Close-coupling analysis of electron-impact excitation of Na(4s)

Electron-impact excitation of sodium from the ground state to the 42S state is investigated for impact energies 5eV≤E≤100 eV. Configuration-interaction-type wave functions are employed in a close-coupling (CC) approximation to determine the significance of the 3p and 3d intermediate states on the 3s→4s excitation cross section by comparing six-, four-, three-, and two-state CC results (6CC, 4CC, 3CC, and 2CC). Surprisingly, the 2CC and 6CC calculated cross sections are in excellent agreement with each other throughout the energy range investigated, and with measurement at 54.4 eV. At other energies the agreement with experimental integral cross sections is less close. Calculations which couple only the 3p or the 3p and 3d intermediate states such as the present 3CC and 4CC approximations may overestimate significantly the Na(4s) cross sections. The second-Born-approximation analysis leads to incorrect results even at 100 eV