Reactions of H2, HD, and D2 with H2 +, HD+, and D2 +: Product-Channel Branching Ratios and Simple Models

We present measurements of the product-channel branching ratios of the reactions (i) HD+ + HD forming H2D+ + D (38.1(30)%) and HD2 + + H (61.9(30)%), (ii) HD+ + D2 forming HD2 + + D (61.4(35)%) and D3 + + H (38.6(35)%), and (iii) D2 + + HD forming HD2 ++ D (60.5(20)%) and D3 + + 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 D3–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.