K shell ionization of C, Al, Ni, Cu, Ag, and Au by electron impact

Abstract

Semirelativistic calculations of the K shell ionization cross sections of C, Al, Ni, Cu, Ag, and Au by electron impact have been performed employing four Born-type approximations. The calculations for C, Al, Ni, Cu, and Ag were carried out for incident electron energies ranging up to about 20 times the K shell ionization potential and those for Au for incident energies of up to 500 keV, The objective of this research was to improve the agreement between experiment and theory by including the effects of electron exchange in the calculations, but without increasing the complexity of the calculations beyond that of the first Born approximation. The approximations employed in these calculations are the following: the Born approximation, the modified Born or Born (b) approximation, the Born-Ochkur approximation, a modified Born-Ochkur approximation. The effects of electron exchange are included in the two Born-Ochkur approximations. The modified Born-Ochkur approximation employed here is obtained by assuming that the Born approximation is valid for collisions in which the scattered electron has a higher energy than the ejected electron and that collisions for which the ejected electron has the higher energy are due solely to electron exchange. The exchange integrals arising in the calculation are then treated in the same way as in the Born-Ochkur approximation. The Born (b) and Born-Ochkur approximations yield cross sections which do not agree with experimental measurements as well as those of the Born approximation. However, the modified Born-Ochkur approximation employed here yields cross sections for the K shell ionization of C, Al, Cu, and Ag by electron impact which are in substantially better agreement with experiment than those predicted by the Born approximation. No improvement in this agreement is obtained for the case of Ni, but the existing experimentally measured cross sections for the K shell ionization of Ni by electron impact are in basic disagreement with those measured for the other atoms considered here. Only a slight improvement in the agreement between experiment and theory is obtained for the case of Au; however, the difficulty here is probably due to the inadequacy of the semirelativistic treatment (Darwin wave functions) employed in these calculations. Much more experimental data is necessary for a conclusive test of these approximations.