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B.D. Sharpee, D.L. Huestis, T.G. Slanger, P.C. Cosby (SRI International)
The [N I] 4S--2D \lambda\lambda5198,5200Å\ transitions are an important electron density diagnostic in the PDR and neutral species-dominated portions of the ISM, planetary nebulae, and H II regions. However, the radiative lifetimes of the J=3/2,5/2 states of the 2D term are known only from theoretical calculations which yield a large range of possible values for I(\lambda5198Å)/I(\lambda5200Å) from 1.5--2.5, assuming statistical populations in the states. We present here new measurements of the intensity ratio, obtained from eight-five 30-60 minute spectral observations of the Earth's atmosphere taken with the Keck II telescope. The measured intensity ratios are remarkably consistent with a value of 1.80±0.04, over a factor of a hundred range in absolute line intensities. This value can be compared directly with theory because of the high pressure/well-mixed conditions that prevail in the terrestrial ionosphere. This value is also in excellent agreement with recent Breit-Pauli calculations yielding a value of 1.78 (Froese Fischer, private communication). Our results demonstrate the precision obtainable for such measurements from high-resolution night-sky spectra taken with large aperture telescopes, rivaling the best-determined laboratory values. This illustrates the need for a comprehensive survey program and central repository for night sky line emission spectra.
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Bulletin of the American Astronomical Society, 35 #3
© 2003. The American Astronomical Soceity.