Coherent Excitation of He Atoms by Proton Impact

Abstract

Theoretical and experimental studies of excitation in ion-atom collisions have been carried out extensively for many years. Most studies were concerned with the cross section of a particular channel. However, these cross sections yield only extremely restricted information about the actual state of the atoms after the collision with the ion, as the collisionally excited states generally differ from the eigenstates of the atomic system. In particular, the center of the charge cloud of the excited electron may not coincide with that of the nucleus. This possible occurrence of electric dipole moments after collisional excitation was considered already in 1915 by N. Bohr as mentioned by W Pauli in his review article about the old quantum theory¹. A complete description of the excitation process can be obtained by introducing excitation matrices². When measuring cross sections, one is measuring the trace of the excitation matrix (the sum of the diagonal elements). The size of the matrix is determined by the energetic resolution of the experimental setup. In order to study the different diagonal elements of the excitation matrix, special experimental techniques are required. The study of polarization and angular distribution of the emitted light after the collision yield some more information, but for a complete determination of the substate population for states with angular momenta L > 1, sophisticated methods, like level-crossing techniques, must be employed³.