High-Resolution Atomic Beam Study of the Helium Excimer PotentialsHe2(A, CΣu,g1+)

The long-range parts of the $A$, $C{{}_{}{}^1{}_{}{}^{}\Sigma _{\mathrm{u},\mathrm{g}}^{}{}_{}{}^{}}^{+}$ excimer potentials of the ${\mathrm{He}}_2$ molecule have been accurately dtermined from high-resolution differential-cross-section measurements at the relative kinetic energy range from 18 to 140 meV. The barrier height of the intermediate maximum of the $A{{}_{}{}^1{}_{}{}^{}\Sigma _{\mathrm{u}}^{}{}_{}{}^{}}^{+}$ potentials is ${47}_{-1\mathrm{}}^{+2\mathrm{}}$ meV at an internuclear distance of 3.14 ± 0.05 Å. The long-range parts of the potential from the recent ab initio calculation by Guberman and Goddard is always 5 to 20 meV higher than the potential determined from our experiments.