This work presents calculations of the parity non-conserving (PNC) electric dipole transitions 6s + 7s in Cs and of the 6p,,, + 7p,,, and 6p,,, + 6p,,, transitions in T1, made possible by the weak interaction between electrons and nucleons. In addition to effects that can be accounted for in an independent-particle description, we have calculated those second-order correlation effects that can be described as modifications of the valence orbitals to approximate Brueckner orbitals, including terms where the PNC correction occurs on intermediate orbitals, as well as on external orbitals. Similar calculations were performed for the electric dipole moment ( EDM) of the ground states of Cs and TI, expressed in terms of a possible electron EDM, which, if found, implies simultaneous violation of symmetry under parity and time reversal.

Ab initio many-body perturbation calculations are presented for the field and the specific mass isotope shifts for caesium and thallium. T h c calculations are fully relativistic and incorporate the random phase approximation core screening corrections together with the lowest order correlation corrections. For the specific mass shift certain higher order corrections are also included. For the Cs 6s,,, ground state we obtain a field shift value which we believe is accurate to 3%. For TI we find relatively poor agreement between our calculated fieldshift valuesat this level of approximation. Current experimental data are re-examined in the light of our results.

In this paper we describe a semi-empirical approach for calculating certain high-order perturbation effects in a simple manner. We use these modified orbitals in RPA type equations to give values for E l transition matrix elements and hyperfine dipole constants that agree well with experiment. Including the spin-independent weak interaction leads to parity non-conserving equations. For the caesium 6s,,, + 7s,,, parity nonconserving E l transition we obtain 0.904 (1 *0.02) x lo-'' (-ieaoQw/N).where for caesium we used c = 5.674 fm and a = 0.52339 fm. These were deliberately chosen to be the same as those used by Johnson et a1 (1986) for comparison with their calculations. PNC H FAs first shown by Sandars (1977), it is possible to include the spin-independent weak interaction directly in the atomic Hamiltonian due to the scalar nature of the operator.

A discrete numerical basis set is a versatile tool for many-body calculations. Here it is used to calculate second-order energy corrections and to construct approximate Brueckner orbitals for Cs and T1 in a relativistic framework. These orbitals, which often account for a large part of the correlation effects, are then used to evaluate the hyperfine structure and electric dipole transition matrix elements for a few low-lying states. The correlation effects were combined with the RPA diagrams, which account for the response of the orbitals to the external perturbation, and the results are compared with other calculations and with experiment. For Cs, the results are in good agreement with earlier work, whereas for the more complicated system T1 we find significantly larger contributions from the modification of the valence orbitals to approximate Brueckner orbitals.

The semi-empirical method proposed by Bouchiat and Piketty for the calculation of the parity non-conserving El transition matrix element for the 65,,*-* 7s,,, transition in caesium is re-examined in terms of many body perturbation theory. A slightly modified formulation is derived and this is used to provide an improved value for the E l amplitude. The value is ElPNC = 0.904 (1 *0.02) x lo-'' (-ieaoQw/N).