Bremsstrahlung and Transition Radiation from Ag Foils Bombarded by Non-Normal Incidence Electrons

Abstract

The radiation emitted by self-supporting Ag foils 700 to 1700 Å in thickness was investigated as a function of angle of electron incidence, foil thickness, and degree of polarization. For a foil 700 Å in thickness, the peak at 3300 Å characteristic of transition radiation was found in the emission spectrum when the foil was bombarded by normally incident electrons (${\theta }_i=0^{\circ }$). The wavelength at which the peak appeared increased monotonically with increasing angles of electron incidence until it appeared at $\lambda =3400\mathrm{}$ Å for ${\theta }_i={87}^{\circ }$. For a foil 1680 Å in thickness, the transition radiation peak at $\lambda =3300\mathrm{}$ Å was found to decrease in intensity as ${\theta }_i$ increased from normal incidence and finally disappeared at ${\theta }_i={80}^{\circ }$. For ${\theta }_i>{75}^{\circ }$, another peak appeared at 3500 Å, which increased in intensity for ${\theta }_i>{85}^{\circ }$, and finally became an intense peak at $\lambda =3600\mathrm{}$ Å, for ${\theta }_i={89}^{\circ }$. The dependence of the peak at 3500-3600 Å for large angles of electron incidence on ${\theta }_i$, electron energy, and polarization agrees with details predicted by a model due to Howe and Ritchie based upon the production of bremsstrahlung in a semi-infinite slab, and leads to the conclusion that the optical emission observed from Ag foils bombarded by grazing-incidence electrons is bremsstrahlung, and not radiation from the decay of surface plasmons, as hypothesized by Boersch et al.