Vibrational-loss EELS and the avoidance of radiation damage.

We discuss vibrational-mode energy-loss spectroscopy using an aloof beam of electrons positioned a small distance b from the edge of a specimen in a probe-forming TEM or STEM equipped with a high-resolution monochromator. Due to the delocalization of inelastic scattering, a strong vibrational-loss signal can be recorded without causing significant damage to a beam-sensitive specimen. Calculations for b=20 nm suggest that damage is reduced by typically a factor of 1000 (relative to electrons of the same energy transmitted through the specimen) for the same signal strength and spatial resolution. About 50% of the vibrational-loss signal comes from material lying within a distance b of the edge of the specimen and extending over a length 2.5b parallel to the edge. Although energy-filtered imaging appears impossible in aloof mode, an undersampling STEM technique is proposed, taking advantage of scattering delocalization to obtain a vibrational-loss image that leaves most of the imaged area undamaged.