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A Computationally Assisted Spectroscopic Technique to measure secondaryelectron emission coefficients ($\gamma$-CAST) in capacitively-coupledradio-frequency plasmas is proposed. This non-intrusive, sensitive diagnosticis based on a combination of Phase Resolved Optical Emission Spectroscopy andparticle-based kinetic simulations. In such plasmas (under most conditions inelectropositive gases) the spatio-temporally resolved electron-impactexcitation/ionization rate features two distinct maxima adjacent to eachelectrode at different times within each RF period. While one maximum is theconsequence of the energy gain of electrons due to sheath expansion, the secondmaximum is produced by secondary electrons accelerated towards the plasma bulkby the sheath electric field at the time of maximum voltage drop across theadjacent sheath. Due to these different excitation/ionization mechanisms, theratio of the intensities of these maxima is very sensitive to the secondaryelectron emission coefficient $\gamma$. This sensitvity, in turn, allows$\gamma$ to be determined by comparing experimental excitation profiles andsimulation data obtained with various $\gamma$-coefficients. The diagnostic,tested here in a geometrically symmetric argon discharge, yields an effectivesecondary electron emission coefficient of $\gamma = 0.066 \pm 0.01$ forstainless steel electrodes.

A computationally assisted spectroscopic technique to measure secondary electron emission coefficients in radio frequency plasmas