Transmission electron diffraction at 200 eV and damage thresholds below the carbon K edge

Transmission electron diffraction patterns from ultra-thin aromatic and ali phatic organic films at beam energies of 200 eV-1 keV have been recorded in a custom low energy electron transmission (LEET) chamber. A significant re duction of the molecular damage cross-section, measured by fading of diffra ction spots, was found for thin films of the aromatic perylene when the bea m energy was reduced from 400 to 200 eV. The corresponding measurements for the aliphatic tetracontane showed a smaller "threshold energy" and the dif ferences are discussed. Electron beam damage from other aromatic materials has also been studied at low energy. Comparison of the carbon K shell ioniz ation cross-section and the measured damage cross-sections show that carbon K-shell ionization is strongly correlated with the damage observed in arom atics at beam energies higher than 284 eV. Calculation of the minimum numbe r of unit cells needed for imaging a single molecule, and comparison of cal culated elastic with measured damage cross-sections both indicate new possi bilities for imaging biomolecules with low energy electrons.