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 dissociatio n limit. The measured fractions of the two dissociation channels are in agr eement with coupled-channels calculations of 1s sigma to 2p sigma transitio ns.