ORTHO-PARA TRANSITIONS IN REACTIVE H+ + H2 COLLISIONS

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 cm 3/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. © 1990 American Institute of Physics.