A rate equation approach towards surface ionisation gas detection

Surface ionisation (SI) is a form of gas response that can be observed on noble and refractory metals as well as on metal oxide surfaces. In SI gas detection, adsorbed analyte molecules are ionised by a transfer of valence electrons to empty electron states inside the adsorbent solid. The so-formed ionic species are extracted from the surface by an oppositely biased counter electrode, positioned at a small distance from the emitting surface. In the present paper, we present a rate equation approach, which describes how analyte gas molecules proceed through the sequence of partial processes of neutral-state adsorption, molecular decomposition and surface ionisation to form analyte ions. Under steady-state conditions, this rate equation approach describes the temperature and the gas concentration dependences of the SI response as well as the current transport across the air gap. The predictions of this theory are compared with experimental data which had been obtained on platinum (Pt) and tin dioxide (SnO2) flat-plate emitters.