Quantum Offset of Velocity Imaging-Based Electron Spectrometry and the Electron Affinity of Arsenic.

Electron imaging has been routinely used for electron spectrometry. It has been ignored, however, that the maximum-intensity circles that surround electric field-produced electron spots do not materialize envelopes of trajectories, but the first interior fringes of a caustic. Neglecting the gap between the fringe and the parent envelope has resulted in spectrometric errors, notably on some reference values of electron affinities. Evidence for the effect is given by photodetachment microscopy of O^{-} and a measurement of the electron affinity of ^{75}As, which is found to be 0.804 486(3) eV.