Electron inelastic mean free path in water

Abstract

Liquid phase transmission electron microscopy (LPTEM) is rapidly developing as a powerful tool for probing processes in liquid environments with close to atomic resolution. Knowledge of the water thickness is needed for reliable interpretation and modelling of analytical studies in LPTEM, and is particularly essential when using thin liquid layers, required for achieving the highest spatial resolutions. The log-ratio method in electron energy-loss spectroscopy (EELS) is often applied in TEM to quantify the sample thickness, which is measured relative to the inelastic mean free path (λ_IMFP). However, λ_IMFP itself is dependent on sample material, the electron energy, and the convergence and divergence angles of the microscope electronoptics. Here, we present a detailed quantitative analysis of the λ_IMFP of water as functions of the EELS collection angle (β) at 120 keV and 300 keV in a novel nanochannel liquid cell. We observe good agreement with earlier studies conducted on ice, but find that the most widely used theoretical models significantly underestimate λ_IMFP of water. We determine an adjusted average energy-loss term E_{m, water}, and characteristic scattering angle θ_{E, water} that improve the accuracy. The results provide a comprehensive knowledge of the λ_IMFP of water (or ice) for reliable interpretation and quantification of observations in LPTEM and cryo-TEM studies.