Presentation 1 : Oral
Depolarization of spectral lines by collisions with atomic hydrogen : semi-classical theory within the O'Mara's interaction potential (PNST)
M. DEROUICH, S. SAHAL-BRÉCHOT
Observations of line linear polarization across the visible part of the solar spectrum at the limb show rich structures. This polarization spectrum (the so-called "second solar spectrum") is entirely new to explore and interpret.
This polarization is due to anisotropic scattering of the incident solar
radiation field, the anisotropy arising from the center to limb darkening. \\
In order to quantitatively interpret the Stokes parameters of the observed lines, a non optically thin and non-LTE model in the presence of a magnetic field has to be taken into account. Collisions of the radiating and absorbing atom with neutral hydrogen atoms of the medium have to be included. In fact collisions between Zeeman sublevels can play a major role for depolarizing the levels (and thus the lines). Very few depolarization rates currently exist, they have been obtained through sophisticated quantum chemistry methods precise but heavy to develop. It would be desirable to develop faster and approached methods capable to give correct results with a short delay. This is the object of the present work.\\
We have extended the O'Mara's theory to the calculation of the depolarization collisional rates which enter the statistical equilibrium equations. We will present and compare our obtained results with the recent ones obtained by quantum chemistry. We will also compare the results to the depolarization rates calculated with the Van der Waals interaction. A discussion of our results will be given by studying the region of the potential curves which play the major role in the variation of the depolarization cross-sections .\\
In conclusion, we will show that our method based on the O'Mara's theory,
owing to the simplicity of the processes taken into account, is a handy
alternative to the accurate but heavy quantum chemistry method for obtaining most of the depolarization rates within a 10-30 \% accuracy.