Autoionizing Lines

Autoionizing lines arise from doubly excited states in which the upper level of the transition is above the ionization energy. Autoionizing lines are characterized by a Fano profile instead of a Voigt profile. Fano profiles depend on three parameters: $\Gamma_{\rm Shore}$, $A_{\rm Shore}$, and $B_{\rm Shore}$, where $\Gamma_{\rm Shore}$ is the profile half width, $A_{\rm Shore}$ is the asymmetry parameter, and $B_{\rm Shore}$ is related to the oscillator strength. More information on these parameters may be found in Shore (1967) and Shore (1968). Most autoionizing lines are in the ultraviolet, but there are a few such lines that appear in the optical. Perhaps the best known of these are the Ca I autoionizing lines in the orange (see below). Beginning with version 2.76, the Fano parameters may be entered into the line list in a way similar to entering the radiation, Stark and van der Waals half widths described in the previous subsection. For instance, for the autoionizing Ca I line at 6343.308Å, enter:

6343.308   20.0  0   35819   51579    0.845  1.000 AI 11.28 -30.00 -13.38  AUTO

where $\log(\Gamma_{\rm Shore}) = 11.28$, $\log(A_{\rm Shore}) = -30.00$, and $\log(B_{\rm Shore}) = -13.38$. Note, however, if the asymmetry parameter $A_{\rm Shore} < 0$, the entry should be - $\log(\vert A_{\rm Shore}\vert)$, which will be a positive quantity. The Fano parameters may be computed from first principles using the references cited above, by fitting resonances in photoionization cross sections (such as those from the Opacity Project^3.2), or they may be derived from the Kurucz line lists or other sources. Note the AI transition type. Autoionizing lines are often extraordinarily broad. Figure shows a synthesis of the region around the three orange Ca I autoionizing lines.

Figure: The three broad, shallow structures seen in this synthetic spectrum (computed with v2.76b using a solar model and shown at full resolution) are the orange autoionizing lines of Ca I. See §  for how to calculate these structures.