Dynamics of conversion of atomic helium(triplet-2S) atoms to molecular helium(odd triplet-a-sigma-plus) molecules in ternary collisions (helium)

The temperature dependence of conversion of He(2$\\sp3$S$\\sb1$) metastable atoms to He$\\sb2$(a$\\sp3\\Sigma\\sp+\\sb{\\rm u}$) metastable molecules in the three-body reaction$$\\rm He(2\\sp3S\\sb1)+2He(1\\sp1S\\sb0) \\to He\\sb2(a\\sp3\\Sigma\\sp+\\sb{u})+He(1\\sp1S\\sb0)$$has been investigated over the temperature range 65K-700K. This reaction is thermally activated as a consequence of a long range repulsive barrier in the He(2$\\sp3$S$\\sb1)$-He(1$\\sp1$S$\\sb0$) interaction potential. The data reveal that there are two reaction channels with distinctly different activation energies. The temperature dependence of the measured rate coefficient $\\delta$(T) is accurately described by$$\\rm \\delta(T) = \\lbrack 87.4\\ T\\ exp({-}750/T){+}4.1T\\ exp({-}200/T)\\rbrack \\times 10\\sp{-37}\\ cm\\sp{-6}sec\\sp{-1}.$$The first activation energy, 750 $\\pm$ 70K(63 $\\pm$ 6meV), is equal to the known He(2$\\sp3$S$\\sb1)$-He(1$\\sp1$S$\\sb0$) repulsive barrier height. The second activation energy is 17 $\\pm$ 2 meV.