Automatic and quantitative measurement of spectrometer aberrations

TL;DR

This paper presents a method for automatic, quantitative measurement of spectrometer aberrations in electron energy-loss spectroscopy, aiding in optimization and alignment of high-resolution spectrometers.

Contribution

It introduces a novel approach combining experimental measurement with wave mechanical simulation to accurately assess spectrometer aberrations.

Findings

Measured geometric aberrations and distortions in EELS

Simulation matches experimental results well

Method can guide spectrometer correction improvements

Abstract

The performance of electron energy-loss spectrometers can often be limited by their electron-optical aberrations. Due to recent developments in high energy-resolution and momentum-resolved electron energy loss spectroscopy (EELS), there is renewed interest in optimizing the performance of such spectrometers. For example, the "{\omega}-q" mode of momentum-resolved EELS, which uses a small convergence angle and requires aligning diffraction spots with a slot aperture, presents a challenge for realigning the spectrometer after adjusting the projection lenses. Automated and robust alignment can greatly benefit such a process. The first step towards this goal is automatic and quantitative measurement of spectrometer aberrations. Here we demonstrate the measurement of geometric aberrations and distortions in EELS within a monochromated scanning transmission electron microscope(STEM). To better understand the results, we present a wave mechanical simulation of the experiment. Using the measured aberration and distortion coefficients as inputs to the simulation, we find a good match between the simulation and experiment, verifying the approach used in the simulation, allowing us to assess the accuracy of the measurements. Understanding the errors and inaccuracies in the procedure can guide further progress in aberration measurement and correction for new spectrometer developments.