The rate of nuclear fusion in the molecule is of the order of 1012 s-1, hence it is too fast to measure experimentally. The reaction is strongly dominated by the resonance 5He , which lies about 55 keV above threshold. Because of the centrifugal barrier of the J=1 states of the molecule, fusion is expected to take place mainly from J=0 states, i.e., (J,v)=(0,1) and (0,0), into which the initial state (1,1) is quickly converted via fast E1Auger transitions.
For , on the other hand, the transition from is strongly suppressed (since it requires spin flip, analogous to the ortho-para transitions in the homonuclear hydrogen molecule), and fusion takes place mainly from the J=1 states, if the molecule is resonantly formed in the (1,1) state. This in fact offers a unique opportunity to study the p wave fusion reaction at very low energy, which is difficult to do in a beam experiment. An interesting feature of this reaction is that the fusion width ratio of the (n+ 3He) channel to the (p + t) channel is about 1.4, apparently violating charge symmetry.
In a simple approach, the fusion rate is calculated by treating three-body
Coulomb physics (molecular wave functions) and nuclear physics
separately , a method known as factorization. For ,
More accurate approaches treat the nuclear force dynamically by directly incorporating it as a complex potential in the three-body Hamiltonian (optical potential method) [55,56], or as complex boundary conditions at the nuclear surface (R-matrix method) [57,58,59]. These elaborate approaches agree rather well with each other, as well as with the simple factorization approach.
In collisions, fusion ``in flight'', i.e., without forming a bound molecule, is also possible, and its rates have been calculated by various methods including, most recently, Faddeev equations (see  and references therein). The reactions are enhanced, compared to bare nuclear collisions, especially if virtual states exist, due to the increased overlap of the nuclear wave functions.