Scanning Electron Microscope Observations of Herbicide Dispersal Using Cathodoluminescence as the Detection Mode

When high energy electrons strike a specimen in an electron microscope a number of phenomena occur. Some electrons are reflected with little or no energy loss. Others actually penetrate the specimen to some degree and excite molecules within a volume of the solid. Some molecules divest themselves of their surplus energy by allowing excited electrons to escape from their orbitals. If these electrons are able to escape from the specimen, they are known as secondary electrons. If the excited electrons do not leave the molecule but instead fall back to their ground state, they must transfer their surplus energy to other molecules or emit it as discrete packages of energy. The emission can be heat, infrared to visible light, or x-rays (for further details of these processes see 1, 4, 7, 8). We will confine ourselves to a consideration of the infrared to visible light production and reflected electron phenomena in this communication. Researchers working with herbicides frequently have a need to know the spatial location of an herbicide in or upon a plant. A common method for obtaining such information is that of radioautography (9). Radioautography, while certainly adequate for many studies, often requires weeks of exposure time to produce a sufficiently intense film image and also involves the expense of radioactively labeled compounds. A number of compounds will fluoresce3 under electron bombardment (3). We have used a scanning electron microscope equipped to detect this fluorescence (cathodoluminescence) and wish to report a new, rapid method for spatially localizing herbicides on leaf surfaces.