hydrogen capacitively coupled microwave plasma

Specrrochrmrca Aclo, Vol. 478. Prmted m Great Bntam.

No. 4. pp. 481-191.

05X+8547/92 $S.(Xl + .tM @ 1992 Pergamon Press plc.

1992

Temperature and electron density measurements in a helium/hydrogen capacitively coupled microwave plasma* W. R. L. MASAMBA,?A. H. ALI+ and J.D. Department

WINEFORDNER§ of Chemistry, University of Florida, Gainesville, FL 32611, U.S.A.

(Received

12 September

1991; accepted 23 September

1991)

Abstract-Various temperature and electron density measurements have been carried out in a capacitively coupled microwave plasma (CMP) with a mixture of hydrogen and helium as the plasma gas using a photodiode array spectrometer for detection. Fe I lines were used for excitation temperature, OH lines for rotational temperature and the H, line for electron number density measurements. Rotational temperatures were found to be between 1800 K and 3000 K, excitation temperatures varied between 2000 K and 5000 K and electron number densities varied between 4 X lOi cm-’ and 9 X 1Or4cm-‘, depending on operating conditions. The influence of power, observation position, solution uptake rate and carrier gas flow have been investigated.

1. INTRODUCTION MICROWAVE plasmas are classified, according to the means by which energy is transferred to the plasma, into microwave induced plasma (MIP) and capacitively coupled microwave plasma (CMP). In the MIPS, the microwave energy is coupled to the gas stream in a discharge tube with an external resonant’ cavity, an antenna or other supporting structure while in a CMP, a magnetron generates microwaves which are conducted through a coaxial waveguide to the tip of a central single electrode where the plasma is formed [l]. The MIPS have better stability than CMPs at comparable power, but usually have to be operated at low power levels in order to avoid melting the quartz tube used to contain the plasma, or overheating of coaxial cables. Low power MIPS are also intolerant to the introduction of molecular species. Introduction of aerosols often cause impedance mismatch of the system, thereby extinguishing the plasma. CMPs suffer from high background, poorer signal to noise ratios and increased interferences from contaminants of the plasma from the electrode material [l-4]. Electrode erosion causes contamination and necessitates replacement of the electrode from time to time. Despite these drawbacks, CMPs are stable over a wide range of power levels, more tolerant to the introduction of molecular species, and can be sustained with a wide range of gases [l]. We recently reported the construction of a high power He CMP utilizing a graphite electrode [5]. This plasma has lower background emission compared to systems employing metallic electrodes. The limits of detection determined using the system were comparable to those by inductively coupled plasma (ICP) emission. This work characterizes this plasma further by the determination of excitation (T,,,) and rotational (T,,,) temperatures and electron number densities (n,). Helium plasmas have been employed in order to take advantage of the high ionization energy of helium which should result in efficient excitation of the elements introduced into the plasma, especially elements of high excitation energies such as arsenic, halogens, nitrogen, oxygen, tellurium and tin [6]. It has been shown that the limits of detection of chlorine and bromine were lower in the He ICP compared to the Ar ICP [6]. The electrical resistivity, heat capacity and thermal conductivity for helium, * Research supported by NIH-5-ROl-GM38434-04. t On study leave from University of Malawi, P.O. Box 280, Zomba, Malawi. $ Present address: Texaco Inc., R & D, P.0 Box 1608, Port Arthur, TX 77641, U.S.A. I Author to whom correspondence should be sent. 481

482

W. R. L. MASAMBA et al.

diode

array

Spectrometer

I r-l

ti

magnetron

torch

Fig. 1. Experimental set-up for the CMP system.

however, are greater than for argon. This leads to faster dissipation of heat towards the torch walls than in the case of argon, raising difficulty in the design of torch configurations [7] as the He plasma tends to adhere to the torch walls. Work on helium plasmas has therefore developed concurrently with special torch designs [7-131. In Ar CMPs, it is necessary to wet the plasma gas with water for stability [3, 141. This is not necessary with He CMPs. Like the He ICPs, there is still the tendency in our system for the plasma to reside on the walls of the torch when pure helium is used. Addition of a small amount of hydrogen (or nitrogen) solves this problem. CHAN and MONTASER [6] mention that He ICPs usually require a mixture of other gases such as Ar and air for stability.

2. EXPERIMENTAL 2.1. Instrumental set-up

The experimental set-up used is shown in Fig. 1 and the torch assembly is shown in Fig. 2. The components are listed in Table 1. The plasma was generated with a system similar to that reported previously [5] except that a different waveguide, the same as described elsewhere [X-17], was used. This waveguide resulted in better long term stability of the plasma. The torch

a

I

I

27

4*

4 /

-

b

44F’O4

Fig. 2. Torch and electrode system (a) and electrode (b). All measurements are in mm.

Electron density measurements Table 1. Instruments

and components

483 for CMP

Instrumen~com~nent

Manufacturer

Diode array: OSMA model IR4-1024 Spectrometer: Jobin-Yvon HR 1000, 1 m, 2400 grooves mm-‘, linear dispersion 0.5 nm mm-’ Software (r-120) OSMA detector controller PC computer High voltage d.c. power supply: Model 8WtA (maximum power ouput 1.6 kW)

Princeton Instruments, Princeton Instruments

Princeton, NJ, U.S.A.

Princeton Instruments Princeton Instruments IBM Hipotronics, Brewster, NY, U.S.A.

Magnetron: model NL10251-2 (frequency 2.45 GHz, maximum power output 1.6 kW)

National Laboratory,

Electrode (Spex graphite rod; grade HPND) Torch: two concentric quartz. tubes

Laboratory made Laboratory made

Orlando, FL, U.S.A.

used was a two tube, tangential flow torch made of quartz, and had a 5.4 cm long graphite electrode inserted in the inner tube. It had been established by ALI [18] that the optimum length of the electrode inside the waveguide should be X/4, where X is the wavelength of the microwaves. In the present system, x14 corresponded to 3 cm. The rest of the electrode was above the waveguide. Under these conditions, approximately 100% of the D.C. power was converted to microwave energy. The microwave power was therefore estimated by taking the product of the voltage and current on the D.C. power supply. A mixture of 92.5% helium and 7.5% hydrogen by volume was used throughout as the plasma gas. Unless specified otherwise, all measurements were carried out at 700 W, with plasma gas and carrier gas flow rates of 6 llmin and 900 ml/min, respectively. The photodiode array had 20 nm spectral range, and no spectral response correction was applied. The use of a photodiode array detection for temperature measurements has several advantages over scanning methods [19]: the spectra can be acquired rapidly and since the intensities of all lines are measured simultaneously, errors in the relative intensities measured are minimized; spectral background resulting from solvent species and plasma background can be conveniently removed in, for example, the iron spectra used for excitation temperature measurements; and lines used were chosen such that (1) they fell within one wavelength window, (2) there was no significant overlap of the lines, and (3) the line intensities should fall within the dynamic range of the array during a fixed integration time. 2.2. Measurement of excitation temperatures Iron excitation temperatures were measured using six emission lines in the 370-377 nm region, Intensities of the following lines (nm) were used for the Boltzmann plot: 372.0; 373.7; 374.8; 374.9; 375.8; and 376.4. In order to increase the resolution, the fourth order setting on the spectrometer was used. Figure 3 shows a typical iron spectrum used in this work. Iron was selected since it has accurately known gf values. The average values from Refs [20-233 were used {see Table 2). 2.3. Measurement

of rotational temperatures

Rotational temperatures were determined from relative intensities of rotational lines of the Qi branch of the (0,O) transition of the OH radical in the 307-310 region. Figure 4 shows the spectrum in this region. The A-values used for the Boltzmann plot were taken from Ref. [24]. Five lines (Qi2, Q,4-Qi6, Qi9) were used. The temperature was obtained from the slope of log (IA/A) vs E (in cm-‘). In all cases, straight lines were obtained. 2.4. Measurement of electron number dens& The method used was that based on the Stark broadening of the He (486.1 nm) line. The Stark full width at half-m~imum (FWHM), AXs, and the electron density are related by the equation [25]: AAs = 2.50 x 10e9 alnn,2’3 where a1,2 is the semi-half-width 5000 K [25] were used.

of the reduced

Stark profile; alIz values corresponding

to

W. R. L.

484

36s

370

371

MASAMBA

et al.

374 372 373 Wavelength (nm)

375

376

‘7

Fig. 3. Spectrum of the neutral iron emitted by the CMP.

Table 2. Wavelengths, excitation energies and log (d for neutral iron used for excitation temperature measurements

A (nm) 373.7 374.8 374.9 375.8 376.4

307

E (cm-‘)

Ref. [20]

Ref. [21]

Ref. [22]

Ref. [23]

Average

27 167 27 560 34 040 34 329 34 547

-0.57 -0.98 0.18 0.00 -0.18

-0.57 -1.01 0.17 0.00 -0.19

-0.58 -1.00 -

-0.58 0.16 -0.03 -0.24

-0.575 -0.997 0.170 -0.010 -0.203

307.5

308 308.5 Wavelength (nm)

309

Fig. 4. Spectrum of the (0,O) band of OH emitted by the CMP.

3ii 9.5

Electron density measurements 5000

485

,

4000

i

z?

_ _L

_ L

T I

T I

IL+*

0’ -5

_ _

a

-

-

+ICf

I

I

,

I

I

I

I

/

I

-4

-3

-2

-1

0

1

2

3

4

Distance

Fig. 5. Excitation temperature

from

plasma

center

(a) and rotational temperature

J 5

(mm)

(b) vs radial distance.

3. RESULTS AND DISCUSSION 3.1. Precision

The relative standard deviation for excitation temperature measurements and electron number density measurements was generally less than 7% while the relative standard deviation for rotational temperature measurements was less than 10% (5 measurements in all cases). The error bars given in the figures correspond to 1 standard deviation for 5 measurements. 3.2. Effect of radial position Figures 5 and 6 show the effect of the radial position on T,,,, Trot and II,. In all three cases, the temperatures and electron number densities are almost constant across

700

i5-

.

560-

‘E

Li

0 c

T L\

T L1

1

I

1

1

I

I

T

n

T 9 L

1

T

n

T

n

11

T

T n

f

1

420-

$fi s; s

280-

b Y

W

140-

I

0 -8

-6

-4 Distance

-2 from

I

I

I

I

J

0

2

4

b

8

plasma

center

(mm)

Fig. 6. Electron number density vs radial position.

486

MASAMBA et al.

W. R. L.

6ooo

I

looI : 0

4

2

6

Distance

Fig. 7. Excitation temperature

from

8 electrode

10

12 top

(a) and rotational temperature

14

16

(mm)

(b) vs axial position.

the plasma within the distances measured. T,,, had a slight dip -1 mm from the center of the plasma, but the difference between the highest and lowest temperatures determined was only 200 K. T,,, showed the same trend except that the slight dip was at +l mm from the plasma center. There is also a decrease of T,,, at -4 mm from the plasma center. Again the variations are rather small, with a temperature range of about 400 K. The electron density showed a very slight increase near the center of the plasma, but the difference between the it, values in the range was still very small. The T,,, results are in qualitative agreement with other workers. WORKMANef al. [26] observed a minimum T,,, at the center of their He MIP. Their excitation temperatures, however, were higher (about 6000 K) than those observed in this work. The range of their temperature values was still small-about 800 K. TANABEef al. [27] also observed a minimum T,,, at the center of their He MIP, but the temperature increased rapidly on one side of the plasma, probably indicating an off-center plasma with substantial wall contact [26]. TANABEet al. [27] observed constant T,,, across the plasma, while WORKMANet al. [26] observed either a minimum or a maximum temperature at the center of the plasma. HELTAI et al. [28] observed a minimum T,,, at the center of the plasma and a maximum at the edges. N: T,,, in a He MIP showed the same trend [29]. BROWNet al. [30] observed maximum IZ, off-center at 360 W, but the electron number density was spatially independent of position at low power. TANABEet al. [27] observed a maximum II, near the center of the plasma. 3.3. Effect of axial position The T,,,, T,,, and n, generally decreased with increase in distance from the electrode (Figs 7 and 8). T,,,, however, increased from 2400 K to 2800 K (at 4 mm above the tip of the electrode), then decreased slowly as the distance was increased. Electron density measurements for 2 and 3 mm observation distances were the same and then decreased. The excitation temperature showed the steepest decrease with observation distance. The plasma gets cooler as the observation distance is increased and this is reflected in the decrease of both the excitation temperature and rotation temperature. Less ionization is also expected in the cooler environment, resulting in low IZ, values. It was not possible to explain the initial increase in the rotational temperature. Although high T,,,, T,,,, and n, values were obtained very close to the electrode tip, all other observations were carried out at 5 mm above the electrode tip in order to avoid the

487

Electron density measurements

600 n^ ‘E u

500

c 2i;j

400

a& 5

300

b u w

200

100

0 1

I

I

1

I

3

5

7

9

Distance

from

electrode

top

11

(mm)

Fig. 8. Electron number density vs axial position.

high background due to the electrode. Obviously, optimum observation heights for different analytes will differ and these have to be determined on an individual basis. 3.4. Effect of carrier gas There was a slight increase of T,,, as the carrier gas flow rate was increased (Fig. 9a). Since water was being aspirated into the plasma, this increase in excitation temperature may be a result of increase in the amount of water entering the plasma together with the carrier gas. Increase in T,,, as amount of water entering plasma is increased has also been observed in Ar plasmas [31-331. T,,, was found to be independent of carrier gas flow rate (Fig. 9b). The same

5000

4000

T

T

T L

T

+

_I_-

T

T

T

T

_L

1

I

1

300

400

600

z

a

T I

Ik

T

,-

TbT

J.

y

1

700

800

900

L

:

z $I

3000

-

2000

-

3 2 b p

9

p

t” 1000

100

200

500

Carrier

Fig. 9. Excitation temperature

gas

flow

10001100

(mL/mln)

(a) and rotational temperature

(b) vs carrier gas flow rate.

488

W. R. L. MASAMBA et al.

1 x

w

z-

400-

$? uw

300-

: z

200-

i

W

loo-

0’

I

,

0

200

400

Carrier

gas

600

1000

800

flow

rate

1200

(mL/mln)

Fig. 10. Electron number density vs carrier gas flow rate.

behavior was observed for nt, values where n, was found to be independent gas flow rate (Fig. 10).

of carrier

3.5. Effect of solution uptake rate T,,, and T,,, (Fig. 11) were found to be almost independent of solution flow rate to the nebulizer (pump flow rate), but the electron number density increased slightly until solution uptake rate of 0.8 mYmin, then it decreased (Fig. 12). The initial increase in IZ, is in agreement with HELTAI et al. [28] and may be due to an increase of the amount of water introduced into the plasma. This is also shown in Fig. 13 where the electron density determined when water was being introduced into the plasma was found to be higher than without water at all power levels. 3.6. Effect of power Both Trot and T,,, increased as the power was increased reaching a plateau at about 600 W (Fig. 14). These results are in agreement with those obtained by other workers [26,27,29] but differ from those obtained by GOODE et al. [34] where the T,,, and T,,,

5000

4000

- T n I

T

;;

;:

I

I

-r In

L

T

T

T

a

I

T 1

2 2

3000

-

1000

0’

0.50

I

0.80

1.10 Solution

Fig. 11. Excitation temperature

I

I

1.40 flow

I

I

rate

1.70

2.00

i 2.30

(mL/mln)

(a) and rotational temperature

(b) vs solution uptakt rate.

489

Electron density measurements

::I

_I:

0.30

1.10

0.70 water

1.90

1.50

flow

rate

2.30

(mL/mbn)

Fig. 12. Electron number density vs solution uptake rate.

A

with water

0

wlthout water

0.80

0.60

0.00

I

200

-I

300

400

500

600 Power

700

800

900

1000

(W)

Fig. 13. Electron number density vs power.

were independent of power, probably because they were operating in the plateau region of the temperature/power plot for their plasma. The n, results showed a different behavior from those exhibited by the temperature curves. The electron number densities were constant with applied power until 700 W and then increased slightly (Fig. 13). In this work, the n, values are always higher with water aspiration. Constant electron number densities were observed in the 350-450 W range for an Ar plasma 2 mm off the central axis by BROWN ef al. [35], but an increase with power was observed when observations were carried out at the plasma center. They observed an initial increase in it, up to 300 W and then a decline to 400 W in a He plasma.

490

W. R. L. MASAMBAet al.

200

300

400

500

600 Power

Fig. 14. Excitation temperature

700

800

900

1000

1100

(W)

(a) and rotational temperature

(b) vs power.

4. SUMMARY

In this work, the excitation temperature, rotational temperature and electron densities were determined and found to be of similar magnitude as those determined by other workers for helium microwave plasmas. The excitation temperature obtained by other workers has been in the range 3000 K-4500 K [7,27] which compares well with our T,,, of about 4000 K. WORKMAN ef al. [26] and ALI [18] obtained T,,, of about 6000 K. Trot values reported for He plasmas generally fall in the range 1300 K-2500 K [7,10, 18,27,29]. T,,, values determined in this work varied between 1800 K and 3000 K. The rotation temperatures determined by WORKMAN et al. [26] using the “lower slope region” were as high as 8500 K indicating the sensitivity of the rotational temperature on wavelength region used for measurements. The electron number densities determined in this work (4 x 1014 cme3-9 x 1014 cm-‘) compare well with those obtained by BROWN et al. [35] (5 x lOi cmp3-2.6 X lOi cm-“) and differ by a factor of approximately 10 compared to those obtained by TANABE et al. [27] (3 X 1013 cm -3-1.4 x 1014 cm-‘). It is remarkable that the different helium plasmas evaluated under various operational conditions have such uniformity in temperatures and electron number densities. Similar results have been obtained by other workers, e.g. [18,26,27]. An interesting feature of the He microwave plasma is the independence of T,,,, T,,, and n, as various parameters are varied. The only parameter which affected these measurements in this work are power and axial observation distance. The difference in T,,, and T,,, indicate non local thermodynamic equilibrium (LTE) conditions in our He CMP. REFERENCES [l] [2] [3] [4] [S] [6] [7] [S] [9]

B. M. Patel, E. Heithmar and J. D. Winefordner, Analyt.Chem. 59, 2374 (1987). Y. K. Zhang, S. Hanamura and J. D. Winefordner, Appl. Spectrosc. 39, 226 (1985). S. Hanamura, B. W. Smith and J. D. Winefordner, Can. J. Spectrosc. 29, 13 (1984). B. Kirsch, S. Hanamura and J. D. Winefordner, Spectrochim. Acta 39B, 955 (1984). J. D. Hwang, W. Masamba, B. W. Smith and J. D. Winefordner, Can. J. Spectrosc. 33, 156 (1988). S. Chan and A. Montaser, Spectrochim. Acta 4OB, 1467 (1985). H. Abdalla and J. M. Mermet, Spectrochim. Acta 37B, 391 (1982). K. Wolnik, D. Miller, C. Seliskar and F. Ficke, Appf. Spectrosc. 39, 930 (1985). C. Seliskar and D. Warner, Appf. Specfrosc. 39, 181 (1985).

Electron density measurements [IO] [ll] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]

[24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35]

491

P. FJeitz and C. Seliskar, Appl. Spectrosc. 41, 679 (1987). D. Miller, C. Seliskar and T. Davidson, Appl. Specfrosc. 39, 13 (1985). D. Miller, H. Fannin, P. Fleitz and C. Seliskar, Appl. Spectrosc. 40, 611 (1986). S. Chart, R. Van Hoven and A. Montaser, Analyt. Chem. 58, 2342 (1986). H. Uchida, W. R. Masamba, T. Uchida, B. W. Smith and J. D. Winefordner, Appl. Spectrosc. 43, 425 (1989). S. Hanamura, B. W. Smith and J. D. Winefordner, Anulyt. Chem. 55, 2026 (1983). B. M. Pate& J. P. Deaver and J. D. Winefordner, Tulunta 35, 641 (1988). S. Hanamura, B. Kirsch and J. D. Winefordner, Analyt. Chem. 57, 9 (1985). A. H. Ah, Electrode heating sample vaporization in capacitively coupled microwave plasma atomic emission spectrometry. PhD Dissertation, University of Florida (1991). M. W. Blades and B. L. Caughhn, Spectrochim. Actu 4OB, 579 (1985). M. C. E. Huber and W. H. Parkinson, Astrophys. 1. 172, 229 (1972). J. M. Bridges and R. L. Komblith, Astrophys. J. 192, 793 (1974). D. E. Blackwell, A. D. Petford, M. J. Shallis and G. J. Simmons, Mon. Not. R. ustron. Sot. 191, 445 (1980). W. L. Wiese and G. A. Marlin, Wavelengths and Transition Probabilities for Atoms and Atomic Ions, Part lZ, Transition Probabilities, NSRDS-NBS 68. The Superintendent of Documents, Washington DC (1980). J. Mermet, Inductively Coupled Plasma Emission Spectroscopy-Part 2 Applications and Fundamentals, Ed. P.W. J. M. Boumans, chap. 10. Wiley, New York (1987). H. R. Griem, Spectral Line Broadening by Plasmas. Academic Press, New York (1974). J. M. Workman, P. G. Brown, D. C. Miller, C. J. Seliskar and J. A. Caruso, Appl. Spectrosc. 40, 857 (1986). K. Tanabe, H. Haraguchi and K. Fuwa, Spectrochim. Actu X?B, 49 (1983). Gy. Heltai, J. A. C. Broekaert, F. Leis and G. Tolg, Spectrochim. Actu 45B,301 (1990). J. M. Workman, P. A. Fleitz, H. B. Fannin, J. A. Caruso and C. J. Seliskar, Appl. Spectrosc. 42, 96 (1988). P. G. Brown, J. M. Workman, D. L. Haas, P. A. Feitz, D. C. Miller, C. J. Seliskar and J. A. Caruso, Appl, Spectrosc. 40, 477 (1986). K. Fallgatter, V. Svoboda and J. D. Winefordner, Appl. Spectrosc. 25, 347 (1971). J. F. Alder, R. M. Bombelka and G. F. Kirkbright, Spectrochim. Acta 35B, 163 (1980). Y. Q. Tang and C. Trassy, Spectrochim. Actu 41B, 143 (1986). S. R. Goode, N. P. Buddin, B. Chambers, K. W. Baughman and J. P. Deaver, Spectrochim. Acta 4OB, 317 (1985). P. G. Brown, T. J. Brotherton, J. M. Workman and J. A. Caruso, Appl. Spectrosc. 41, 774 (1987).