Electron-impact-excitation cross sections of hydrogenlike ions

Convergent close-coupling (CCC) and Coulomb-Born with exchange and normalization (CBE) methods are used to study electron-impact excitation of hydrogenlike ions. The nl-->n'l' cross sections demonstrate (i) good agreement between the CCC and CBE results, (ii) a scaling over ion nuclear charge z, (iii) a domination of the dipole (l'=l+/-1) contributions in total n-->n' cross sections, and (iv) significant effect of electron exchange in the energy range x<3 (here x is the ratio of the incident electron kinetic energy epsilon to the transition energy E(n,n')). For ions with z>5 the n-->n' cross sections obtained in the CCC and CBE approximations agree with each other to better than 10% for any x. An accuracy of the cross sections scaling over z(4) depends on z: for z=6-18 the scaling is accurate to better than 10% (quantitative analysis is done for n'<7), for ions with z<6 the cross sections deviate from the z(4) scaling more significantly (at n about unity). The n-->n' cross sections are presented by a formula which fits our CCC and CBE results with an accuracy to better than 10% (for transitions with n<n'<7 in ions with z>5). The new Gaunt factor G(x) suggested for the widely used Van Regemorter formula [Astrophys. J. 136, 906 (1962)] makes this formula accurate to better than 50% in the x>3 range and to better than 20% in the x>100 range. It is shown that the semiempirical formula by Vainshtein, Sobelman, and Yukov [Electron-impact Excitation Cross Sections of Atoms and Ions (Nauka, Moscow, 1973)] provides an accuracy to better than 50% for any incident electron energy. For x<2 this formula is accurate to better than 30%. These accuracy assessments are based on a comparison with our CCC and CBE results.