LFA: A Lattice Fourier Analyzer for Quantitative In Situ EC‐STM of Adsorbate–Substrate Superstructures

In situ electrochemical scanning tunneling microscopy (EC‐STM) can alternately resolve an adsorbate layer and the underlying substrate by stepping the electrode potential across adsorption/desorption regimes or by tuning tunneling conditions to favor substrate states. Quantitative lattice metrology, however, is hindered by drift, anisotropic scanner distortions, and the coexistence of multiple periodicities (substrate, adsorbate, and superstructures). We present LFA (Lattice Fourier Analyzer), a free, open‐source Python application for reproducible reciprocal‐space analysis of STM/EC‐STM images through a graphical user interface. LFA integrates sub‐pixel Bragg‐peak localization by 2D‐Gaussian fitting, substrate‐anchored affine calibration (rotation, anisotropic scaling, shear), extraction of real‐space lattice vectors and misorientation angles with propagated uncertainties, satellite‐peak quantification for superstructure periods, and mask‐based real‐space reconstructions. We demonstrate LFA on two EC‐STM benchmarks. For iodide on Au(100), LFA quantifies potential‐dependent electrocompression of the c(2√2×p√2)R45° phase via nearest‐neighbor spacings, geometric superperiods, and coverage trends referenced to the substrate. For a porphyrin monolayer on commensurate I/Au(100), bias‐dependent contrast enables transfer of the same substrate‐anchored calibration to the molecular lattice, yielding unit‐cell parameters, registry, and intramolecular orientations in distortion‐corrected real space. Ultimately, LFA provides the rigorous metrological foundation necessary for establishing quantitative structure–potential correlations at electrified interfaces.