Non-dissociative C2-and C2H-ion formation by double electron transfer in one collision

Positive ions produced from C 2 H 2 by electron impact were collided with Cs, K and Na vapours in the keV energy range. Negative ions formed by two electron transfers were mass-analysed. C - , CH - , C 2 - and C 2 H - ions formed by dissociative and/or nondissociative processes were observed. The dependences of each ion on target density were measured. Formations of nondissociative C 2 - ions from C 2 + ions and C 2 H - ions from C 2 H + ions were determined to be double electron transfers in one collision from the density dependence for the Cs target. The cross sections of nondissociative C - and CH - ion formations were much less than those of C 2 - and C 2 H - ion formations. The order of cross sections for nondissociative C 2 - and C 2 H - ions in the condition of a Cs target at the collision energy of 3.0 keV was estimated to be 10 -18 cm 2 . Nondissociative peaks of C 2 - and C 2 H - ions which were detected with a Cs target were not, however, observed with either the K or Na targets. The double electron transfer cross sections are discussed in relation to the endothermicity.