Electron Affinity of Atomic Iodine

Photodetachment of I— ions has been observed in a crossed-beam experiment. The apparatus used was essentially similar to that in previous photodetachment experiments. Use of four Sharp Cut filters provided good ``effective'' resolution close to threshold but did not permit an independent determination of the behavior of the cross section as a function of wavelength. A step-function cross section, shown by Berry and co-workers to approximate closely the true cross section behavior, was used to provide an upper energy limit to the electron affinity shown to be close to the actual value. The value of the electron affinity thus determined, 3.076±0.005 eV, is in excellent agreement with the shock-wave determination of Berry, 3.078±0.005. The absolute magnitude of the cross section in the region 0.3 eV above threshold was determined to be 2.1±1.1×10—17 cm2.