Effects of Electron-Gun and Laser Parameters on Collection Efficiency and Packet Duration in Ultrafast Electron Microscopy

Real-space imaging of full-morphological, angstrom-scale structural dynamics occurring on the femtosecond (fs) timescale is possible with ultrafast electron microscopy (UEM) via the stroboscopic pump-probe methodology enabled by interfacing an otherwise standard TEM with a short-pulsed laser system [1-3]. The technology has matured to the point that commercial systems, such as the FEI Tecnai Femto, are now available [4]. The base platform for the Tecnai Femto is FEI’s Tecnai G2 20 200 kV instrument with a thermionic electron gun. In UEM mode, the LaB6 is operated cold such that electron emission occurs only during photo-illumination. Despite this, nanosecond single-shot imaging and diffraction – wherein >10 7 electrons per packet reach the specimen – are possible with a Tecnai-based UEM equipped with a conventional thermionic electron gun [5,6]. While this illustrates that sufficient photoelectrons for such experiments are generated in an otherwise unmodified Tecnai TEM, the parameter space for application-specific optimal electron-gun and photoelectron-generating laser properties remains ill-defined. Quantitatively and systematically mapping such parameter space is challenging due to the large number of variables affecting photoelectron packet properties [7-9].