  We show that isospin non-diagonal terms should appear in the mean field
Hamiltonian when neutron-proton symmetry is broken. They give rise to charge
mixing in the single-particle wave-functions. We study the Time Dependent
Hartree-Fock response of a charge-exchange excitation which generates a charge
mixing in Ca isotopes. We find an enhancement of the low energy proton emission
in neutron-rich isotopes interpreted in terms of a charge oscillation below the
barrier.
