On the surface condition of Langmuir probes in reactive plasmas

The edge effect of a planar probe induces an elliptic-like sheath structure that acts as an electrostatic lens, which then focuses the charged particles on distinct regions of the probe surface. Positive-ion sputtering, chemical adsorption, and/or plasma deposition divide the probe surface into distinct regions with different work functions, which cause a double-hump structure (DHS) in the second derivative of the probe current. Thus, the DHS cannot be correlated with a distinct group of charged particles.