, Gas Discharge Physics, Springer, Berlin, 1991. [26] A.P. Bokhan, P.A. Bokhan, D.E. Zakrevsky, Peculiarities of electron emission from the cathode in an abnormal glow discharge, Appl. Phys. Lett. 15 (2005). [27] G. Auday, P. Guillot, J. Galy, H. Brunet, Experimental study of the effective secondary emission coefficient for rare gases and copper electrodes, J. Appl. Phys. 11 (1998) 5917–5921. [28] Y. Chutopa, B. Yotsombat, I.G. Brown, Measurement of secondary electron emission yields, IEEE Trans. Plasma Sci. 5 (2003) 1095–1099. [29] A.V. Phelps, Z.L. Petrovic, Cold-cathode discharges and breakdown in argon: surface and gas phase production of secondary electrons, Plasma Sources Sci. Technol. 3 (1999) R21–R44. [30] A. Bogaerts, R. Gijbels, The ion- and atom-induced secondary electron emission yield: numerical study for the effect of clean and dirty cathode surfaces, Plasma Sources Sci. Technol. 1 (2002) 27–36. [31] A.P. Bokhan, P.A. Bokhan, D.E. Zakrevsky, Electron emission from a cathode doped by fast particles of a working gas, Tech. Phys. 9 (2005) 1233–1235. [32] A. Bogaerts, R. Gijbels, G. Gamez, G.M. Hieftje, Fundamental studies on a planarcathode direct current glow discharge. Part II: numerical modeling and comparison with laser scattering experiments, Spectrochim. Acta Part B 4 (2004) 449–460. [33] G. Gamez, A. Bogaerts, F. Andrade, G.M. Hieftje, Fundamental studies on a planarcathode direct current glow discharge. Part I: characterization via laser scattering techniques, Spectrochim. Acta Part B 4 (2004) 435–447. [34] C.V. Budtz-Jorgensen, J. Bottiger, P. Kringhoj, Energetic ion bombardment of the grounded anode in pulsed DC-glow discharges, Surf. Coat. Technol. 1 (2001) 104–110. [35] M.M. Pejovic, G.S. Ristic, J.P. Karamarkovic, Electrical breakdown in low pressure gases, J. Phys. D: Appl. Phys. 10 (2002) R91–R103. [36] H. Eichhorn, K.H. Schoenbach, T. Tessnow, Paschen Law for a hollow-cathode discharge, Appl. Phys. Lett. 18 (1993) 2481–2483. [37] P.D. Mixon, S.T. Griffin, J.C. Williams, Breakdown voltage scaling characteristics of an Argon hollow-cathode discharge, J. Appl. Spectrosc. 10 (1993) 1567–1570. [38] P.A. Bokhan, D.E. Zakrevsky, Surface condition and electron emission from cold cathodes in vacuum and in noble gas glow discharge, Tech. Phys. 1 (2007) 104–112. [39] H.B. Smith, C. Charles, R.W. Boswell, Breakdown behavior in radio-frequency argon discharges, Phys. Plasmas 3 (2003) 875–881. [40] P. Guillot, P. Belenguer, L. Therese, V. Lavoine, H. Chollet, Secondary electron emission coefficients of standard samples for GDOES, Surf. Interface Anal. 7 (2003) 590–592. [41] S.C. Brown, W.P. Allis, Basic Data of Electrical Discharges, MIT, Cambridge, 1956. [42] A.A. Garamoon, A. Samir, F.F. Elakshar, E.F. Kotp, Electrical characteristics of a DC glow discharge Plasma Sources, Sci. Technol. 3 (2003) 417–420. [43] M.D. Wagenaars, M.D. Bowden, G.M.W. Kroesen, Plasma emission imaging of a low-pressure argon breakdown, Plasma Sources, Sci. Technol. (2005) 342–350.
637
[44] O.M. Stepanovic, M.K. Radovic, C.A. Maluckov, The glow duration time influence on the ionization rate detected in the diodes filled with noble gases on mbar pressures, Facta Universitatis, Phys. Chem. Technol. 5 (2003) 259–267. [45] I. Popovic, V. Rajovic, M. Zlatanovic, Dynamic Voltage–Current Characteristics of Unipolar Pulse Glow Discharge, 2005, pp. 315–320. [46] S.L. Tong, W.W. Harrison, Glow-discharge sputtering of chromium and niobium disk cathodes in Argon, Anal. Chem. 12 (1984) 2028–2033. [47] D. Fang, R.K. Marcus, Parametric evaluation of sputtering in a planar, diode glowdischarge.1. Sputtering of oxygen-free hard copper (Ofhc), Spectrochim. Acta Part B 12 (1988) 1451–1460. [48] M. Dogan, K. Laqua, H. Massmann, Spectrochemical analysis with glow discharge lamp as light source.1. Electric properties, sample decomposition and spectral characteristic, Spectrochim. Acta Part B 10 (1971) 631–&. [49] X.B. Tian, B.Y. Tang, P.K. Chu, Accurate determination of pulsed current waveform in plasma immersion ion implantation processes, J. Appl. Phys. 7 (1999) 3567–3570. [50] G.P. Jackson, C.L. Lewis, S.K. Doorn, V. Majidi, F.L. King, Spectral, spatial and temporal characteristics of a millisecond pulsed glow discharge: metastable argon atom production, Spectrochim. Acta Part B (2001) 2449–2464. [51] C.L. Lewis, G.P. Jackson, S.K. Doorn, V. Majidi, F.L. King, Spectral, spatial and temporal characterization of a millisecond pulsed glow discharge: copper analyte emission and ionization, Spectrochim. Acta Part B 5 (2001) 487–501. [52] A. Bogaerts, R. Gijbels, Two-dimensional model of a direct current glow discharge: description of the argon metastable atoms, sputtered atoms, and ions, Anal. Chem. 15 (1996) 2676–2685. [53] C.K. Pan, F.L. King, Ion formation processes in the afterpeak time regime of pulsed glow-discharge plasmas, J. Am. Soc. Mass Spectrom. 9 (1993) 727–732. [54] A. Bogaerts, The afterglow mystery of pulsed glow discharges and the role of dissociative electron–ion recombination, J. Anal. At. Spectrom. (2007) 502–512. [55] M.K. Levy, D. Serxner, A.D. Angstadt, R.L. Smith, K.R. Hess, Optical investigations of excitation processes responsible for ionized sputtered species in a low-pressure, low current, coaxial geometry glow-discharge, Spectrochim. Acta Part B 2 (1991) 253–267. [56] K.A. Hardy, J.W. Sheldon, Metastable atom density in helium, neon, and argon glow-discharges, J. Appl. Phys. 12 (1982) 8532–8536. [57] G.P. Jackson, F.L. King, Probing excitation/ionization processes in millisecondpulsed glow discharges in argon through the addition of nitrogen, Spectrochim. Acta Part B 2–3 (2003) 185–209. [58] A. Bogaerts, R. Gijbels, Effects of adding hydrogen to an argon glow discharge: overview of relevant processes and some qualitative explanations, J. Anal. At. Spectrom. 4 (2000) 441–449.