Electron Affinity of Lithium

Abstract

The positive and negative surface ionization of a lithium atom beam on a thin, heated, molybdenum ribbon was studied using a specially designed mass spectrometer. At a constant incident beam flux, Li+ yields were determined as a function of surface temperature in the 1400°–2100°K range. Using 5.39 eV for the first lithiumionization potential (I) , this data yielded an average positive ion work function (φ̄+) of 4.38 eV with a standard deviation of 0.01 eV. The [Li+/Li−] ratio was then determined at beam intensities and surface temperatures made sufficiently large to give measurable Li− yields. From the above values for I , φ̄ + , T , and [Li+/Li−], it was found that A (Li) = φ̄ − − (3.35 ± 0.05) eV . The quantity φ̄ − is an average negative ion work function. This could not be measured with any precision because of the very small Li− yields. A recent determination of the single‐plane work functions of molybdenum shows a variation from 4.0 eV for the (116) plane to 5.0 eV for a (110) surface. Since the negative ion work‐function average accentuates the low work‐function patches, the maximum uncertainty in φ̄ − is given by 4.0 < φ̄ − < 4.4 eV . Consequently, the electron affinity of lithium is bounded by the following values: 0.65 < A(Li) < 1.05 eV. From previously reported measurements of the polycrystalline work function of molybdenum, it is concluded that A(Li) is more likely to be near the upper rather than the lower limit of this range of uncertainty.