H3++HD↔H2D++H2: low-temperature laboratory measurements and interstellar implications

The system of reactions H3++HD↔H2D++H2 has been studied in a low-temperature multipole ion trap at a nominal temperature of 10 K. The rate coefficient k1 for the forward reaction has been found to be 3.5×10−10 cm3 s−1 at 10 K, a value significantly smaller than the currently accepted value of 1.5×10−9 cm3 s−1. The rate coefficient k−1 for the backward reaction has been found to be much higher than the value derived from the equilibrium coefficient of ∼10−18 cm3 s1. For normal-hydrogen, a value of k−1=4.9×10−11 cm3 s−1 has been deduced while for almost pure para-hydrogen (purity 99±1%) the value drops to k−1=7.3×10−13 cm3 s−1. The results are discussed on the basis of the well-known energy levels of the involved molecules and the potential energy surface of the H4D+ intermediate. Zero-point energy plays a key role; however, there are additional complications due to the formation of rotationally excited H2D+, if even traces of ortho-H2 (o-H2) are present. The consequences of the results for the chemistry of cold clouds are illustrated using an evolutive gas-phase chemical model. There is strong evidence, that the new results significantly reduce the efficiency of isotope fractionation via gas-phase reactions. The experimental results also indicate the need for state-to-state rate coefficients in order to correctly simulate the o-H2 induced non-equilibrium conditions prevailing both in the low temperature ion trap and in interstellar clouds.