Attenuation length in ion-induced kinetic electron emission: A key to an understanding of angular-dependent yields

The mean attenuation length, < L >, of electrons emitted from ion bombarded solids was derived from measured angular-dependent electron yields gamma(theta) in combination with Monte Carlo simulations of inelastic (electronic) energy deposition. The transport controlled contributions of excited electrons to the measured electron yields were derived as the integral g(L) over S-e exp(-z/L), where S-e (theta, z) is the electronic energy deposition and z the depth from the surface. The unknown attenuation length L < L > reflects the average over the energy spectrum and the angular distribution of those internally excited electrons that can reach the solid-vacuum interface and overcome the surface barrier. To determine L, the ratios g(L) (theta)/g(L) (0), calculated for 0 <= L <= 10 nm, were compared with measured yield ratios gamma(theta)/gamma(0) for a wide variety of projectile-target combinations and impact energies between 1 and 50 keV (velocity-proportional electronic stopping). The procedure works well at angles at which S-e (theta, z) decreases smoothly in the depth region between 1 and 3 nm. The result is < L > = 1.5 +/- 0.3 nm, a number basically in accordance with expectation based on estimated data for the inelastic mean free path of low-energy electrons (< 25 eV) but a factor of 10 lower than the numbers recently advocated (10-15 nm) to rationalize " internal" electron yields observed with metal-insulator-metal sandwich structures.