Determination of the E2/M1 amplitude ratio in atoms from the polarisation dependence of two-colour mixing

It is shown that the polarisation dependence of two-colour mixing in free atoms can be used to measure the E2/M1 ratio of contributions to the amplitude of a dipole-forbidden radiative transition. The dependence is calculated for indium and thallium atoms, in which this ratio plays an important role in the description of parity nonconservation effects in optical atomic spectra.