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

Details H3++HD<-- >H2D++H2: low-temperature laboratory measurements and interstellar implications H3++HD<-- >H2D++H2: low-temperature laboratory measurements and interstellar implications

Oct 2002

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002p%26ss...50.1275g&link_type=abstract

Planetary and Space Science, Volume 50, Issue 12-13, p. 1275-1285.

Computer Science

85

Scientific paper

The system of reactions H3++HD<-- >H2D++H2 has been studied in a low-temperature multipole ion trap at a nominal temperature of 10K. The rate coefficient k1 for the forward reaction has been found to be 3.5×10-10cm3s-1 at 10K, a value significantly smaller than the currently accepted value of 1.5×10-9cm3s-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 ~10-18cm3s1. For normal-hydrogen, a value of k-1=4.9×10-11cm3s-1 has been deduced while for almost pure para-hydrogen (purity /99+/-1%) the value drops to k-1=7.3×10-13cm3s-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.

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