{{\rm{H}}}_{2}\,X{}^{1}{{\rm{\Sigma }}}_{g}^{+}-c{}^{3}{{\rm{\Pi }}}_{u} Excitation by Electron Impact: Energies, Spectra, Emission Yields, Cross-sections, and H(1s) Kinetic Energy Distributions
Abstract
The c{}3{{{\Pi }}}u state of the hydrogen molecule has the second largest triplet-state excitation cross-section, and plays an important role in the heating of the upper thermospheres of outer planets by electron excitation. Precise energies of the H2, D2, and HD c{}3{{{\Pi }}}u-(v,N) levels are calculated from highly accurate ab initio potential energy curves that include relativistic, radiative, and empirical non-adiabatic corrections. The emission yields are determined from predissociation rates and refined radiative transition probabilities. The excitation function and excitation cross-section of the c{}3{{{\Pi }}}u state are extracted from previous theoretical calculations and experimental measurements. The emission cross-section is determined from the calculated emission yield and the extracted excitation cross-section. The kinetic energy (E k ) distributions of H atoms produced via the predissociation of the c{}3{{{\Pi }}}u state, the c{}3{{{\Pi }}}u- - b{}3{{{Σ }}}u+ dissociative emission by the magnetic dipole and electric quadrupole, and the c{}3{{{\Pi }}}u - a{}3{{{Σ }}}g+ - b{}3{{{Σ }}}u+ cascade dissociative emission by the electric dipole are obtained. The predissociation of the c{}3{{{\Pi }}}u+ and c{}3{{{\Pi }}}u- states both produce H(1s) atoms with an average E k of ∼4.1 eV/atom, while the c{}3{{{\Pi }}}u- - b{}3{{{Σ }}}u+ dissociative emissions by the magnetic dipole and electric quadrupole give an average E k of ∼1.0 and ∼0.8 eV/atom, respectively. The c{}3{{{\Pi }}}u - a{}3{{{Σ }}}g+ - b{}3{{{Σ }}}u+ cascade and dissociative emission gives an average E k of ∼1.3 eV/atom. On average, each H2 excited to the c{}3{{{\Pi }}}u state in an H2-dominated atmosphere deposits ∼7.1 eV into the atmosphere while each H2 directly excited to the a{}3{{{Σ }}}g+ and d{}3{{{\Pi }}}u states contribute ∼2.3 and ∼3.3 eV, respectively, to the atmosphere. The spectral distribution of the calculated continuum emission arising from the X{}1{{{Σ }}}g+ - c{}3{{{\Pi }}}u excitation is significantly different from that of direct a{}3{{{Σ }}}g+ or d{}3{{{\Pi }}}u excitations.
- Publication:
-
The Astrophysical Journal Supplement Series
- Pub Date:
- October 2017
- DOI:
- 10.3847/1538-4365/aa89f0
- Bibcode:
- 2017ApJS..232...19L
- Keywords:
-
- molecular data;
- molecular processes