Reactions of H 2 , HD, and D 2 with H 2 + , HD + , and D 2 + : Product-Channel Branching Ratios and Simple Models.

We present measurements of the product-channel branching ratios of the reactions (i) HD + + HD forming H 2 D + + D (38.1(30)%) and HD 2 + + H (61.9(30)%), (ii) HD + + D 2 forming HD 2 + + D (61.4(35)%) and D 3 + + H (38.6(35)%), and (iii) D 2 + + HD forming HD 2 + + D (60.5(20)%) and D 3 + + H (39.5(20)%) at collision energies E coll near zero, i.e., below k B × 1 K. These branching ratios are compared with branching ratios predicted using three simple models: a combinatorial model (M1), a model (M2) describing the reactions as H-, H + -, D-, and D + -transfer processes, and a statistical model (M3) that relates the reaction rate coefficients to the translational and rovibrational state densities of the H n D 3- n + + H/D ( n = 0, 1, 2 or 3) product channels. The experimental data are incompatible with the predictions of models M1 and M2 and reveal that the branching ratios exhibit clear correlations with the product state densities.