The Van der Waals interaction between a radiator and a light neutral perturber explains the broadening and shift of spectral lines only partly satisfactorily. Comparison with solar observations suggests that these theoretical damping constants are systematically too low. Moreover, the theoretically predicted shifts of the lines agree only incidentally with those obtained in laboratory experiments.
We used a combination of a Smirnov-Roueff (S-R) interaction potential and a Van der Waals (VdW) potential. The damping constants, thus obtained, are always somewhat higher than those obtained by a VdW potential alone. The agreement between theory and experiment for the shift of the line is -although not completely satisfactory -better than the very poor agreement resulting from a VdW potential. However, the S-R potential may not be used in the case that heavy perturbers are involved. In that case the VdW potential yields the better results.
Fairly good agreement is found between theoretically and empirically determined values of 7 for the solar centre. * Now at the Utrecht Observatory. The financial support was granted by a fund of the Belgian-Dutch cultural treaty.