State specificity in association reaction of H3+ with H2

The three-body association reaction of H3+ with H2 assisted by helium and H2 was studied using a 22-pole radiofrequency ion trap instrument in the temperature range of 15-35 K. The role of the rotational excitation of H2 was investigated in measurements with normal and para-enriched H2. From the dependence of the measured three-body reaction rate coefficients on the character of the third body and on the H2 nuclear spin state, we found that rotationally excited (normal) H2 is less efficient in stabilizing the (H5+)* intermediate complex than both He and ground-state (para) H2. The lifetime of the (H5+)* complex was found to be ∼3 times higher in para-enriched H2 than in normal H2.