SPECTRA, EMISSION YIELDS, CROSS SECTIONS, AND KINETIC ENERGY DISTRIBUTIONS OF HYDROGEN ATOMS FROM H2 X 1Σg+- d 3Πu EXCITATION BY ELECTRON IMPACT

Electron-impact excitation of H2 triplet states plays an important role in the heating of outer planet upper thermospheres. The d (3)Pi(u) state is the third ungerade triplet state, and the d (3)Pi(u)-a (3)Sigma(+)(g) emission is the largest cascade channel for the a (3)Sigma(+)(g) state. Accurate energies of the d (3)Pi(-)(u)(v, J) levels are calculated from an ab initio potential energy curve. Radiative lifetimes of the d (3)Pi(u)(v, J) levels are obtained by an accurate evaluation of the d (3)Pi(u)-a (3)Sigma(+)(g) transition probabilities. The emission yields are determined from experimental lifetimes and calculated radiative lifetimes and are further verified by comparing experimental and synthetic d (3)Pi(u)-a (3)Sigma(+)(g) spectra at 20 eV impact energy. Spectral analysis revealed that multipolar components beyond the dipolar term are required to model the X (1)Sigma(+)(g)-d (3)Pi(u) excitation, and significant cascade excitation occurs at the d (3)Pi(u)(v = 0,1) levels. Kinetic energy (E-k) distributions of H atoms produced via predissociation of the d (3)Pi(u) state and the d (3)Pi(u)-a (3)Sigma(+)(g)-b (3)Sigma u(+) cascade dissociative emission are obtained. Predissociation of the d (3)Pi(u) state produces H atoms with an average E-k of 2.3 +/- 0.4 eV/atom, while the E-k distribution of the d (3)Pi(u)-a (3)Sigma(+)(g)-b (3)Sigma(+)(u) channel is similar to that of the X (1)Sigma(+)(g)-a (3)Sigma(+)(g)-b (3)Sigma(+)(u) channel and produces H(1s) atoms with an average E-k of 1.15 +/- 0.05 eV/atom. On average, each H-2 excited to the d (3)Pi(u) state in an H-2-dominated atmosphere deposits 3.3 +/- 0.4 eV into the atmosphere, while each H-2 directly excited to the a (3)Sigma(+)(g) state gives 2.2-2.3 eV to the atmosphere. The spectral distribution of the calculated a (3)Sigma(+)(g)-b (3)Sigma(+)(u) continuum emission due to the X (1)Sigma(+)(g)-d (3)Pi(u) excitation is significantly different from that of direct a (3)Sigma(+)(g) excitation.