Proton-induced fine-structure transitions in O IV

Proton-impact excitation of fine-structure transitions in ${\mathrm{O}}^{3+}{}_{}{}^2{}_{}{}^{}P_{}^0{}_{}{}^{}$ ions is studied in a quasimolecular representation. The ${}_{}{}^2{}_{}{}^{}\Pi$ and ${}_{}{}^2{}_{}{}^{}\Sigma _{}^{+}{}_{}{}^{}$ states of the O${\mathrm{H}}^{4+}$ molecule formed by the approach of ${\mathrm{H}}^{+}$ and ${\mathrm{O}}^{3+}{}_{}{}^2{}_{}{}^{}P_{}^0{}_{}{}^{}$ are represented with self-consistent-field wave functions which include the effects of polarization. Accurate, close-coupled solutions of the nuclear scattering equations are obtained and used to assess the accuracy of some approximate scattering descriptions. The elastic approximation is found to overestimate the cross sections at all energies. The Coulomb-Born approximation is accurate at both high and low energies but greatly overstimates the cross sections at intermediate energies. A simple unitarity correction to the Coulomb-Born approximation reduces the errors substantially. For high partial waves, the Coulomb-Born approximation is satisfactory at all energies and cross sections for fine-structure excitation by proton impact can be calculated accurately and efficiently with the use of a combination of close-coupled and Coulomb-Born results. Cross sections accurate to within 25% can be obtained with very little computational effort by a combination of the elastic and Coulomb-Born approximations.