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

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: 1 Σ 1 s σ, np σ( B , B ', B '', n = 2, 3, 4)→ X 1 Σ and 1 Π 1 s σ, np π( C , D , D ', D '', n = 2, 3, 4, 5)→ X 1 Σ . A model spectrum of HD, based on newly calculated transition probabilities and line positions including rovibrational coupling for the strongest band systems, B 1 Σ - X 1 Σ , B ' 1 Σ - X 1 Σ , C 1 Π u - X 1 Σ , and D 1 Π u - X 1 Σ , is in excellent agreement with observed intensities. The cross sections for direct excitation at 100 eV of the B 1 Σ , B ' 1 Σ , C 1 Π u , and D 1 Π 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 1 Σ , B ' 1 Σ , C 1 Π u , and D 1 Π 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 cm 2 , 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) × 10 -18 and (0.40 ± 0.10) × 10 -18 cm 2 , 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 (2 s ) production cross section values by Möhlmann et al., the H, D ( n = 2) cross section is estimated to be 1.6 × 10 -17 cm 2 at 100 eV.