Production ofHe+inHe2+-He Collisions near Threshold

The relative cross section for production of ${\mathrm{He}}^{+}$ in ${\mathrm{He}}_2^{+}$-He collisions has been measured near threshold. Convolution integrals taking account of the thermal motion of the target He atoms are numerically calculated for several forms of the threshold cross section and compared to the data. For the threshold laws considered, threshold is in the range 2.0-2.4 eV, in agreement with theoretical values of the dissociation energy of $X{}_{}{}^2{}_{}{}^{}\Sigma _u^{+}{}_{}{}^{}$ ${\mathrm{He}}_2^{+}$. By using Liu's recent rigorous lower bound for this dissociation energy, a possible form of threshold law is obtained. The experiments also indicate that significant amounts of ${\mathrm{He}}_2^{+}$ resulting from He*-He interactions are formed in excited vibrational-rotational states of the $X{}_{}{}^2{}_{}{}^{}\Sigma _u^{+}{}_{}{}^{}$.