Electronic and rotational energy relaxation in molecular helium

We have selectively pumped the He₂(a³Σᵤ⁺, v=0, J=5)→He₂(e³Πg, v=0, J=6) transition of molecular helium with the light produced by a pulsed dye laser, and we have subsequently followed by fluorescence measurements on a nanosecond time scale both the relaxation of the electronic excitation energy and the energy redistribution among rotational sublevels of the e³Π state. The radiative deexcitation rate of the upper electronic state is 6.2×10⁷ sec^-1; the corresponding two-body quenching rate by collisions with neutral helium atoms at 295^∘{}K is 7.1×10^-11 cm³{sec}^{{-}1}$. The total two-body rotational relaxation rate of the $J=6$ level by collisions with neutrals is ${5.8×{}10}^{{-}10}$ ${cm}³sec^-1. While collisions with ΔJ=±1 account for more than 60% of total rotational transfer, it is necessary to include a substantial probability of multiquantum rotational transitions in order to explain the observed results.