Astronomy and Astrophysics – Astrophysics
Scientific paper
May 2000
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000rmxac...9..148c&link_type=abstract
"Astrophysical Plasmas: Codes, Models, and Observations, Proceedings of the conference held in Mexico City, October 25-29, 1999,
Astronomy and Astrophysics
Astrophysics
2
Scientific paper
In the visible and near infrared, the photoabsorption spectrum of H-, the negative ion of hydrogen, is relatively featureless, with a threshold near 1644 nm and a broad maximum at about 840 nm. However, in the vacuum ultraviolet one finds, embedded in the continuum, a ``resonance region'' of doubly-excited, singlet P, H- states corresponding to electrons bound to excited hydrogen states. The most prominent of these resonances, the ``shape resonance,'' is formed by an electron bound in a potential with three classical turning points formed by the first excited state of hydrogen. This short-lived state interferes, mostly constructively, with the continuum, to form an asymmetric Fano profile with a width of about 0.22 nm. The shape resonance peak cross section is about 8×10-17 cm2. Upon autodetachment the n=2 state is preferentially populated, in effect, pumping Lyman alpha emission. A nearby narrow, but distinctive, Feshbach resonance could enhance the absorption feature's visibility under certain conditions. In spite of the presumed role of this ion in stellar atmospheres, so far no clear signature for these resonances has been found in astrophysical phenomena.
Bryant H. C.
Cohen Samuel
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