ON THE DYNAMICS OF THE REACTION OF POSITIVE HYDROGEN CLUSTER IONS (H-5(+) TO H-23(+)) WITH PARA AND NORMAL HYDROGEN AT 10 K

The dynamics of clustering and fragmentation reactions H(i)(+) + 2H(2) reversible arrow H(i+2)(+) + H(2), for odd i, was studied at a nominal temperature of 10 R in a 22-pole radio-frequency ion trap in normal hydrogen and para-enriched hydrogen. Ternary association rate coefficients, k(3), and binary fragmentation rate coefficients, k(f), were extracted from the measured temporal evolution of the hydrogen cluster ion intensity, I(H(i)(+)), for i = 3, ..., 23. Pure para hydrogen enhances the rate coefficients for association and fragmentation. For i > 9 this general trend is explained by a difference in the capture cross-sections, k(c), for the two hydrogen nuclear spin modifications. Significant differences in k(3) which remain for small clusters (i < 9) are due to the availability of the J = 1 rotational energy of the ortho modification when merging into the cluster. This surprising result is discussed in the framework of simple dynamical and energetic considerations. Possible structures of the cluster can be classified and estimates for the bond energy of the outermost H(2) in the cluster as a function of cluster size are derived.