Theoretical low-energy scattering cross sections forHe(2S1)+He(1S1)⇄He(2P1)+He(1S1)

Abstract

Ab initio potential-energy calculations have been performed on the $C{}_{}{}^1{}_{}{}^{}\Sigma _g^{+}{}_{}{}^{}$ state of $\mathrm{He}\left(2\mathrm{}{}_{}{}^1{}_{}{}^{}S\right)+\mathrm{He}\left(2\mathrm{}{}_{}{}^1{}_{}{}^{}S\right)$ and on the $B{}_{}{}^1{}_{}{}^{}\Pi _g^{}{}_{}{}^{}$ state of $\mathrm{He}\left(2\mathrm{}{}_{}{}^1{}_{}{}^{}P\right)+\mathrm{He}\left(1\mathrm{}{}_{}{}^1{}_{}{}^{}S\right)$; the $L_y$ rotational-coupling matrix elements were also calculated. The potential curves were found to cross at $R=3.89\mathrm{}{a}_0$, where $L_y=0.68\mathrm{}$. Close-coupled calculations were then used to predict the cross sections for collision energies $E\le 100$ eV. The excitation total cross section was found to rise rapidly from the threshold at $E=0.60\mathrm{}$ eV to values greater than 1 × ${10}^{-16\mathrm{}}$ ${\mathrm{cm}}^2$ at collision energies $E\gtrsim 4$ eV. The inelastic- scattering total cross section for the reverse reaction, deexcitation of $\mathrm{He}\left(2\mathrm{}{}_{}{}^1{}_{}{}^{}P\right)$ by $\mathrm{He}\left(1\mathrm{}{}_{}{}^1{}_{}{}^{}S\right)$, was found to be greater than 1 × ${10}^{-17\mathrm{}}$ ${\mathrm{cm}}^2$ at thermal energies and yields a Boltzmann-averaged rate of $k\left(300\mathrm{}°\mathrm{K}\right)=2.9\mathrm{}\times {10}^{-12\mathrm{}}$ ${\mathrm{cm}}^3$/sec. At energies $E\lesssim 3$ eV, sharp peaks are found on the inelastic-scattering total cross sections and are attributed to quasibound states present at these low energies. The inelastic-scattering differential cross sections are forwardly peaked with most of the scattering appearing at reduced angles $\tau =E\theta \lesssim 250$ eV deg.