We report extensive theoretical calculations on the rotation-inversion excitation of interstellar ammonia (NH 3 ) due to collisions with atomic and molecular hydrogen (both para-and ortho-H 2 ). Close-coupling calculations are performed for total energies in the range 1-2000 cm −1 and rotational cross sections are obtained for all transitions among the lowest 17 and 34 rotation-inversion levels of ortho-and para-NH 3 , respectively. Rate coefficients are deduced for kinetic temperatures up to 200 K. Propensity rules for the three colliding partners are discussed and we also compare the new results to previous calculations for the spherically symmetrical He and para-H 2 projectiles. Significant differences are found between the different sets of calculations. Finally, we test the impact of the new rate coefficients on the calibration of the ammonia thermometer. We find that the calibration curve is only weakly sensitive to the colliding partner and we confirm that the ammonia thermometer is robust.

We present quantum close-coupling calculations for the rotational excitation of the interstellar amidogen radical NH due to collisions with H molecules. The calculations are based on a recent, high-accuracy full-dimensional NH potential energy surface adapted for rigid-rotor scattering calculations. The collisional cross section calculations are performed for all transitions among the first 15 energy levels of both ortho- and para-NH and for total energies up to 1500 cm. Both para- and ortho-H colliding partners are considered. The cross sections for collision with para- and ortho-H are found to differ significantly, the magnitude of the ortho-H ones being dominant. No strong propensity rules are observed but transitions with Δk=0 are slightly favored.

We present a new three-dimensional potential energy surface (PES) for the NH(X(3)Σ(-))-Ne van der Waals system, which explicitly takes into account the NH vibrational motion. Ab initio calculations of the NH-Ne PES were carried out using the open-shell single- and double-excitation coupled cluster approach with non-iterative perturbational treatment of triple excitations [RCCSD(T)]. The augmented correlation-consistent quadruple zeta (aug-cc-pVQZ) basis set was employed. Mid-bond functions were also included in order to improve the accuracy in the van der Waals well. Using this new PES, we have studied the collisional excitation of NH(X(3)Σ(-)) by Ne. Close-coupling calculations of the collisional excitation cross sections of the fine-structure levels of NH by Ne are performed for energies up to 3000 cm(-1), which yield, after thermal average, rate coefficients up to 350 K. The propensity rules between fine-structure levels are reported, and it is found that F-conserving cross sections are larger than F-changing cross sections even if the propensity rules are not as strong as for the NH-He system. The calculated rate coefficients are compared with available experimental measurements at room temperature and a fairly good agreement is found between experimental and theoretical data, confirming the good quality of the scattering calculations and also the accuracy of the potential energy surface used in this work.

Accurate estimation of the abundance of the NH2 radical in the interstellar medium requires accurate radiative and collisional rate coefficients. The calculation of hyperfine-resolved rate coefficients for the collisional (de-)excitation of NH2 by both ortho- and para-H2 is presented in this work. Hyperfine-resolved rate coefficients are calculated from pure rotational close-coupling rate coefficients using the Mj randomizing approximation. Rate coefficients for temperatures ranging from 5 to 150 K were computed for all hyperfine transitions among the first 15 rotational energy levels of both ortho- and para-NH2 in collisions with ortho- and para-H2. The new data were then employed in radiative transfer calculations to simulate the excitation of NH2 in typical star-forming regions such as W31C, where NH2 is seen in emission. We compared the excitation and brightness temperatures for different NH2 transitions obtained using the new and the previously available collisional data. It is found that the new rate coefficients increase the simulated line intensities by a factor ∼10–30. As a consequence, NH2 abundance derived from the observations will be significantly reduced by the use of the present rate coefficients.

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