Electron impact ionisation of C/sup 4+/ and O/sup 6+/

Absolute cross-sections for single ionisation of helium-like C/sup 4+/ and O/sup 6+/ by electron impact have been measured for incident electron energy ranging from below the expected metastable state ionisation threshold to 3 keV. A crossed electron-ion beam setup has been used in this experiment. From analysis of the ion time of flight, it is concluded that the signal observed below the ground state ionisation threshold can only be due to the triplet state 2 /sup 3/ /b S/. The importance of the various ionisation processes is discussed for this particular state. The authors present also a calculation of the branching ratio for autoionisation following the electron impact excitation from one of the two metastable states to the doubly excited states of the 2/b l/,2/b l/' configurations. The metastable population has been estimated at 16.3% for C/sup 4+/ and 3.9% for O/sup 6+/.Anglai