Messungen und Berechnungen zum elektronenmikroskopischen Streukontrast für 17 bis 1200 keV-Elektronen

Electron transmission experiments under electron microscope conditions were done with C-, Ge- and Pt-films to get information about the influence of electron energy and objective aperture. One gets an exponential law of transmission for small mass thicknesses x of the objects and can calculate the “contrast thickness” x_k (in T = exp(— x/x_k)). Using the Lenz theory, two constants X_a and ϑ₀ are determined, for each element and electron energy, from the measurements of x_k at four different objective apertures. x_a is related to the total elastic scattering cross section and ϑ₀ is the half width of the atomic scattering amplitude ƒ(ϑ). The variation of x_a and ϑ₀ with electron energy is not in agreement with the theory using the first Born approximation and a simple screened atomic potential. But using these two experimentally determined constants in a plural scattering theory of LENZ, good agreement between calculated and experimental deviations from the exponential law of transmission up to mass thicknesses of 300 µg · cm^-2 is obtained.

To get better theoretical values of x_k, the complex atomic scattering amplitudes were calculated quantum theoretically with the WKB-method and Hartree potentials. The values agree with the experimental results for Ge- and Pt-films. For carbon there is a large contribution by inelastic scattering making a direct comparison with experimental results difficult.

The energy dependence of x_a shows saturation for high voltages, as expected by theory. At high voltages the difference in the x_k-values for films with different atomic numbers is larger, resulting in lower x_k-values for platinum. But at very low voltages the x_k-values of carbon are lower than those of platinum. Some measurements of x_k at 60 keV on targets of noble gases confirm the absence of a large difference in contrast between atoms in the gaseous and condensed amorphous state.