Energy and impact angle dependence of sub-threshold external electron emission

The kinetic electron emission induced by Ar+ ions impinging onto a polycrystalline Ag surface is studied as a function of the projectile impact angle in the energy range 5-10 keV well below the classical threshold. The resulting emission yield is compared to published model descriptions of sub-threshold electron emission. We find reasonable agreement with the prediction of the hot spot model proposed by Sroubek, which is shown to become equivalent with a Richardson-Dushman-type description of thermionic emission when converted to three dimensions. The model is further extended to describe the impact angle dependence and shown to fit the corresponding experimental data reasonably well.