Hyperfine collisional excitation of ammonia by molecular hydrogen

TL;DR

This paper presents the first quantum mechanical calculations of hyperfine excitation rate coefficients for ammonia colliding with molecular hydrogen, crucial for interpreting interstellar ammonia spectra with hyperfine structure anomalies.

Contribution

It introduces a recoupling approach for calculating hyperfine excitation rates of NH₃ by H₂, providing data up to 100 K and all hyperfine levels within certain rotational states.

Findings

Rate coefficients differ significantly from statistical estimates.

No simple propensity rules govern hyperfine transitions.

Radiative transfer models show impact on ammonia spectral line intensities.

Abstract

Ammonia is one of the most widely observed molecules in space, and many observations are able to resolve the hyperfine structure due to the electric quadrupole moment of the nitrogen nucleus. The observed spectra often display anomalies in the satellite components of the lines, which indicate substantial deviations from the local thermodynamic equilibrium. The interpretation of the spectra thus requires the knowledge of the rate coefficients for the hyperfine excitation of NH$_3$ induced by collisions with H$_2$ molecules, the dominant collider in the cold interstellar medium. In this paper we present the first such calculations using a recoupling approach. The rate coefficients are obtained for all hyperfine levels within rotation-inversion levels up to $j=4$ and temperatures up to 100 K by means of quantum scattering close-coupling calculations on an accurate, five-dimensional, potential energy surface. We show that the rate coefficients depart significantly from those obtained with the statistical approach and that they do not conform to any simple propensity rules. Finally, we perform radiative transfer calculations to illustrate the impact of our new rate coefficients by modelling the hyperfine line intensities of the inversion transition in ground state para-NH$_3$ ($j_k=1_1$) and of the rotational transition $1_0\rightarrow 0_0$ in ortho-NH$_3$.