Gas discharge response to light: dependence of linearity on space charge for optogalvanic and excited-state photoionization signals.

Experiments involving He-Cd and Ar laser beam interactions with an Ar discharge indicate that space charge effects generated by the incident light itself are the physical mechanism responsible for nonlinearity of response in discharge regions of excited atom concentration spatial gradients. This occurs with both excited atom photoionization and optogalvanic signals. Such optically generated space charges explain the role of electrode geometry in forming effective cross sections which have been used to describe mathematically the nonlinearity. The optogalvanic effect here is greater in Townsend than in glow discharges, particularly in the cathode fall. This is advantageous because of the extremely low noise and higher responsivities of Townsend discharges.