Emission spectra of titanium and argon in argon/hydrogen glow discharge ´, M. M. Kuraica, I. P. Dojc ˇinovic ´ B. M. Obradovic Faculty of Physics, University of Belgrade POB 368, 11001 Belgrade, Serbia and Montenegro e-mail:
[email protected]
´ N. Cvetanovic Faculty of Transport and Traffic Engineering, University of Belgrade Vojvode Stepe 305, Belgrade, Serbia and Montenegro Received 27 April 2006 Experimental results examining the influence of hydrogen introduced into argon glow discharge on spectral line intensities are presented. The cathode was made of titanium, an element with low atomic energy levels. Spectral lines intensity of sputtered material (Ti I) and carrier gas (Ar I and Ar II) change differently with progressive addition of molecular hydrogen. PACS : 32.70.Fw, 52.70.Kz, 52.80.Hc Key words: glow discharge,spectral line intensity
1
Introduction
In recent years, there has been a lot of interest in the effects of small amounts of hydrogen added to argon discharges (see, e.g., [1] and references therein). The addition of hydrogen was found to cause a drop in the ionization in the discharge, and in the Ar ion and electron densities. It is also well-recognized that the addition of hydrogen affects the sputter rates in glow discharges [2]. The effect of hydrogen has been investigated on glow discharges used for the spectrochemical analysis of solid materials by cathode sputtering. It has been demonstrated in glow discharge optical emission spectrometry that some optical emission line intensities increase while others decrease when hydrogen is added [3] to [5]. This has some very important implications for the analysis of “real samples”. Some traces of hydrogen are always detected in a glow discharge, arising from residual moisture in the source and on the sample surface, gaseous hydrocarbons coming from the pre-vacuum oil-pumps, leakage of water vapour through porous samples, etc. Because of the effects of hydrogen on the optical emission line intensities, a good understanding of the role of hydrogen in glow discharge plasma is important in order to include corrections of the hydrogen effect in quantification algorithms, especially for the analysis of thin films and other surface layers containing hydrogen. On the other hand, small amounts of hydrogen added in argon glow discharge are used for measurement of hydrogen Balmer lines in order to obtain information on reactions in plasma, the electron density, and the electric field distribution. Titanium is an element that has Czechoslovak Journal of Physics, Vol. 56 (2006), Suppl. B
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B. M. Obradovi´ c, I. P. Dojˇ cinovi´ c M. M. Kuraica and N. Cvetanovi´ c
low electron energy levels, down to 2.3 eV. Electron energy distribution function (EEDF) has a maximum value for electron energies of couple of eV which is typical for glow discharges, therefore change of EEDF due to addition of small amount of hydrogen can be detected by change of energy levels population i.e. by examining the change in intensity of spectral lines. 2
Experimental setup
Modified Grimm-type glow discharge source (GDS), used in this work is laboratory made and described in details elsewhere [7], [8]. Here, for completeness, minimum details will be given. The hollow anode, 30 mm long with inner and outside diameters 8.00 and 13 mm, has a longitudinal slot (15 mm long and 1 mm wide) for side-on observations along the discharge axis. The water-cooled cathode holder has an exchangeable titanium (purity 99.5 %), 18 mm long and 7.60 mm in diameter, which screws tightly onto its holder to ensure good cooling. A gas flow of about 300 cm3 /min of argon (purity 99.9995 %) or argon-hydrogen previously prepared mixtures (Ar + 0.5 % H2 and Ar + 3.0 % H2 ) are sustained at a pressure 5.5 mbar by means of a needle valve and a two–stage mechanical vacuum pump. In order to prevent back streaming of oil vapours, the zeolite trap is mounted between discharge vessel and vacuum pump. To run the discharge a 0 ÷ 1 000 V current stabilized power supply is used. In series with the power supply 10 kΩ ballast resistor is used. Trough all measurements current was sustained at 18 mA. Stabilisation time of 5 minutes supplied accurate intensity values for the investigated lines at different hydrogen concentrations in the GDS. Spectroscopic observations were performed along the discharge axis (end–on), using a 1 m Czerny–Turner spectrometer equipped with one-dimensional CCD detector. Recorded spectra are obtained after 10 averages. 3
Results and discussion
It is well known that when hydrogen is present in the GDS even in small concentrations, changes of the macroscopic parameters of the glow discharge are observed, see for instance [1] and references therein. Addition of hydrogen may be presumed to affect the EEDF and electron density by vibrational excitations of hydrogen molecular levels [1]. Keeping pressure and current as constant parameters to control the glow discharge, we have found that the dependent parameter, discharge voltage, is considerably affected by the presence of hydrogen. With the progressive addition of hydrogen into the GDS, the discharge voltage was observed to increase. In many papers voltage was kept as a constant parameter [3] to [5], but from the texts is not clear if the voltage was constant on the GDS or on a power supply. If the voltage was constant on the power supply (what we suppose because commercial GDS for the optical emission spectrometry was used), decreasing of current decreased voltage on ballast resistor and consequently increased voltage on GDS. Finally, hydrogen addition induced a mutual change of current and voltage in GDS. B972
Czech. J. Phys. 56 (2006)
Emission spectra of titanium and argon in argon/hydrogen glow discharge
Since one of the electric discharge parameters must be kept constant for investigations we have fixed the current. Ten emission lines of titanium are recorded with the instrumentation and under the conditions described above. Their intensities are measured as a function of the hydrogen content (in % of partial pressure) in the GDS. Addition of hydrogen also affected lines of working gas, so 4 atomic and 17 ionic argon spectral lines intensities are recorded as a function of hydrogen content. All presented intensities of emission lines are normalized for the sake of comparison. Some general characteristics of line intensities vs. hydrogen content may be noticed. – Ti I. The intensity of all Ti I emission lines presented on Fig 1 increases strongly with the addition of 0.5 % of hydrogen. At 3 % hydrogen content in the gas mixture intensities of all Ti I lines decrease to values smaller than in the pure argon. However two groups of Ti I line intensities on Fig. 1 are noticed to behave differently according to their upper energy levels. The spectral lines originated from energy levels 2.5 eV are more strongly changed than the lines from levels 3.1 eV. This effect could be connected with change of EEDF with addition of hydrogen due to vibrational excitations of molecular levels.Straight lines connecting the experimental points are only for the sake of clarity of the figure and have no physical meaning. Ti I lines
Normalized Intensity
1.0
0.8
0.6
2.40 eV, 517.4 nm 2.41 eV, 519.3 nm 2.43 eV, 521.0 nm 2.48 eV, 504.0 nm
0.4
2.50 eV, 506.5 nm 3.11 eV, 398.2 nm 3.13 eV, 399.0 nm 0.2
3.15 eV, 399.9 nm 3.15 eV, 395.6 nm 3.18 eV
395.8 nm
0.0 0.0
0.5
1.0
1.5
% H
2
2.0
2.5
3.0
in Ar - H
2
Fig. 1. Intensity of normalized Ti I emission lines at different percentage of hydrogen in argon. Discharge conditions: I = 18 mA, p = 5.5 mbar.
– Ar I. Decreases of Ar I line normalized intensities are shown on Fig. 2. There are two possible mechanisms for intensity decreasing. First, excitation of Ar I Czech. J. Phys. 56 (2006)
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B. M. Obradovi´ c, I. P. Dojˇ cinovi´ c M. M. Kuraica and N. Cvetanovi´ c
lines via metastable states is disabled when metastable population is quenched by the addition of hydrogen i.e. Penning excitation of the H2 excited state 1sσ 2sσ 3 Σ g ; and second taking into account competitive direct electron impact with the same H2 state.[3] Ar I lines 1.0
Normalized Intensity
0.8
0.6
0.4
13.09 eV, 801.5 nm 13.15 eV, 810.4 nm 0.2
13.17 eV, 800.6 nm 13.28 eV, 794.8 nm
0.0 0.0
0.5
1.0
1.5
% H
2
2.0
2.5
3.0
in Ar - H
2
Fig. 2. Intensity of Ar I emission lines at different percentage of hydrogen in argon. Discharge conditions: I = 18 mA, p = 5.5 mbar.
– Ar II. Most of the Ar II lines presented in Fig. 3 show an increase of intensity with addition of 0.5 % of hydrogen. Increasing of intensity is not as strong as for Ti I lines but the dependence on upper levels energy is more pronounced. As can be seen on Fig. 3a, and especially on Fig. 3b, lines with the higher excitation energy have more evident increase and lesser decrease. Such energy dependent intensity increase (and decrease) can only explained by change of EEDF. There are three exceptions: 393.25 nm, 386.85 nm and more evident 380.95 nm. We propose that excitation of these lines is connected to metastables and quenched by hydrogen in process similar to one mentioned for Ar I lines. Further experimental work with use of more previously prepared gas mixtures (for example with 0.1 %, 0.3 %, 0.8 % 1.0 % and 1.5 % of hydrogen) and analyze of obtained results will give a better insight of the above effects. 4
Conclusion
Addition of molecular hydrogen in the abnormal glow discharge in argon dissimilarly changed the emission line intensities for titanium atom, argon atom and B974
Czech. J. Phys. 56 (2006)
Emission spectra of titanium and argon in argon/hydrogen glow discharge 1.6
a
Ar II lines
b 1.0
1.4
Normalized Intensity
Normalized Intensity
1.2
1.0
0.8
19.97 eV, 385.1 nm 19.97 eV, 392.8 nm 0.6
22.51 eV, 380.9 nm 23.08 eV, 397.9 nm
0.4
23.12 eV, 393.3 nm 23.17 eV, 386.9 nm
0.8
0.6
19.22 eV, 480.6 nm 19.26 eV, 473.6 nm 19.31 eV, 484.8 nm
0.4
19.76 eV, 472.6 nm 19.80 eV, 465.8 nm 19.87 eV, 476.5 nm
0.2
21.35 eV 473.2 nm
24.28 eV, 394.6 nm
0.2
Ar II lines
24.28 eV, 392.6 nm 0.0
0.0 0.0
0.5
1.0
1.5
% H
2
2.0
in Ar-H
2
2.5
3.0
0.0
0.5
1.0
1.5
% H
mixture
2
2.0
in Ar - H
2.5
3.0
2
Fig. 3. Intensity of normalized Ar II emission lines at different percentage of hydrogen in argon. Discharge conditions: I = 18 mA, p = 5.5 mbar
argon ion. Intensity variations are attributed to the change of EEDF, for Ti I and the most of Ar II lines. Change in Ar I lines intensity is explained by quenching of argon metastabiles by added hydrogen. This work is supported by the Ministry of Science and Environmental Protection of the Republic of Serbia within the Project 141043 “Spectroscopic diagnostics of plasma in sources important for application”
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