Dissociative excitation of molecular hydrogen plays an important role in the heating of outer planet upper thermospheres. This paper addresses the role of one of the triplet states involved in the process. H2 excited to the a3Σ+g state, or higher triplet-ungerade states, is dissociated via the a3Σ+g−b3Σ+u continuum. The kinetic energy distribution of H(1s) produced from direct X1Σ+g–a3Σ+g(v, J) excitation by electrons is investigated by an accurate theoretical evaluation of spontaneous transition probabilities of the a3Σ+g(v, J)−b3Σ+u continuum transition. It is shown that the X1Σ+g(0)–a3Σ+g(v, J) excitation primarily produces H(1s) atoms with kinetic energies lower than 2 eV. In addition to the continuum a3Σ+g(v, J)−b3Σ+u transition probabilities, spontaneous emission lifetimes of the a3Σ+g(v, J) (v = 0–20, J ⩽ 14) levels have been calculated by considering both the a3Σ+g−b3Σ+u and a3Σ+g–c3Πu transitions. The calculated lifetimes show a moderately strong rotational dependence, and the lifetimes for the J = 0 rotational level of the low v levels agree well with previous calculations and experimental measurements. Calculations of the a3Σ+g−b3Σ+u continuum emission spectra from electron impact X1Σ+g–a3Σ+g excitation are included.
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