Autoionisation of N⁵⁺ (nln'l') with n=2,3,4 and n'⩾n measured by electron spectrometry in collisions of N⁷⁺ with He and H₂, at 4.9 keV amu^-1

A spectroscopic investigation of electrons coming from autoionising states N⁵⁺(nln'l'), with n=2,3,4 and n'>or=n, of the fast ion has been made at 11.6 degrees and 4.89 keV amu^-1. These excited states are formed by two-electron capture in a single collision between a bare atom ¹⁵N⁷⁺ and a two-electron target (He or H₂). Only exothermic reactions are observed. It is shown that the ionisation potential of the target has a strong influence on the more probable values of n and n'. In N⁷⁺-He collisions, the n=3, n'=3 configurations are the more excited. In N⁷⁺-H₂, the capture process is less selective since n=3, n'>3 and also n=4, n'>or=4 configurations are strong populated. In all these cases, the residual N⁶⁺ ion is left in an excited state (n=2 or 3). The n=2, n'>2 excitation probability is found to be much smaller for the two collisional systems.