Theory of united-atom rotational coupling inH++He collisions

United-atom 2pσ-2pπ rotational coupling in asymmetric collisions is influenced by the avoided crossing between the 2pσ and 2sσ orbits. We study this influence using the ${\mathrm{HeH}}^{+}$ system as a prototype. The time-dependent Schrödinger equation is solved numerically in the basis of ${\Sigma }_2$-${\Sigma }_3$-${\pi }_1$ molecular states. We find substantial population of the 2sσ state, in disagreement with estimates based on the Landau-Zener model. The ${\Sigma }_3$ level is populated directed by transitions near the avoided crossing at b=0.4 a.u. and indirectly by ${\Sigma }_2$-${\pi }_1$-${\Sigma }_3$ rotational coupling for b>0.4 a.u.