Some Atomic Effects of an Electronic Electric Dipole Moment

A parity-violating perturbation, corresponding physically to a permanent electric dipole moment of an electron parallel to its spin, is introduced into the Dirac equation for an electron. For positrons two choices of the relative sign of electric moment and spin are considered. Some numerical consequences of such a perturbation for hydrogenic atoms and for positronium are calculated.From various experiments carried out previously, rough upper limits are obtained for ξ, the electric dipole moment expressed in units of emc: From the numerical value of the Lamb shift, ξ<0.004; from the metastability of the $2s$-state in hydrogen, ξ<0.03. From the absence of K→LI x-ray transitions in the heaviest atoms, the upper limit for ξ is about 0.005 or, probably, even smaller. From the value of the hyperfine splitting of the positronium ground state, ξ<0.02.