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