Scattering polarization of molecular emission lines in the quiet solar chromosphere

We present scattering polarization measurements performed with THEMIS in July 2000 near the south polar limb. The low level of scattered light at THEMIS, which is the only large solar telescope to include a superpolished primary mirror, allows, in good seeing conditions, to observe the emission spectrum of the low chromosphere above the limb. These are, as far as we know, the first high spectral resolution observations of the intensity and the first measurements of the polarization of ${\rm C}_2$ and ${\rm MgH}$ molecular lines in emission above the limb. Molecules are present in a thin layer in the region of the temperature minimum between the photosphere and the chromosphere. We present a very simple model for the formation of the polarized lines and we relate the observed polarization rates to the so-called intrinsic line polarizability coefficients W2. Those quantities may be derived from quantum mechanical computations involving the solution of the Schrödinger equation for the molecular potential. Solar observations provide a direct way of checking these heavy computations and contribute to the improvement of our knowledge in molecular physics. Nine ${\rm C}_2$ molecular transitions and two ${\rm MgH}$ transitions are present in our spectral window; we find that for the ${\rm C}_2$ transitions, the polarizability is between 0.13 and 0.26 and that it takes higher values, namely 0.41 and 0.46, for the two ${\rm MgH}$ transitions