The mechanism of the contrast in ‘environmental’ or ‘gaseous’ secondary electron images in the environmental scanning electron microscope is at best poorly understood. The original theory suggested a simple gas amplification model in which emitted secondary electrons ionise the chamber gas, leading to signal amplification and finally measurement at a biased detector. This theory is being advanced but little attention has as yet been paid to the factors which influence the actual secondary emission, although unusual contrast effects have been noted in one case. The conven-tional view is that the positive ion product of the gas-electron interaction results in charge neu-tralisation at the sample surface.

The implantation and trapping of charge in non-conductive materials was recently described, in reference to electron range measurements. This work demonstrated that trapped charge influ-enced the secondary electron yield, with enhanced secondary electron emission above the region of trapped charge. The consequence is that the distribution of the trapped charge is seen as a bright circle on the surface of the specimen, centred on the point of beam exposure (Fig.l).