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Radiofrequency Argon Plasma 51

4.3

Spectroscopic Evaluation of the Effect of the Mi-

croparticles on a Radiofrequency Argon Plasma

4.3.1

Objectives

The objective of this work is to evaluate changes of number densities of metastable and resonant levels in an argon rf discharge induced by the presence of microparticles. For this purpose the spatial distribution of plasma emission spectra is observed in a pure plasma and in a plasma with particles. The measurements of the number density of the 1s states were based on a single mirror self-absorption method [149], [150]. In order to use this method in the presence of particles in the discharge a correction for the extinction of light on the microparticles is introduced.

4.3.2

Experimental method

Experiments are carried out in the ”PK-3 Plus” laboratory setup at working pressures of 15, 30 and 60 Pa in argon buffer gas. The light from the discharge was collected by an optic fiber with a collimator attached to it. A mirror opposing the fiber was installed in such a way so that reflected light from the discharge could be used to probe the plasma. For each position in the discharge spectra with and without mirror were recorded. For the same plasma conditions measurements were done twice for each position for the microparticle free plasma and plasma with a large cloud of particles levitated in it.

Z

rf electrodes rf electrodes

particles _detector

mirrorshuter

Figure 4.10: Sketch of the experimental setup for measuring the selfabsorption using single- mirror method.

The evaluation of the relative absorption was done by measuring a portion of the reflected light, transmitted through the plasma. In the microparticle-free plasma the only

mechanism contributing to the relative absorption is the absorption of the radiation by bottom levels of the respective transitions, i.e. self-absorption. In addition to that, in complex plasmas microparticles will also effectively contribute to the absorption due to extinction. Therefore, the absorption profile and optical thickness had to be rewritten as a sum of the plasma and microparticle components. Estimations, using the experimental data on the broadening of Ar lines, shows that collisional broadening is negligible and pure Doppler profile can be used [119]. Using the Doppler profile allows us to combine the relative absorption with the optical thickness of the plasma which is directly connected to the number density of the bottom state of the observed transition [151].

4.3.3

Results

To correctly determine the number density of the argon states in complex plasmas, the correction for the extinction of the light by the microparticles had to be taken into account. Due to the lack of data on the refractive index of melamine-formaldehyde in the observed spectral range it was impossible to make any estimations on light-particle interaction. For this reason we estimated this values experimentally using an argon lamp to evaluate the extinction on microparticles levitated in the neon buffer gas under the same plasma conditions and the same microparticle density. Neon plasma was used in order to avoid the absorption of source argon lines by the plasma therefore only extinction on particles can be measured. Typical values of the measured attenuation of argon lines by microparticles are presented in figure 4.11.

According to our results, for the number densities of the argon states to be determined in presence of a microparticle cloud, the attenuation Kd

ij (Fig.4.11) must be taken into

account. It was shown that neglecting this effect can lead up to 25 % overestimation of the number density of states.

Under the present conditions the discharge is strongly non-uniform and exhibits a clas- sical α-form of the rf discharge [152] with increased light intensity close to the powered electrodes and a darker area in the center presented in figure 4.3.3a. Due to the influence of gravity, the injected particles in the discharge concentrate themselves in the glow region above the bottom electrode figure 4.3.3c, which results in breaking the spatial symmetry of the light emission, figure 4.3.3b. A bright region above the bottom electrode significantly shifts into the central, relatively dark area of the discharge, while the upper half of the

4.3 Spectroscopic Evaluation of the Effect of the Microparticles on a

Radiofrequency Argon Plasma 53

Figure 4.11: Measured spectral dependence of the attenuation Kd

ij and relative absorbtion

1₋!Kd ij

"−1

of the light by a cloud of 2.55 µm diameter microparticles. In the spectral range of interest the relative absorbtion varies more than twice. The higest value of Kd

ij,

obtained in our experiments, is _≈1.13

discharge remains almost undisturbed. Using the proposed method and correction due to the presence of microparticles in the discharge the observed changes in the light intensities were used for the evaluation of the number density of metastable and radiative states.

The axial distribution of the number density of states undergoes practically the same changes as the light distribution of the line intensities in the presence of microparticles. The strongest relative changes (twice or more with respect to microparticle-free plasma) are in the center of the discharge. This effect is much less pronounced at 15 Pa than at 30 and 60 Pa.

The observed effects of microparticles on the plasma can have different explanations. A non local increasing of the light emission and metastable densities were tried to be explained with the few mechanisms:

• Disturbance in the ionization balance due to the additional depletion of electrons on the particles surfaces.

• Secondary electron emission from ions and metastables interaction with particle sur- face.

• Sputtering of particles.

However, deeper insight into the role of each mechanism would require further experi- mental and theoretical investigations.

Figure 4.12: Influence of the presence of the microparticle cloud on the parallel-plate RF-discharge in argon at 30 Pa. (a) videoimage of a microparticle-free discharge; (b) videoimage of a discharge containing a microparticle cloud in the vicinity of the bottom electrode; (c) videoimage of a laser-illuminated microparticle cloud; (d) axial profiles of the intensities of argon spectral lines; (e) axial profile of the number density of metastable 1s5

state; (f) axial profile of the number density of radiative 1s2 state. In figures (d)-(f) solid line corresponds to the microparticle-free case and dashed line with circles corresponds to the case of a plasma, containing a microparticle cloud.

4.3.4

Conclusion

This work clearly demonstrated the influence of the microparticles presence in the plasma on the light emission and metastable density distribution. Under the considered experimen- tal conditions its is clear that the cloud of microparticles, levitating in the plasma produces a significant non-local influence on the discharge. Presence of such a microparticle cloud leads to the enlargement of the bottom glow region, containing the microparticles. Mea- surements of the densities of 1s excited states of argon did not reveal significant changes in the amplitudes of their variation, whereas some local values increased twice and more in presence of microparticles. The underlying processes that cause these effects are not completely understood showing the importance of further investigations in this direction. In addition, it was shown that more attention should be focused on microparticle-light interaction in further spectroscopic observations of complex plasmas.