rare gas microwave plasmas

Spectrochunrco Acfo, Vol Printed m Great Bntam.

47B. No. 6, PP 835-841,

1992 0

n58‘68547/92 $5.00 + .oo 1992 Pergamon Press Ltd

Factors influencing iron excitation in nitrogen/rare gas microwave plasmas J. T. CLAY and T. M. NIEMCZYK* Department

of Chemistry, University of New Mexico, Albuquerque,

NM 87131,

U.S.A.

(Received 14 August 1991; accepted 20 November 1991) Abstract-The results of experiments involving the use of a microwave-induced plasma operating at atmospheric pressure are discussed. The plasma gases range from pure argon, helium or nitrogen, to mixtures of rare gas and nitrogen. When nitrogen is present, the excitation mechanisms can change dramatically due to the influence of metastable nitrogen molecules. The evidence indicates that nitrogen metastables become important when the nitrogen concentration in the plasma exceeds 3%. Further, data shows that the vibrational states of the nitrogen molecules involved in the excitation process are very important, and can be justified on the basis of Franck-Condon factors.

1. INTRODUCTION PURE N2 or rare gas/N, mixed plasmas are of interest because of their utility as analytical sources. These plasmas exist in a variety of forms ranging from high-power atmospheric pressure discharges to low-power low-pressure afterglows. Nitrogen and argon/nitrogen afterglows have been employed in reduced pressure systems for both atomic emission spectroscopy (AES) and molecular emission spectroscopy. These afterglows have been shown to give low detection limits and wide linear ranges for many elements [l-6]. Active nitrogen afterglows at atmospheric pressure have also been employed for analytical purposes by FASSELet al. [7,8]. In an attempt to exploit the collisional environment of an active nitrogen plasma, SELTZERand GREEN have explored an atmospheric pressure microwave-induced nitrogen plasma as an atom reservoir for laser-induced ionization spectrometry [9]. The chemical and spectroscopic nature of an afterglow changes significantly with parameters such as the discharge used to generate the afterglow, the pressure, and the temperature of the system. One of the most common discharges employed to initiate an afterglow is the microwave discharge. Recently, effort has been directed toward employing a pure nitrogen microwave induced plasma (MIP) to be operated in the open atmosphere for AES [lO,ll]. Generally, the nitrogen plasma is viewed radially so that the intense molecular emission from the discharge inside the cavity does not complicate the background spectrum. Since fairly small quantities of hydrocarbons entering a microwave discharge can destabilize or extinguish the discharge, the afterglow region of nitrogen containing plasma is also of interest in chromatographic applications. Present MIP systems used for gas chromatography detection generally consist of a helium plasma and employ complicated venting systems to keep the solvent front from extinguishing the plasma. The tail section of an open-atmosphere nitrogen-containing MIP, and the other nitrogen afterglows discussed here, represent a fundamental change from pure rare gas plasmas, since both nitrogen metastables and atoms display a high degree of chemical reactivity [12]. The reactivity, as well as the ability to excite atoms and molecules, is the reason a flow of nitrogen molecules, atoms, and ions (some in excited states) is widely referred to as active nitrogen, This reactivity can be exploited to effectively derivatize elements such as carbon, phosphorus, and sulfur to nitrogencontaining radicals which can yield strong molecular emission that can be monitored * Author to whom correspondence

should be addressed. 835

836

J. T. CLAY and T. M. NIEMCZYK

for analytical purposes [13,14]. By employing the afterglow, hydrocarbons need not be routed through the discharge inside the microwave cavity. This alleviates the problem associated with destabilization of the discharge due to sample introduction. Despite the applications of nitrogen-containing afterglows, the fundamental spectroscopic character of mixed argon/nitrogen microwave-induced plasmas is not well known. The work reported here includes a description of a cavity that allows operation at atmospheric pressure using pure or mixed gases. Of greater importance are experiments that point out factors that influence excitation in active nitrogen plasmas. The primary excitation mechanism in active nitrogen is a collision of the second kind between a nitrogen metastable and a ground state atom. This results in a ground (electronic) state nitrogen molecule and an excited state atom. Studies show that excitation involving nitrogen metastables can be extremely efficient [1,3,4]. It has been claimed the high efficiency of excitation occurs because the ground state nitrogen molecule, produced in the collisional excitation, can take up any non-resonance between the excited atom and the nitrogen metastable in the form of vibronic energy. This is certainly the case, but the process is more restrictive than first thought. We will present evidence that Franck-Condon factors between the vibronic states of the nitrogen molecules are more important than initially realized and must be considered when active nitrogen is responsible for, or contributes to, excitation in systems such as nitrogen or mixed gas plasmas. 2. EXPERIMENTAL A TMolo Beenakker type microwave cavity was constructed so that a variety of gases could be used as the plasma gas. The cavity diameter was 90.5 mm. This dimension was suitable for sustaining a variety of different plasmas using quartz or ceramic torches. Pure argon, helium, or nitrogen plasmas were sustained as well as various mixtures of helium and nitrogen or argon and nitrogen. Two quartz tuning rods, diametrically opposed, assisted in achieving a resonant frequency in the cavity. The cavity was made of brass and electrolytically coated with silver. The antenna was a 3.2 mm diameter brass welding rod fastened to the floor of the cavity with low temperature silver solder. The cavity face plate was 9 mm thick, and dual tuning stubs were threaded through the face plate into the brass antenna to permit impedance matching. A Micro Now Model 420B microwave power supply was used as the power source. The light from the plasma was viewed radially since the afterglow was of primary interest. The plasma was imaged onto the entrance slit of a 0.3 m GCA McPherson Model 218 monochromator using a 10 cm focal length plano-convex quartz lens in a configuration designed to fill the grating with light, Line intensities were determined using a 2400 line/mm grating blazed at 300 nm in the first order. The plasma region from 3 mm above the faceplate to 8 mm above the faceplate was sampled by the monochromator. The signal from an RCA Model IP28 photomultiplier was sent to a Keithley Model 427 current-to-voltage amplifier. The current-toamplifier output was digitized and recorded using a laboratory computer. Integration of line intensities and background correction were performed via software routines. Gas flows were metered using Matheson Model 603 and 605 rotometers which were laboratory calibrated for argon and nitrogen. The torches used for this study were quartz and consisted of two concentric tubes and a side arm for admitting gas to the larger diameter tube (see Fig. 1). The iron was introduced into the plasma through the center tube only, which was flared at the outlet in an attempt to create a more stable, centered discharge. Careful torch design and construction were found to be crucial in the case of nitrogen-containing plasmas because any prolonged contact of the plasma with the quartz etches the torch. Improperly aligned torches resulted in plasma contact with a torch wall, hence rapid etching. As the concentration of nitrogen in the plasma increased, this problem was more noticeable. Iron was introduced into the plasma in the form of Fe(C0) 5 which was allowed to evaporate from a container into the gas flow introduced to the center tube. This compound was chosen as prior studies have shown it to be a precisely controllable and continuous method of introducing Fe to a plasma that might be affected by more conventional sample introduction methods [15]. The rate of evaporation was controlled by suspending the container in a constant temperature bath. The argon (Matheson, SNS)/nitrogen(Matheson, 5N5) mixtures were used for both side arm and center tube inlets of the torch, and the total flow rate was kept at a constant 13 Urnin

Iron excitation in N,/rare gas microwave plasmas

837

I 8mmI I

14mm

Fig. 1. Cross section view of the quartz torch.

for all mixtures. After a few hours of operation with Fe(CO)S flowing through the torch, the center tube blackened with carbonaceous deposits. These deposits were removed by soaking the torch in aqua regia for a few hours.

3.

RESULTS AND DISCUSSION

The temperature determinations were made using the two-line methods of KIRKBRIGHT al. [16]. Using this procedure, line intensities are related to the electronic temperature of the plasma by: ef

IJ_ 12

-

&fljG exp

-~ &tA:

(Ek - E,)/kT

where g, and & are the degeneracies of the upper energy levels leading to emission at Al and X2, respectively, and fj and fk(are the oscillator strengths of the respective transitions. E, is the upper energy level of the transition corresponding to Al, while Ek corresponds to A2. This expression assumes that local thermodynamic equilibrium (LTE) prevails in the plasma. Adherence to the LTE conditions are questionable, even when operating such a plasma at atmospheric pressure. The use of molecular gases should aid in the establishment of equilibrium, but the temperatures measured should be considered on a relative scale. Although iron emits a complex line-rich spectrum when introduced into MIP, choosing a line pair for the temperature measurements was complicated by the background spectrum of the molecular nitrogen. The iron lines at 371.99 nm (5F”5 + 5D,) and 373.49 nm (5F5 + 5F”5) were chosen for this study because this region of the spectrum showed no significant background. In addition, the optimum temperature of the line pair for minimizing self absorption effects was calculated as 5850 K using the oscillator strength data of BRIDGES and KORNBLITH [17]. Further, when metastables are potentially involved in the excitation process, transitions involving identical spin changes must be used. The selection rules for an excitation process involving a collision of the second kind, e.g. an energy transfer between a nitrogen metastable and an iron atom, are strongly dependent on spin [l&19]. Anomalous

J. T.

838

1

3000

0

1

CLAY

and T. M. NIEMCZYK

r;,C&&lTSRA~ONi%) 8 9

Fig. 2. Excitation temperature

vs N, concentration

10

in the plasma support gas.

results are obtained when transitions involving different spin changes are used in the excitation temperature measurement [ 151. Argon/nitrogen plasmas containing from 100% Ar/O% N2 to 90% Ar/lO% N2 were used to excite iron emission. A plot of electronic excitation temperature vs plasma composition is shown in Fig. 2. The error bars in Fig. 2 represent the standard deviations in a series of repetitive measurements. The input power was kept at 185 W for all the temperature measurements shown in this figure. Although the intensity of the emission lines was affected by the power level, the excitation temperature was unaffected over the range of input power level studied from 145 to 225 W. The excitation temperature of the pure argon plasma is over 5700 K, and remains high until the plasma contains 3% NZ. Then, a precipitate.drop in temperature occurs. This drop may mark the composition of the plasma at which the nitrogen becomes a major energy donor for exciting iron emission. Figure 3 shows that the intensities of the lines at 371.99 nm and 373.49 nm reach maxima at 3% Nz. When more than 3% N2 is present in the plasma, the intensities of the lines begin to drop while the electronic temperature remains well below that of a pure argon plasma. The intensities of eleven iron lines other than those used for the temperature determinations shown in Fig. 2 were measured as a function of nitrogen concentration. All iron lines monitored were between 371 nm and 384 nm so as to avoid worrying about grating and detector efficiency changes. The intensities of all lines monitored showed identical features to those of the lines at 371.99 nm and 373.49 nm plotted in Fig. 3, i.e. an increase in intensity with nitrogen concentration until a maximum at 3%, followed by a decrease in intensity with further increase in nitrogen concentration. All line combinations with like spin were evaluated for temperature determinations, but the line combination used for the data in Fig. 2 gave the-best fit to the criteria of line selection for the two-line method [16]. The dramatic intensity changes seen in Fig. 3, when the nitrogen concentration in

7

. . . .

p1

s

9 (,

El-

a

54.

.

. .

.

* . .

.

. l

0

.

3 0

& CONOhRAT!ON (%I B

10

Fig. 3. Intensity of iron line emission vs N, concentration in the plasma support gas: 7, 373.49 nm; 0, 371.99 nm.

Iron excitation in N,/rare gas microwave plasmas

839

24-

INTERNUCLEAR DISTANCE (A) Fig. 4. Nitrogen energy levels involved in active nitrogen excitation.

the plasma reaches 3%, are analogous to behavior noted in prior research into mixed argon/nitrogen plasmas [20]. These prior studies were at low pressure. Only small changes in metal atom intensities occurred as the nitrogen concentration in the plasma was changed from 0% to 3-4%; after this point dramatic intensity changes were seen. In contrast to the changes noted here, the intensity changes were positive and lines that involved a spin change of +2 increased more dramatically than did lines where no spin change was involved. This is explained by the excitation by N2 (A3Z:), which favors transitions that conserve spin [18,19]. The lines monitored here were carefully chosen to avoid any complication due to spin states. The fact that the Fe line intensities change abruptly, beginning at about 3% nitrogen, suggests that nitrogen begins to play an important role in the excitation process at that point. Further evidence of this is the change in pattern seen in the intensities of the two Fe lines shown in Fig. 2. Note that the intensity for the line at 371.99 nm decreases more dramatically than does the line at 373.49 nm. The upper state of the line at 371.99 nm is 4.18 eV above the ground state, while the upper state of the line at 373.49 nm is 3.33 eV above the ground state. When considering excitation into these levels, one must take into account the excited levels of nitrogen available for energy transfer. The excitation of nitrogen molecules in the afterglow region of a discharge might either be due to electron bombardment, recombination of nitrogen atoms produced in the discharge, or energy transfer from argon metastables. In any case, all routes produce significant concentrations of Nz (A32:). In low pressure discharges, the highest populations of N2 (A3C:) are produced in vibronic levels 2-8 [21,22]. The populations of these excited vibronic levels are subject to rapid quenching when molecules that are efficient quenchers are present. Ground state nitrogen molecules, Nz (XK’Z:), are effective vibronic quenchers. Indeed, the population of the vibronic levels of the N2 (A32:) has been shown to be dependent on pressure, with almost all N2 (A32:) vibrational levels quenched to the 0 or 1 vibrational states at pressures above 20 torr [22,23]. Thus, excitation due to N2 (A3Z:) in the atmospheric pressure system discussed here would be due mainly to energy transfer from the 0 or 1 vibronic level. The energy levels of the nitrogen molecule that play a role in active nitrogen excitation processes are shown in Fig. 4. As can be seen in this figure, N2 (A32,+) has more than enough energy to excite either state of the Fe atom considered here, even

840

.I. T. CLAYand T. M. NIEMCZYK

WAVELENGTH

(nm)

Fig. 5. Background spectra of the afterglow region of the microwave induced plasma: A, 100% Ar; B, 96% Ar14% N,.

if the entire population of the N2 (A3Z:) is in the lowest vibrational level. The high efficiency of excitation of metal atoms due to collision with N2 (A3C:) is stated to be a consequence of the ability of N2 (XlZb) to act as a sink for any excess energy. Examination of Fig. 4 indicates that the vibronic energy that can be accommodated by N2 (X1x:) spans a wide range, hence the above statement is reasonable at first glance. When considering transitions between electronic energy levels of the nitrogen molecule, the geometries of the initial and final states (i.e. the vibrational states), must be taken into account. The geometries are reflected in the Franck-Condon (FC) factors for the various transitions. The role of FC factors in the collisional energy between helium metastables and nitrogen molecules [24] and between active nitrogen species and copper atoms [25,26] has been discussed. FC factors should influence the energy transfer between N2 (A3C,+) and iron atoms as well. When the N2 (A3Cz) molecule is in the lowest vibrational level, as would be expected here, then excitation of the 371.99 nm Fe line results in N2 (XlZg, v = 7) and excitation of the 373.49 nm Fe line results in N2 (X!ZC,+,v = 11). The FC factors for these transitions are 0.16 and 0.010, respectively [27]. If these factors play a role, then the differences in the intensity will change with increasing nitrogen content, as expected. Indeed, the intensities for the two Fe lines shift from being nearly equal, to a ratio near that of the FC factors for the two transitions. The excitation temperature determination, which is a measure of the relative populations of the two Fe excited states, reflects the trend. Further evidence that N2 (A”Z:) plays a significant role in the afterglow region is shown in Fig. 5. The spectra in Fig. 5 are the background spectra of discharges containing pure Ar and Ar/3% Nz. In the afterglow of a pure argon discharge, many Ar lines are present. The Ar line intensities decrease slightly as the nitrogen concentration in the plasma is increased from 0 to 2%. These lines are abruptly quenched when the nitrogen is increased to 3% (Fig. 5, spectrum B). The afterglow region is considerably downstream from the primary discharge so radiation emanating from the afterglow must be the result of long-lived species or species that are populated in the afterglow region. Ar has metastable levels at 11.54 eV and 11.72 eV that are reported to be precursors of Ar lines in Ar afterglows. Also, these Ar metastable levels readily transfer their energy to N2 to produce N2 (C’II,) [28,29]. Both the first and second positive band systems of nitrogen are present in the 3% nitrogen spectrum which suggests that, indeed, the Ar metastables are quenched in a collision with N2 (X1x;) resulting in ground state Ar and Nz (C311,). The v = 1 and v = 2 levels of N2 (C”II,) are nearly resonant with two Ar metastable levels, and the FC factors for the transitions N2 (X1x:,+) + Nz (C31’I,) are large [27]. Indeed, it has been argued that one of the reasons why the energy exchange between Ar and Nz is efficient is because of the favorable FC factors for these transitions 1301. A result analogous to that shown in Fig. 5 was noted when He was used as the major component of the plasma gas. When pure He was used, He lines were present in the afterglow spectrum. When nitrogen was introduced, the He lines decreased

Iron excitation in N,/rare gas microwave plasmas

841

slightly at first and then totally quenched when N2 was increased from 3 to 4%. The contained belonging to nitrogen afterglow spectrum bands the 4% NT (B*Z:) -j N2+ (X*IZg+)system indicating that the He metastables transferred their energy to nitrogen in a Penning-type reaction. There is a close energy match between N2+ (B*ZT) and the He metastables, a fact that makes this reaction efficient. Again, the FC factors for this reaction are very favorable [27].

4. CONCLUSION Excitation of metal atoms as a result of a collision of the second kind between a nitrogen metastable N2 (A3Z:), and a metal atom is known to be highly efficient. The high efficiency has been accounted for by assuming that the product nitrogen molecules, N2 (XrCl), could absorb any excess energy in vibronic states. Franck-Condon factors that take into account the geometries of the initial and final states of molecular transitions are known to be very important in optical transitions. Although the nitrogen transition of major importance in active nitrogen excitation is the result of a collision, and resonance certainly plays a role, the evidence presented here indicates Franck-Condon factors can be used to predict the transition efficiency. REFERENCES [l] G. A. Capelle and D. G. Sutton, Appl. Phys. Lerr. 30, 407 (1977). Anal. Chem. 54, 826 (1982). [3] W. B. Dodge and R. 0. Allen, Anal. Chem. 53, 1279 (1981). [4] H. C. Na and T. M. Niemczyk, Anal. Chem. 54, 1839(1982). [5] H. C. Na and T. M. Niemczyk, Appl. Specrrosc. Rev. 19, 363 (1983). [6] W. H. Hood and T. M. Niemczyk, Anal. Chem. 59, 2468 (1987). [7] A. P. D’Silva, G. W. Rice and V. A. Fassel, Appl. Specrrosc. 34, 578 (1980). [8] G. W. Rice, A. P. D’Silva and V. A. Fassel, Appl. Specfrosc. 38, 148 (1984). [9] M. D. Seltzer and R. B. Green, Specrrosc. Len. 20, 601 (1987). [lo] G. Hieftie and R. Deutsch, Appl. Specrrosc. 39, 214 (1985). [ll] K. G. Michlewicz, J. J. Uhr and J. W. Carnahan, Specrrochim. Acra 4OB,493 (1985). [12] M. P. Ianuzzi and F. Kaufman, J. Phys. Chem. 85, 2163 (1982). [13] W. H. Hood and T. M. Niemczyk, Anal. Chem. 58, 210 (1986). [14] D. G. Sutton, K. R. Westberg and J. E. Melzer, Anal. Chem. 51, 1399 (1979). [15] W. H. Hood and T. M. Niemczyk, Appl. Specrrosc. 41, 674 (1987). [16] G. F. Kirkbright, M. Sargent and S. Vetter, Specrrochim. Acra 25B, 465 (1970). [17] J. M. Bridges and R. L. Kornblith, Asrrophys. J. 192, 793 (1974). 1181 A. C. G. Mitchell and M. W. Zemansky, Resonance Radiaripn and Excited Atoms. Cambridge University Press, London (1961). [19] C. J. Duthler and H. P. Broida, J. Chem. Phys. 59, 167 (1973). [20] B. D. Thompson, M.Sc. Thesis, University of New Mexico (1986). [21] D. E. Shemansky, J. Chem. Phys. 64, 565 (1976). [22] W. Brennan, R. V. Gutowski and E. C. Shane, Chem. Phys. Lerr. 27, 138 (1974). [23] J. F. Noxon, J. Chem. Phys. 36, 926 (1962). [24] W. W. Robertson, J. Chem. Phys. 44, 2456 (1966). [25] I. Nadler and S. Rosenwaks, Chem. Phys. Len. 69. 266 (1980). [26] I. Nadler, G. Rawnitzki and S. Rosenwaks, J. Phys. Chem. 86, 1503 (1982). [27] A. Lofthus and P. H. Krupenie, J. Phys. Chem. Ref. Daru 6, 113 (1977). [28] D. H. Stedman and D. W. Setser, Chem. Phys. Lerr. 2, 542 (1968). [29] D. W. Setser, D. H. Stedman and J. A. Coxon. J. Chem. Phys. 53, 1004 (1970). [30] E. S. Fishburne, J. Chem. Phys. 47, 58 (1967).

[2] T. H. Ramsey and M. D. Nelson,