Formation and Collisional Dissociation of Heteronuclear Rare-Gas Associative Ions

Ratios R of the cross sections for associative to the sum of associative and Penning ionization have been measured using molecular beams of thermal energy (330°K) metastable atoms, A*, incident on a gas B confined in a collision chamber at low pressures, where A* is either 21,3S helium or 3P2,0 neon and B is argon or krypton. By determining R as a function of the collision chamber pressure in the range 0.05–0.4 μ and by measuring the relative intensity of the secondary product ion, B2 +, cross sections σI and σII for destruction of the associative ions, AB+ + B → A + B+ + Band AB+ + B → A + B2 +,have been determined. Extrapolation to zero pressure gives the true value of R for the primary processes. The effect of a change in the ratio of 21S/23S helium in the metastable atom beam has also been investigated by changing the exciting electron beam energy. In collisions of He* with both argon and krypton, R is greater for 23S helium than for 21S helium by about 0.05. Within the experimental error, σI and σII for destruction of AB+ are independent of whether this ion is formed by collisions of 23S or 21S helium. The large size of the cross sections σI, which range from 210 Å2 for NeAr+–Ar to 470 Å2 for HeKr+–Kr, suggests that the associative ions are formed very close to the dissociation limit.