The assessment of field ionization detectors for molecular beam use

An overly simplistic but nonetheless useful model is employed to explore basic attributes of field ionization detection. The base cross-sectional detection area of a field ionization tip is set by a centrifugal barrier: molecules impinging at too high a velocity or too large an impact parameter cannot reach the tip and therefore cannot be ionized. For those that do reach the tip, the probability of ionization depends critically on whether they lose sufficient energy on their initial impact to be captured in the polarization field of the tip. Capture is enhanced if the tip radius lies well within the centrifugal barrier and the impinging species is highly polarizable. Scaling relations are developed within the context of this model, allowing recent measurements of field ionization yields to be assessed.