Electronic and rotational energy relaxation in molecular helium

We have selectively pumped the ${\mathrm{He}}_2(a{}_{}{}^3{}_{}{}^{}\Sigma _u^{+}{}_{}{}^{}, v=0, J=5)\to {\mathrm{He}}_2(e{}_{}{}^3{}_{}{}^{}\Pi _g^{}{}_{}{}^{}, v=0, J=6)$ 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 $e{}_{}{}^3{}_{}{}^{}\Pi$ state. The radiative deexcitation rate of the upper electronic state is ${6.2\times 10}^7$ ${\sec }^{-1\mathrm{}}$; the corresponding two-body quenching rate by collisions with neutral helium atoms at 295°K is ${7.1\times 10}^{-11\mathrm{}}$ ${\mathrm{cm}}^3$ ${\sec }^{-1\mathrm{}}$. The total two-body rotational relaxation rate of the $J=6\mathrm{}$ level by collisions with neutrals is ${5.8\times 10}^{-10\mathrm{}}$ ${\mathrm{cm}}^3$ ${\sec }^{-1\mathrm{}}$. While collisions with $\Delta J=\pm 1$ 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.