Differential Scattering of Metastable He(2 S3) on He(1 S1) at Energies between 5 and 10 eV

Differential cross sections of the scattering of $\mathrm{He}(2 ^{3}S)$ on $\mathrm{He}(1 ^{1}S)$ at seven energies between 5 and 10 eV have been measured in a new apparatus. The cross sections display several types of structure. A large rainbow peak is observed at a reduced angle $\ensuremath{\tau}=E\ensuremath{\theta}$ of approximately 161 eV deg for all energies. Fine structure on the rainbow peak and at smaller angles is also resolved. From the rainbow scattering we can deduce that the depth of the ${\mathrm{He}}_{2}(a ^{3}\ensuremath{\Sigma}_{u}^{+})$ potential curve is ${D}_{e}=2.00$ eV. This value translates to a well depth for the $A ^{1}\ensuremath{\Sigma}_{u}^{+}$ state [$\mathrm{He}(2 ^{1}S)+\mathrm{He}(1 ^{1}S)$] of 2.50 eV, which is in agreement with the latest spectroscopic analysis. At reduced scattering angles less than 120 eV deg the observed structure is not reproduced in the two-state elastic-scattering theoretical calculations based on the adiabatic states $a ^{3}\ensuremath{\Sigma}_{u}^{+}$ and $c ^{3}\ensuremath{\Sigma}_{g}^{+}$. It is suggested that this disagreement is due to a strong interaction between the $c ^{3}\ensuremath{\Sigma}_{g}^{+}$ state of $\mathrm{He}(2 ^{3}S)+\mathrm{He}(1 ^{1}S)$ and the $b ^{3}\ensuremath{\Pi}_{g}$ state of $\mathrm{He}(2 ^{3}P)+\mathrm{He}(1 ^{1}S)$. This interaction severely modifies the differential elastic-scattering cross sections at angles less than 120 eV deg.