O r t h o–p a r a transitions in reactive H++H2 collisions

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Abstract
An experimentally well-proven dynamically biased statistical theory is used to calculate state specific cross sections and product state distributions for the proton–hydrogen scrambling reaction. Conservation of parity and nuclear spin and the anisotropy of the interaction potential are explicitly taken into account. The influence of the permutation symmetry is explained with a simple statistical model. The results are used to calculate rate coefficients for state to state transitions, and for conversion of ortho-H2 into para-H2. At low energies (<43 meV), the decay of a H+⋅H2( j=1) complex into the exothermic H++H2( j=0) channel is strongly hindered, the probability is smaller than 1/12. Therefore, this j=1 to j=0 transition occurs with a rate coefficient of only 2.2×10−10 cm3 /s at temperatures of interstellar space. A van’t Hoff plot of the results shows that the ortho–para coupling via proton exchange leads to an equilibrium abundance ratio n(ortho)/n(para)=9.35⋅exp(−169.4 K/T). The concept of separate conservation of nuclear spin during the lifetime of the intermediate complex was tested experimentally in a high resolution molecular beam experiment. The partially resolved rovibrational distributions are in good agreement with populations calculated with the statistical model.