A technique for absolute measurements in near threshold excitation of positive ions: application to excitation of

An electron-ion crossed beams experiment has been developed in which the energy loss of scattered electrons is used to determine cross sections for excitation in positive ions. Scattered electrons are confined by an axial magnetic field and are channelled into a detector using reflecting potentials and an (trochoidal) deflection. Complete collection of these electrons along with monitoring of the overlap and current of the beams enables absolute total cross sections to be determined. This technique has been used to study near threshold excitation of the D resonance transition in . , CII, trochoidal spectrometer