4.9. Proton Impact Excitation
In another pioneering paper, Seaton (1955) pointed out the importance of proton impact excitation for transitions at temperarures where E/kT << 1, e.g. the 2s-2p transition in H (and other Rydberg transitions) at nebular temperatures. Even for highly charged ions, fine structure transitions may be driven to a considerable extent by proton scattering at high temperatures, such as the excitation of coronal lines Fe+13 3p1/2 - 3/2 (Seaton 1964). A number of studies have since been carried out and it has been demonstrated by semi-classical and quantum mechanical calculations that proton impact excitation may contribute significantly to the formation of spectral lines due to transitions between closely spaced levels (Dalgarno 1983). A review by Walling and Weisheit (1988) given many references to the relevant data on ion-ion excitation.