Image resolution in high-voltage electron microscopy

Abstract

An examination is made of the effect of lens aberrations on image resolution for electron microscopy in the 200-1000 keV energy range. In particular a new assessment is made of the effect of chromatic aberration on the inelastic image resolution neglecting relativistic effects. At the optimum lens defocus for the elastic image, the inelastic image contributes mainly as an unstructured background displaying a resolution in the range 0·3-1·0 nm, when the resolution of the elastic image is 0·10-0·26 nm, the lower limit corresponding to an incident electron energy of 1000 keV. At the optimum defocus for the inelastic image, -CcEmp/E0, for a most probable energy loss Emp and an incident electron energy E0, the inelastic image resolution is in the range 0·2-0·4 nm, and the corresponding elastic image resolution is inferior to this. A comparison is made for a specimen thickness of 50 nm, corresponding to a typical section thickness of an amorphous biological section, of the inelastic image and the dark- and bright-field elastic images. Although the ratio of elastic to inelastic scattering is significantly less than unity, the elastic dark-field and inelastic image intensities are comparable at the elastic optimum defocus. In bright-field microscopy, phase contrast effects, which increase with incident electron energy, for the elastic component are significant, and the contribution of the inelastic image to the total image intensity is only as a slowly varying background, again considered at the optimum elastic defocus. The relevance of these calculations to practical high-voltage electron microscopy is discussed.