Investigation On The Surface De-excitation Probability Of The Metastable Atoms In Argon Plasmas

Argon is widely used as the feed gas to produce plasmas in both industry and scientific research. In argon-containing plasmas, argon metastable atoms play an important role. They are a significant source for electrons, ions and atoms in high-lying levels, and they can dissociate oxygen-, chlorine- and fluorine-containing molecules. Therefore, it is indispensable in argon-containing plasma research to investigate the density of argon metastable atoms, its temporal and spatial evolution and their major processes.The de-excitation on the wall is an important loss process in the particle balance of argon metastable atoms, especially in low-pressure plasmas. However, the surface de-excitation probability on some usually used wall materials has not been measured, such as on aluminum, stainless steel, silicon and quartz. In order to measure the surface de-excitation probability of argon metastable atoms, the density of the metastable atoms needs to be measured accurately under a plasma condition, where the surface de-excitation process is the dominant process. Both laser absorption and self-absorption method can be used to measure the density of argon metastable atoms. The setup of self-absorption method is simpler, which makes it suitable to be used in industry. In the model of the self-absorption method, the spatial profiles of both the metastable atoms and the emission from the upper level need to be known, in order to obtain the density of the metastable atoms. However, in most plasma, the two distributions are nonuniform and cannot be measured. The uniform profiles are usually assumed in the previous works, but the error due to the real nonuniform distributions is never analyzed.To solve the above problems, firstly a model analysis is performed to investigate the influence of the plasma nonuniformity on the result in the density measurement of argon metastable atoms with self-absorption method. And the conclusion of the model analysis is verified through a comparison between the results of laser absorption and self-absorption method under the same condition. Then the time-resolved laser absorption method is used in the afterglow of a low-pressure pulsed plasma to measure the surface de-excitation probability of argon metastable atoms on some usually used wall materials. The results are compared and discussed. The main differences between this work and the previous ones are as follows:1. In the density measurement of argon metastable atoms with self-absorption method, both the condition in which the plasma nonuniformity will have an influence on the result and the possible error range are determined. The obtained densities with laser absorption and self-absorption method are compared and analyzed under the same condition.2. A method is proposed to measure the surface de-excitation probability of argon metastable atoms in plasma environment. The de-excitation probability of argon metastable atoms on aluminum, stainless steel, silicon and quartz surfaces is measured for the first time with this method.