Contrast in the electron spectroscopic imaging mode of a TEM. IV. Thick specimens imaged by the most-probable energy loss.

When the zero-loss transmission falls below 10(-3) for biological sections of mass-thickness greater than 70 micrograms/cm2, the energy window in the electron spectroscopic imaging (ESI) mode of an energy-filtering electron microscope (EFEM) can be shifted to the most-probable energy loss of the electron energy-loss spectrum. This enables mass-thicknesses up to 150 micrograms/cm2 or thicknesses of 1.5 microns to be examined. Electron energy-loss spectra of thick carbon films calculated by a Fourier method agree with experimental spectra. Measurements of the electron energy-loss spectroscopy and ESI image intensities with an additional platinum film confirm a scattering model for the calculation of the image intensity. This model considers the angular broadening at the most-probable energy loss by introducing an effective illumination aperture of the order of the full-width at half-maximum of the angular distribution.