ELECTRON AFFINITY OF LITHIUM

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 lithium ionization potential (I), this data yielded an average positive ion work function (φ+) 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, φ+, T, and [Li+/Li-], it was found that A (Li) =φ- (3.35±0.05) eV. The quantity φ_ 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 φ_ is given by 40>φ_< 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.