2005ApJS..159..314A

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: ¹Σ⁺_u1sσ, npσ(B, B', B'', n=2, 3, 4)⟶X ¹Σ⁺_g and ¹Π⁺_u1sσ, npπ(C, D, D', D'', n=2, 3, 4, 5)⟶X¹Σ⁺_g. A model spectrum of HD, based on newly calculated transition probabilities and line positions including rovibrational coupling for the strongest band systems, B ¹Σ⁺_u-X ¹Σ⁺_g, B '¹Σ⁺_u-X ¹Σ⁺_g, C ¹Π_u-X ¹Σ⁺_g, and D ¹Π_u-X ¹Σ⁺_g, is in excellent agreement with observed intensities. The cross sections for direct excitation at 100 eV of the B ¹Σ⁺_u, B' ¹Σ⁺_u, C ¹Π_u, and D ¹Π_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 ¹Σ⁺_u, B' ¹Σ⁺_u, C ¹Π_u, and D ¹Π_u states were found to be (2.57±0.26)x10^–17, (0.22±0.06)x10^–17, (2.54±0.25)x10^–17, and (0.17±0.04)x10^–17 cm², respectively. We have also determined the dissociative excitation cross sections at 100 eV for the emission of Lyα at 1216 Å and Lyβ at 1025 Å lines, which are (7.98±1.12)x10^–18 and (0.40±0.10)x10^–18 cm², respectively. The summed excitation function of the closely spaced pair of lines, H Lyα and D Lyα, 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 Lyα 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 1.6x10^–17 cm² at 100 eV.