The quadrupole moment of the 39K and 41K nuclei from microwave data for KF and KCl

Accurate calculations of the electric field gradient at the potassium nucleus in KF and KCl, combined with nuclear quadrupole coupling constants derived from microwave spectra, are used to determine the electric quadrupole moments of the 39K (0.0585 barn) and 41K (0.0711 barn) nuclei. The accuracy of these `molecularʹ values of the nuclear quadrupole moment is expected to be better than 1 per cent and surpasses the accuracy of the corresponding atomic data. Also the quadrupole moment of the 40K nucleus is determined to be equal to about −0.073 barn. The quadrupole moments of the potassium nuclei are compatible with the accurate ab initio data recently derived from atomic spectra.