Long-range helium excimer potentials (A,CΣu,g+1anda,cΣu,g+3) from high-resolution differential cross sections for He(2S1,2S3)+He

The long-range parts of the ($A$,$C{}_{}{}^1{}_{}{}^{}\Sigma _{u,g}^{+}{}_{}{}^{}$ and $a$,$c{}_{}{}^3{}_{}{}^{}\Sigma _{u,g}^{+}{}_{}{}^{}$) excimer potentials of the ${\mathrm{He}}_2$ molecule have been accurately determined from high-resolution differential-cross-section measurements in the relative kinetic-energy range from 18 to 140 meV. The barrier heights and internuclear distances of the intermediate maxima of the $A$ and the $a$ state have been obtained with an accuracy of better than 2-5 meV and 0.1 Å, respectively. The long-range parts of the singlet potentials from the ab initio calculation by Gubermann and Goddard are always 5-20 meV higher than the potential determined from our experiments, but the splitting of the potentials coincides with our results within experimental accuracy for $R\ge 3$ Å. The experimental setup, data evaluation, and the construction of the potential are described in detail. The complicated interference structure of the angular distribution is discussed with the help of the quantal deflection function. Total and excitation-transfer cross sections are calculated and compared to other experimental results, and the similarity to other exchange processes is pointed out.