A second-order distorted-wave model for the excitation of the 21P state of helium by electron and positron impact

The second-order distorted-wave amplitude is evaluated in a high-energy approximation. In this approximation, the non-local second-order distorting potential is replaced by a local second-order potential obtained in the closure approximation. It is shown that for the case of electron excitation of the 2 1 P state of helium, the second-order effects are fairly large and improve agreement with experimental data. It is also noted, however, that the best agreement with experimental data may be obtained through a hybrid first-order calculation. Results for positron excitation of helium are also given and discussed.