PhotoelectricK-Absorption Cross Section of Lithium

The photoelectric absorption cross section of metallic lithium has been measured in the spectral region extending from 80 to 280 Å. The wavelength dependence deduced from the present experiment is in general agreement with that reported by previous investigators. A more refined technique has made it possible to determine the absolute value of the cross section. Theoretical calculations of the atomic cross section have also been carried out and the results compare favorably with observations. All the measurements were obtained with the use of a grazing incidence spectrometer which was provided with a specially designed auxiliary vacuum system in which the reactive samples were prepared and then transferred to the spectrometer chamber. Thus, the absorption measurements were carried out with a minimum of contamination risk. The surface densities of the thin absorbing films used were determined by observing the change in the resonant frequency of a quartz crystal as lithium vapor was condensed onto it. Spectrograms were obtained for different values of the surface density. The internal agreement of these results provided an indication of the consistency of the crystal monitor. The cross section reached a maximum of 2.60±0.3 mb at an incident photon wavelength of 193 Å. (The $K$ edge of metallic lithium falls at 227 Å.)