X-Ray Polarization in Ion-Atom Collisions

Excited atomic states with spatially anisotropic charge distributions will generally emit polarized anisotropic radiation. The study of K x rays has been a valuable tool in the quest for understanding the ion-atom collision. In addition to cross section and spectroscopic information, which have added much to the total knowledge of the ion-atom collision, K x rays will also yield information about the relative substate populations. The determination of the ratio of magnetic substates from x-ray observations requires either a measurement of the polarization or the angular distribution of the emitted radiation. Recently both polarization and angular distribution measurements have been performed for projectile x rays and the results indicate a strong preference for populating the mL = 0 substate in P states. Further, the preferential substate population (alignment) was shown to be independent of the method of production, either capture or excitation. Other studies of the polarization of target K x rays have shown preferred production of the mL = 0 magnetic substate of the states of the 1s-12p-1 vacancy configuration. The target x-ray polarization is reasonably well described by two vacancy production theories which separately describe the substate production cross sections. These studies of the previously ignored substate populations in ionatom collisions enhance current efforts in the understanding of ion-atom collisions