Abstract
The polarimetric properties of the phenomenon of resonance scattering in molecular bands are analyzed in detail for the general case where both the lower and upper electronic states of the molecule are described in the coupling scheme intermediate between Hund's cases (a) and (b). A semi-analytical expression is derived for the line polarizability factor in the special case where atomic polarization is neglected in the lower electronic state, and particular attention is devoted to clarifying the role played by the phenomenon of Λ -doubling, which turns out to be particularly relevant in the case of $\Pi - \Sigma$ transitions. The expression for the line polarizability factor is derived for both cases where the energy eigenvectors are expressed as a linear combination of the eigenvectors relative to Hund's case (a) and of those relative to Hund's case (b). Such expressions are then combined with the corresponding expressions of the Einstein coefficient and of the Lande factors to obtain an estimate of the linear polarization signals to be expected in molecular bands observed at the solar limb either in the quiet atmosphere or in magnetic regions. Illustrative results are shown for the bands of MgH and of C 2 around 5100 A.
Highlights
Systematic measurements of linear polarization in the solar spectrum, observed over the disk at close distances from the limb, have revealed the unexpected existence of strong polarization signals in molecular bands (Stenflo & Keller 1996; Gandorfer 2000, 2002)
The polarimetric properties of the phenomenon of resonance scattering in molecular bands are analyzed in detail for the general case where both the lower and upper electronic states of the molecule are described in the coupling scheme intermediate between Hund’s cases (a) and (b)
A semi-analytical expression is derived for the line polarizability factor in the special case where atomic polarization is neglected in the lower electronic state, and particular attention is devoted to clarifying the role played by the phenomenon of Λ-doubling, which turns out to be relevant in the case of Π − Σ transitions
Summary
Systematic measurements of linear polarization in the solar spectrum, observed over the disk at close distances from the limb, have revealed the unexpected existence of strong polarization signals in molecular bands (Stenflo & Keller 1996; Gandorfer 2000, 2002) This phenomenon, an obvious manifestation of resonance scattering in an astrophysical context, has motivated a revival of interest in the theoretical study and modeling of resonance polarization in molecular lines (see, e.g., the proceedings of the workshop “Solar Polarization 3” edited by Trujillo Bueno & Sánchez Almeida 2003). The concept of a polarizability factor was introduced in the literature to characterize the polarization properties of the continuous radiation scattered by anisotropic classical particles (Chandrasekhar 1950) and of the resonance line radiation scattered by atoms (Hamilton 1947).
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