= Photodetachment threshold
{title2=$E_{\rm th}$}
In the published <chlorine> experiment, negative <ions> were accelerated and mass-selected, then overlapped with a tunable <laser> beam. A downstream <electric field> deflected charged <ions>; neutral <chemical atoms> continued to a plate, whose secondary <electrons> were counted. Scanning the <laser> <frequency> produced an onset in this neutral-atom signal. The apparatus therefore inferred an <electron affinity> from a threshold, rather than measuring the <heat> of attachment.<Isotope shift in the electron affinity of chlorine by U. Berzinsh and others>[§2, printed pp. 2–4, Figure 1: ion preparation, deflectors, neutral-atom secondary-emission detector and threshold fitting]{ref}
Reversing the <laser> direction and averaging the two thresholds corrected the first-order <Doppler shift>. A <Fabry–Pérot interferometer> and reference spectral lines calibrated the <frequency>. Finite <laser> bandwidth, unresolved <quantum state> splitting and shifts of the calibration lines limited the inference; the authors included these effects in fitting and the error estimate. The level diagram shows only the extracted gap, not a measured signal curve.<Isotope shift in the electron affinity of chlorine by U. Berzinsh and others>[§§2.1 and 2.3, printed pp. 3–7, equation (2), Figures 2 and 5: direction reversal, calibration, bandwidth and reference-line systematic shifts]{ref}
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