Rotational excitation of molecular ions in interstellar clouds

Rate constants for rotational excitation of N2H(+) by low-energy collisions with He have been obtained from accurate quantum-mechanical calculations. Rates for excitation of HCO(+) and rates for excitation by collisions with H2 are expected to be qualitatively similar. The excitation rate for molecular ions is found to be only slightly faster than that for similar neutral species, and not much faster as had been previously suggested. The implications of this for interpreting microwave observations of interstellar N2H(+) and HCO(+) are discussed.