Scattering of NH3 by <i>ortho</i>- and <i>para</i>-H2: Expansion of the potential and collisional propensity rules

Rist, Claire; Alexander, Millard H.; Valiron, P.

doi:10.1063/1.464970

Cited by 44 publications

(47 citation statements)

References 29 publications

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“…For each rotational state j ǫ k , the umbrella inversion symmetry is equal to −(−1) j ǫ and the parity index of the rotation-inversion wave function is −ǫ(−1) j+k (Rist, Alexander & Valiron 1993). Because of the symmetry under permutation of the three identical protons, the rotational states are split into two k stacks : ortho-NH3 which correspond to k=3n (with n an integer) and para-NH3 with k = 3n (hereafter denoted as o-NH3 and p-NH3 respectively).…”

Section: Scattering Calculationsmentioning

confidence: 99%

“…In the calculations of both Danby et al (1988) and Maret et al (2009), H2 was restricted to its (spherically symmetrical) para j2 = 0 state. As a result, there is no rate coefficient available for collisions with ortho-H2, even if some ortho-H2 cross sections were computed at a few selected energies (Offer & Flower 1989;Rist, Alexander & Valiron 1993) with the PES of Danby et al (1986). In addition, the calculations of Maret et al (2009) for para-H2 need to be improved by including j2 = 2 in the scattering basis set.…”

Section: Introductionmentioning

confidence: 99%

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Collisional excitation of NH3 by atomic and molecular hydrogen

Bouhafs

Rist

Daniel

et al. 2017

Monthly Notices of the Royal Astronomical Society

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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.

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Section: Scattering Calculationsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Collisional excitation of NH3 by atomic and molecular hydrogen

Bouhafs

Rist

Daniel

et al. 2017

Monthly Notices of the Royal Astronomical Society

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“…Energy-level diagram of NH 3 , where the rotational quantum number, J, is placed to the left of each level, the symmetry index, = + or −, is placed to the right of each level. The symmetry index is that defined by Rist et al (1993), which might differ from common spectroscopic notation. The transitions are marked with colored arrows, where the direction corresponds to absorption.…”

Section: Spectral Propertiesmentioning

confidence: 99%

On the accretion process in a high-mass star forming region

Hajigholi

Persson

Wirström

et al. 2016

A&A

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Aims. Our aim is to explore the gas dynamics and the accretion process in the early phase of high-mass star formation. Methods. The inward motion of molecular gas in the massive star forming region G34.26+0.15 is investigated by using high-resolution profiles of seven transitions of ammonia at THz frequencies observed with Herschel-HIFI. The shapes and intensities of these lines are interpreted in terms of radiative transfer models of a spherical, collapsing molecular envelope. An accelerated Lambda Iteration (ALI) method is used to compute the models. Results. The seven ammonia lines show mixed absorption and emission with inverse P-Cygni-type profiles that suggest infall onto the central source. A trend toward absorption at increasingly higher velocities for higher excitation transitions is clearly seen in the line profiles. The J = 3 ← 2 lines show only very weak emission, so these absorption profiles can be used directly to analyze the inward motion of the gas. This is the first time a multitransitional study of spectrally resolved rotational ammonia lines has been used for this purpose. Broad emission is, in addition, mixed with the absorption in the 1 0 -0 0 ortho-NH 3 line, possibly tracing a molecular outflow from the star forming region. The best-fitting ALI model reproduces the continuum fluxes and line profiles, but slightly underpredicts the emission and absorption depth in the ground-state ortho line 1 0 -0 0 . An ammonia abundance on the order of 10 −9 relative to H 2 is needed to fit the profiles. The derived ortho-to-para ratio is approximately 0.5 throughout the infalling cloud core similar to recent findings for translucent clouds in sight lines toward W31C and W49N. We find evidence of two gas components moving inwards toward the central region with constant velocities: 2.7 and 5.3 km s −1 , relative to the source systemic velocity. Attempts to model the inward motion with a single gas cloud in free-fall collapse did not succeed.

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“…[26][27][28]42 We approximate the inversion doublet wave functions of NH 3 /ND 3 as even and odd combinations of the two rigid equilibrium structures multiplied by rigid rotor wave functions, namely,…”

Section: Scattering Calculationsmentioning

confidence: 99%

“…[25][26][27][28][29][30][31][32][33][34][35] Earlier work has focused on collisions out of the ground rotational level of ortho-NH 3 , which is not suitable for Stark selection. Strong resonance peaks do appear in the computed energy dependence of state-to-state cross sections (see Fig.…”

Section: Introductionmentioning

confidence: 99%