Theory and experiment in the electron impact excitation of Li+

The low-energy electron impact excitation of helium-like Li+ is investigated using the R-matrix method. The lowest nineteen states (i.e. with a principal quantum number n ≤ 4) are explicitly included in the target description. Collision strengths for exciting the n = 2 states are compared with other methods, particularly five-state (n ≤ 2) and eleven-state (n ≤ 3) calculations. Attention is paid to the 11S-23P transition, for which experimental total emission cross sections are available. We show the importance in calculating reliable collision strengths for exciting target states with a principal quantum number n of including coupling to higher n states. There is now no significant difference between theory and experiment at low impact energies (up to 6 Ryd), providing cascade contributions are accounted for in the interpretation of the experimental measurements. (author)