Speaker
Description
For thirty years, slow electron velocity imaging, a.k.a. SEVI [1], has relied on the idea that the maximum intensity circles obtained when monoenergetic electrons are projected on a detection plane are a direct visualization of transverse velocities. Quantitatively, the squared radii of those circles would directly provide a measure of the electron energy [2].
When the projected electrons emerge from an atomic process, however, they do so as electron waves, so the electron intensity maxima SEVI relies on are no stigmatic images, but the main fringes formed near caustic surfaces. To put it in optical terms, these are electron rainbows and, as Airy observed, “the maximum illumination does not take place at the Geometrical Caustic (...) but (...) on the luminous side of the geometrical position of the rainbow” [3]. Interpreting the radii of the electron rings, classically, as radii proportional to transverse velocities, has thus resulted in a systematic underestimation of the electron energies.
The underestimation does not depend on the energy of the detected electron. It is a constant energy $\Delta = 1.01879... \times\sqrt[3]{\left(\hbar qF\right)^2/\left(2m\right)}$, which, apart from the fundamental constants ħ (the reduced Planck constant), q (the elementary charge) and m (the electron mass), only depends on the electric field F, in which electron emission has taken place [4]. Numerically, the bias is 18 μeV in a 380 V/m field, which is the typical order of magnitude of the fields used in SEVI measurements (when not larger ones). Correspondingly, SEVI-measured electron affinities, which are about one third of the electron affinities of reference nowadays, have probably been overestimated by a similar amount. The electron affinities measured by photodetachment microscopy [5], which has always relied on fitting high-resolution electron images with the actual form of the squared electron wave function, are not affected.
[1] A. Osterwalder, M. J. Nee, J. Zhou & D. M. Neumark, J. Chem. Phys. 121, 6317 (2004)
[2] C. Bordas, F. Paulig, H. Helm & D. L. Huestis, Rev. Sci. Instrum. 67, 2257 (1996)
[3] G. B. Airy, Trans. Cambridge Philos. Soc. 6, 379 (1838)
[4] C. Blondel & C. Drag, Phys. Rev. Lett. 134, 043001 (2025)
[5] C. Valli, C. Blondel & C. Delsart, Phys. Rev. A 59, 3809 (1999)
