Free electron charging of ultrafine aerosol particles

Experiments of free electron charging of aerosol particles in the free molecule regime are reported. Monodisperse ultrafine silver particles of 5-30 nm were exposed to known concentrations of low energy electrons produced by ionization by α-particles in a unipolar aerosol charger. The kinetic energy of the electrons was varied by changing the electric field intensity in the charger (i.e. between 93 and 279 V cm^-1). The range of Knudsen number for aerosol charging (i.e. the ratio of the electron mean free path to the particle radius) was from 30 to 261. The charged fraction was measured as a function of particle size in high-purity helium and nitrogen under different charging conditions. The experimental results for the combination coefficient between neutral particles and electrons suggested a free-molecule diffusion charging mechanism which was dependent on the electron mobility, transverse diffusion coefficient, mean free path, and mean kinetic energy (i.e. electron temperature). The functional dependence was similar to that given by the ionic charging theories of Natanson (1960, Sov. Phys. 5, 538-551), and Fuchs (1963, Geofis. Pura Appl. 56, 185-193) when the appropriate electron properties were used. The electron charging models of O'Hara et al. (1989, J. Aerosol Sci. 20, 313-330), and Zagnit'ko et al. (1989, Russ. J. Phys. Chem. 63, 883-888) did not fit the experimental results because they were not derived for the free-molecule regime. A modified Fuchs charging theory that uses an empirical accommodation coefficient for the electrons at the surface of the particle was used to fit the experimental results. Good agreement was found by using an accommodation coefficient of 0.4 for both helium and nitrogen.