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Title: Collisional excitation of CH 2 rotational/fine-structure levels by helium

Abstract

Accurate determination of the abundance of CH 2 in interstellar media relies on both radiative and collisional rate coefficients. Here, we investigate here the rotational/fine-structure excitation of CH 2 induced by collisions with He. We employ a recoupling technique to generate fine-structure-resolved cross-sections and rate coefficients from close coupling spin-free scattering calculations. The calculations are based on a recent, high-accuracy CH 2–He potential energy surface computed at the coupled clusters level of theory. The collisional cross-section calculations are performed for all fine-structure transitions among the first 22 and 24 energy levels of ortho- and para-CH 2, respectively, and for temperatures up to 300 K. As a first application, we simulate the excitation of CH 2 in typical molecular clouds. The excitation temperatures of the CH 2 lines are found to be small at typical densities of molecular clouds, showing that the non-local thermodynamic equilibrium approach has to be used to analyse interstellar spectra. We also found that the fine-structure lines connected with the 404 – 313 and 505 – 414 rotational transitions show possible maser emissions so that they can be easily seen in emission. In conclusion, these calculations show that CH 2 may have to be detected mainly throughmore » absorption spectra.« less

Authors:
 [1];  [2]
  1. Johns Hopkins Univ., Baltimore, MD (United States)
  2. CNRS-Université du Havre (France); Univ. of Maryland, College Park, MD (United States)
Publication Date:
Research Org.:
Univ. of Maryland, College Park, MD (United States)
Sponsoring Org.:
USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22)
OSTI Identifier:
1512963
Grant/Contract Number:  
SC0002323
Resource Type:
Accepted Manuscript
Journal Name:
Monthly Notices of the Royal Astronomical Society
Additional Journal Information:
Journal Volume: 473; Journal Issue: 4; Journal ID: ISSN 0035-8711
Publisher:
Royal Astronomical Society
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; 79 ASTRONOMY AND ASTROPHYSICS; molecular data; molecular processes; ISM: abundances

Citation Formats

Dagdigian, P. J., and Lique, F. Collisional excitation of CH2 rotational/fine-structure levels by helium. United States: N. p., 2017. Web. doi:10.1093/mnras/stx2697.
Dagdigian, P. J., & Lique, F. Collisional excitation of CH2 rotational/fine-structure levels by helium. United States. doi:10.1093/mnras/stx2697.
Dagdigian, P. J., and Lique, F. Sat . "Collisional excitation of CH2 rotational/fine-structure levels by helium". United States. doi:10.1093/mnras/stx2697. https://www.osti.gov/servlets/purl/1512963.
@article{osti_1512963,
title = {Collisional excitation of CH2 rotational/fine-structure levels by helium},
author = {Dagdigian, P. J. and Lique, F.},
abstractNote = {Accurate determination of the abundance of CH2 in interstellar media relies on both radiative and collisional rate coefficients. Here, we investigate here the rotational/fine-structure excitation of CH2 induced by collisions with He. We employ a recoupling technique to generate fine-structure-resolved cross-sections and rate coefficients from close coupling spin-free scattering calculations. The calculations are based on a recent, high-accuracy CH2–He potential energy surface computed at the coupled clusters level of theory. The collisional cross-section calculations are performed for all fine-structure transitions among the first 22 and 24 energy levels of ortho- and para-CH2, respectively, and for temperatures up to 300 K. As a first application, we simulate the excitation of CH2 in typical molecular clouds. The excitation temperatures of the CH2 lines are found to be small at typical densities of molecular clouds, showing that the non-local thermodynamic equilibrium approach has to be used to analyse interstellar spectra. We also found that the fine-structure lines connected with the 404 – 313 and 505 – 414 rotational transitions show possible maser emissions so that they can be easily seen in emission. In conclusion, these calculations show that CH2 may have to be detected mainly through absorption spectra.},
doi = {10.1093/mnras/stx2697},
journal = {Monthly Notices of the Royal Astronomical Society},
number = 4,
volume = 473,
place = {United States},
year = {2017},
month = {10}
}

Journal Article:
Free Publicly Available Full Text
Publisher's Version of Record

Figures / Tables:

Figure 1 Figure 1: Energies of the low-lying rotational levels of CH2(X̃3B1) and the approximate frequencies (in GHz) of the allowed transitions between these levels. The ortho 220 level is connected to the 211 and 111 levels by transitions at 4925 and 5934 GHz (labelled A and B), respectively, and the paramore » 221 level is connected to the 212 and 110 levels by transitions at 5022 and 5895 GHz (labelled C and D), respectively, The fine-structure splittings are too small to be visible in the figure.« less

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