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

The system of reactions H-3(+) + HD <---->H2D+ + H-2 has been studied in a low-temperature multipole ion trap at a nominal temperature of 3 10 K. The rate coefficient k(1) for the forward reaction has been found to be 3.5 X 10(-10) cm(3) s(-1) at 10 K, a value significantly smaller than the currently accepted value of 1.5 x 10(-9) cm(3) 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 similar to 10(-18) cm(3) s(1). For normal-hydrogen, a value of k(-1) = 4.9 x 10(-11) cm(3) s(-1) has been deduced while for almost pure para-hydrogen (purity 99 +/- 1%) the value drops to k(-1) = 7.3 x 10(-13) cm(3) 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-H-2 (o-H-2) 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-H-2 induced non-equilibrium conditions prevailing both in the low temperature ion trap and in interstellar clouds. (C) 2002 Elsevier Science Ltd. All rights reserved.