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Rotational excitation and de-excitation of interstellar ammonia in collisions with helium

We present quantal close coupling (CC) calculations of rotational excitation rate coefficients of ammonia NH3 by collision with He. Calculations have been performed with the new accurate interaction potential energy surface of Hodges and Wheatley (2001 J. Chem. Phys. 114 8836). Comparison with previous theoretical and experimental studies is performed. The MOLSCAT molecular collision code of Hutson and Green has been used at two different levels of approximation. We find that the coupled states (CS) approximation gives results for the rotational excitation collision cross sections which are consistent with CC calculations. The sensitivity to the potential energy surface is emphasized.