Collisional rate coefficients for the iron ions Fe viii, Fe ix, and Fe x

Relative values of rate coefficients for excitation by electron collisions were determined experimentally for various transitions in Fe viii, Fe ix, and Fe x at two plasma conditions in a small $\theta$-pinch. Collisional ionization rates were also obtained for these ions. The maximum errors of the relative values of the excitation rate coefficients are estimated to be ± 30% to ± 40% and those of the ionization rates to be a factor of 1.5. Iron pentacarbonyl [Fe ${\mathrm{}\left(\mathrm{CO}\right)\mathrm{}}_5$] was used as an impurity in hydrogen or helium gases. The electron temperature and density were obtained from the analysis of scattered laser light. They were in the range 60-150 eV and ${10}^{15}$ to 2 × ${10}^{16}$ ${\mathrm{cm}}^{-3\mathrm{}}$, although this range was not available for each ion. The excitation rate coefficients were calculated by the use of the Coulomb-Born approximation. Comparison with experiment is made within each ion and also between ionization stages. A discrepancy in previous Coulomb-Born calculations is noted for the case of Fe viii.