Signal-to-noise in femtosecond electron diffraction.

Pump-probe electron diffraction can directly record atomic-scale motion within molecules or materials. However, the available current in femtosecond experiments is limited, making it challenging to reach the sensitivity required for detecting the fastest structural dynamics, which are encoded in time-dependent diffraction intensities. Here we present a unified analysis of signal-to-noise for an ultrafast electron diffraction apparatus. We characterize the noise of realistic ultrafast electron sources and detectors, test the performance on crystalline and polycrystalline samples and discuss practical approaches for improving measurement sensitivity. The analysis is found sufficient to predict the achievable signal-to-noise ratio in pump-probe electron diffraction before actually starting an investigation.