The Electron Excited Ultraviolet Spectrum of HD: Cross Sections and Transition Probabilities

Joseph M. Ajello (1) , Prahlad Vatti Palle (1) , Hervé Abgrall (2) , Evelyne Roueff (2) , Anil Bhardwaj (3) , Jacques Gustin (4)

1 JPL - Jet Propulsion Laboratory
2 LUTH (UMR_8102) - Laboratoire Univers et Théories
3 NASA Marshall Space Flight Center, Space Science Branch, NSSTC/XD12
4 University of Liège

We have analyzed the high-resolution ultraviolet (UV) emission spectrum of molecular deuterium hydride (HD) excited by electron impact at 100 eV under optically thin, single-scattering experimental conditions. The high-resolution spectrum (FWHM=160 mÅ) spans the wavelength range from 900 to 1650 Å and contains the two Rydberg series of HD: ¹Sigma⁺_u1ssigma, npsigma(B, B^', B^'', n=2, 3, 4)-->X ¹Sigma⁺_g and ¹Pi⁺_u1ssigma, nppi(C, D, D^', D^'', n=2, 3, 4, 5)-->X ¹Sigma⁺_g. A model spectrum of HD, based on newly calculated transition probabilities and line positions including rovibrational coupling for the strongest band systems, B ¹Sigma⁺_u-X ¹Sigma⁺_g, B^{' 1}Sigma⁺_u-X ¹Sigma⁺_g, C ¹Pi_u-X ¹Sigma⁺_g, and D ¹Pi_u-X ¹Sigma⁺_g, is in excellent agreement with observed intensities. The cross sections for direct excitation at 100 eV of the B ¹Sigma⁺_u, B^{' 1}Sigma⁺_u, C ¹Pi_u, and D ¹Pi_u states were derived from a model analysis of the experimental fluorescence spectrum to the ground state. The absolute cross section values for excitation to the B ¹Sigma⁺_u, B' ¹Sigma⁺_u, C ¹Pi_u, and D ¹Pi_u states were found to be (2.57+/-0.26)×10^-17, (0.22+/-0.06)×10^-17, (2.54+/-0.25)×10^-17, and (0.17+/-0.04)×10^-17 cm², respectively. We have also determined the dissociative excitation cross sections at 100 eV for the emission of Lyalpha at 1216 Å and Lybeta at 1025 Å lines, which are (7.98+/-1.12)×10^-18 and (0.40+/-0.10)×10^-18 cm², respectively. The summed excitation function of the closely spaced pair of lines, H Lyalpha and D Lyalpha, resulting from dissociative excitation of HD, has been measured from the threshold to 800 eV and is analytically modeled with a semiempirical relation. The model cross sections are in good agreement with the corrected Lyalpha cross sections of Möhlmann et al. up to 2 keV. Based on measurements of H, D (2s) production cross section values by Möhlmann et al., the H, D (n=2) cross section is estimated to be 1.6×10^-17 cm² at 100 eV.

Domaines: Astrophysique [astro-ph]

Joseph M. Ajello, Prahlad Vatti Palle, Hervé Abgrall, Evelyne Roueff, Anil Bhardwaj, et al.. The Electron Excited Ultraviolet Spectrum of HD: Cross Sections and Transition Probabilities. The Astrophysical Journal, 2005, 159, pp.314-330. ⟨10.1086/431309⟩. ⟨hal-03732476⟩