Parity violation effects in diatomics

Discussion of the parity violation and the break of the time-reversal invariance in diatomic molecules has continued for a number of years. Experiments on the TlF molecule gave one of the most stringent limits on the electric-dipole moment of the proton and on the T-violating nuclear forces. At present, a new generation of experiments with paramagnetic diatomic molecules is underway. These experiments are aimed mainly at the search for the electric-dipole moment of the electron. In this topical review we examine theoretical aspects of parity non-conservation in diatomic molecules. We focus on molecular theory leaving aside the nuclear part of the problem. In this approach the nucleus is characterized by the number of P-odd and P,T-odd moments, namely anapole moment, Schiff moment and magnetic quadrupole moment. Molecular theory has to link these moments to the experimentally measured quantities, such as frequency shifts, etc. The other possible sources of the parity non-conservation in molecules are the electron-nuclear neutral current interactions and the electric-dipole moment of the electron. They are also discussed in this review.