A NEW VERSION OF THE DISTORTED-WAVE IMPULSE APPROXIMATION - APPLICATION TO COPLANAR SYMMETRICAL IONIZATION

The exact calculation of the distorted-wave impulse approximation (DWIA) for ionization presents a very difficult task. The DWIA formula has therefore always been evaluated using a further simplification, the so-called factorization approximation. One effect of this simplification is to restrict the validity of the method to cases in which the momentum transfer, N, to the residual ion is small. The authors propose a new approximation to DWIA which goes beyond the factorization method. The new approximation, called DWWIA, can treat situations in which N is not small. Such a case exists in coplanar symmetric ionization. Here a secondary maximum should be observed at large angles of about 135'. This maximum is a result of a double scattering mechanism in which the incident electron first makes a hard elastic collision with the atom before ionizing it. They apply the new DWWIA approximation to coplanar symmetric ionization of H(1s), H(2s) and He(1s2). Agreement with the available experimental data for He(1s2) is very encouraging and can probably be improved by using more suitable wavefunctions for the final state. New experiments to study the systematics of the large angle secondary peak are proposed.