Symmetry breakdown in ground state dissociation of HD

Experimental studies of the dissociation of the electronic ground state of HD+ following ionization of HD by fast proton impact indicate that the H+ + D(1s) dissociation channel is more likely than the H(1s) + D+ dissociation channel by about 7%. This isotopic symmetry breakdown is due to the finite nuclear mass correction to the Born-Oppenheimer approximation which makes the 1s sigma state 3.7 meV lower than the 2p sigma state at the dissociation limit. The measured fractions of the two dissociation channels are in agreement with coupled-channels calculations of 1s sigma to 2p sigma transitions.