Core radial polarization and the contact hyperfine structure of ⁴S state of nitrogen

The contact hyperfine structure constant for the ⁴S state of the nitrogen atom is computed variationally by a multiconfiguration self-consistent-field method. The present calculations take account of radial-correlation contributions to spin density, but do not take account of any significant part of angular correlations. It is found that the calculated value of 0.0977 for the spin density is in excellent agreement with the experimental value of 0.0972. A mechanism called "core radial polarization," for providing contributions mainly to the spin density at the nucleus, is proposed. It is noted that the mechanism provides a main contribution for the ⁴S state of nitrogen as well as for the ²P state of lithium. It is also claimed that the proper description of radial correlations should be a prerequisite for a good spin density.