Selective hyperfine excitation of N2H+ by He:: Potential energy surface, cross sections, and propensity rules

We present potential energy surfaces for the He-N2H+ system adiabatically corrected for the zero-point motion along the intermolecular stretching vibrations v(1)=0 and v(1)=1. The potentials are extended to shorter He-N2H+ separations which makes them useful for scattering calculations. Close coupling calculations of the spinless S matrices for the rotational excitation of N2H+ by He are presented, and recoupling techniques to obtain collisional excitation cross sections between the N2H+ hyperfine levels are used. The propensity rules between hyperfine levels are investigated for the case where two nuclear spins are involved. It is found that the only well defined propensity rule is DeltaF=DeltaF(1)=Deltaj and that calculations are required in order to obtain the relative intensities of the two-spin hyperfine cross sections. (C) 2004 American Institute of Physics.