The absolute cross section, $\sigma \mathrm{}\left(E\right)\mathrm{}$, for electron attachment in iodine is determined from the measured values of the average cross section, $\overline{\sigma }\left(E^{\prime }\right)$, described in parts I and II of this series. This is accomplished by solution of the integral equation relating the average cross section, the actual cross section, and the energy distribution of the electrons. The derived cross section $\sigma \mathrm{}\left(E\right)\mathrm{}$ exhibits a much sharper energy dependence than that of $\overline{\sigma }\left(E^{\prime }\right)$ given in II, decreasing from a maximum value at zero electron energy to a half-value in ∼0.01 ev. The maximum cross section, $\sigma \mathrm{}\left(0\right)\mathrm{}\sim 3\times {10}^{-15\mathrm{}}$ ${\mathrm{cm}}^2$, is orders of magnitude larger than the values obtained by previous investigators. It is proposed that the difference in values can be attributed to deficiencies in measurements involving electron distributions of appreciable energy spread.

• Received 18 November 1957

DOI:https://doi.org/10.1103/PhysRev.109.2012