C + +H2( j) --> CH + +H: The effect of reagent rotation on the integral cross section in the threshold region

Integral cross sections for the reaction of C⁺ with hydrogen have been determined as a function of translational and rotational energy. Measurements were made with a guided beam apparatus, using a temperature variable scattering cell or alternatively a nozzle beam and different ortho-para mixtures of H₂. The detailed results are in good agreement with statistical phase space calculations provided that the nuclear spins are assumed to be completely decoupled (frozen spin approximation). One consequence of fundamental interest is that H₂( j=1) is less reactive than H₂( j=0) in the threshold region. Further, the theory predicts an oscillatory structure for the kinetic energy dependence of the reaction cross section which has been observed for the first time. The calculated cross sections are used to predict reliable rate coefficients for H₂ in selected rotational ( j=0-7) states. An analytical approximation of these results allows a simple calculation of rate coefficients for nonequilibrium rotational populations as can be found, e.g., in interstellar space.