Spectra, emission yields, cross sections, and kinetic energy distributions of hydrogen atoms from H{sub 2}X{sup 1}Σ{sub g}{sup +}–d{sup 3}Π{sub u} excitation by electron impact
- Planetary and Space Science Division, Space Environment Technologies, Pacific Palisades, CA 90272 (United States)
- Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109 (United States)
Electron-impact excitation of H{sub 2} triplet states plays an important role in the heating of outer planet upper thermospheres. The d{sup 3}Π{sub u} state is the third ungerade triplet state, and the d{sup 3}Π{sub u}–a{sup 3}Σ{sub g}{sup +} emission is the largest cascade channel for the a{sup 3}Σ{sub g}{sup +} state. Accurate energies of the d{sup 3}Π{sub u}{sup −}(v, J) levels are calculated from an ab initio potential energy curve. Radiative lifetimes of the d{sup 3}Π{sub u}(v, J) levels are obtained by an accurate evaluation of the d{sup 3}Π{sub u}–a{sup 3}Σ{sub g}{sup +} transition probabilities. The emission yields are determined from experimental lifetimes and calculated radiative lifetimes and are further verified by comparing experimental and synthetic d{sup 3}Π{sub u}–a{sup 3}Σ{sub g}{sup +} spectra at 20 eV impact energy. Spectral analysis revealed that multipolar components beyond the dipolar term are required to model the X{sup 1}Σ{sub g}{sup +}–d{sup 3}Π{sub u} excitation, and significant cascade excitation occurs at the d{sup 3}Π{sub u}(v = 0,1) levels. Kinetic energy (E{sub k}) distributions of H atoms produced via predissociation of the d{sup 3}Π{sub u} state and the d{sup 3}Π{sub u}−a{sup 3}Σ{sub g}{sup +}−b{sup 3}Σ{sub u}{sup +} cascade dissociative emission are obtained. Predissociation of the d{sup 3}Π{sub u} state produces H atoms with an average E{sub k} of 2.3 ± 0.4 eV/atom, while the E{sub k} distribution of the d{sup 3}Π{sub u}−a{sup 3}Σ{sub g}{sup +}−b{sup 3}Σ{sub u}{sup +} channel is similar to that of the X{sup 1}Σ{sub g}{sup +}–a{sup 3}Σ{sub g}{sup +}−b{sup 3}Σ{sub u}{sup +} channel and produces H(1s) atoms with an average E{sub k} of 1.15 ± 0.05 eV/atom. On average, each H{sub 2} excited to the d{sup 3}Π{sub u} state in an H{sub 2}-dominated atmosphere deposits 3.3 ± 0.4 eV into the atmosphere, while each H{sub 2} directly excited to the a{sup 3}Σ{sub g}{sup +} state gives 2.2–2.3 eV to the atmosphere. The spectral distribution of the calculated a{sup 3}Σ{sub g}{sup +} –b{sup 3}Σ{sub u}{sup +} continuum emission due to the X{sup 1}Σ{sub g}{sup +}–d{sup 3}Π{sub u} excitation is significantly different from that of direct a{sup 3}Σ{sub g}{sup +} excitation.
- OSTI ID:
- 22887023
- Journal Information:
- Astrophysical Journal, Vol. 818, Issue 2; Other Information: Country of input: International Atomic Energy Agency (IAEA); Since 2009, the country of publication for this journal is the UK.; ISSN 0004-637X
- Country of Publication:
- United Kingdom
- Language:
- English
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