Secondary Electron Yield Measurements as a Means for Probing Organic Films on Aerosol Particles

Secondary electron yields of salt particles (KCI and NaCI) coated with organic films (dioctyl sebacate and octacosane) were measured by impacting aerosol particles with monoenergetic electrons (10-600 eV) inside a particle beam apparatus. The results demonstrate that secondary electron yield measurements can be used to obtain information on the structure of organic films on aerosol particles. The technique relies upon the large differences in the yields of organic compounds and certain salts in order to obtain a measurable contrast in the yields of clean and coated particles. Secondary electron yields measured as a function of primary electron energy and film coverage show that the probe depth of electrons increases with energy and is ~ 10-20 nm for the range of energies investigated. The technique is sufficiently sensitive that submonolayer films can be detected. Measurements made within the monolayer regime suggest that vapor-deposited dioctyl sebacate (DOS) forms uniformly thick films, and that the thickness of one monolayer is ~O. 7 nm. Similar measurements made for octacosane suggest that multilayer films are formed by this compound, even below the nominal monolayer point. The utility of the technique for studies of aerosol chemistry is demonstrated in an investigation of the behavior of octacosane-coated NaCI particles subjected to a hydration-dehydration cycle