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Panel Discussion on the Quantum Statistical and Thermal Treatments of Processes in Molecule-Sized PAHs

Published online by Cambridge University Press:  23 September 2016

Sydney Leach*
Affiliation:
Département Atomes et Molécules en Astrophysique Meudon, France Laboratoire de Photophysique Moléculaire du C. N. R. S. Orsay, France

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This discussion followed the two talks, one by John Barker on “Grains or Molecules, Thermal or Non-Thermal,” the second, by Louis d'Hendecourt on “The Particle/Thermal Approach to the Physics of Emission from Interstellar PAHs.” The presentations discussed the factors involved in the process of absorption of an ultraviolet photon by a PAH molecule or ion, followed by infrared and/or visible emission. Model calculations were presented for various stages of those sequential events, as a function of PAH size. The material presented applied to large molecules in general and was not restricted to the PAH case. The models sought to investigate the radiative, and nonradiative intramolecular processes, including dissociation, that are competitive for conversion of the absorbed ultraviolet photon energy. The essential differences between the Barker and d'Hendecourt approaches were brought out by estimations of the range of physical parameters which delimited the approximate validity of a thermal treatment using Boltzmann statistics or the Planck function as compared with a more formally correct quantum statistical (i. e. non-thermal) approach to energy conversion and dissipation. It was concluded that the thermal approximation was reasonably satisfactory at high (absorbed) photon energies in explaining conversion to an infrared emission situation, but was unsatisfactory at low energies, when the (vibrational) excitation of the excited molecule was of the same order of magnitude as the emitted infrared photons. It was also not valid for discussions of the competitive unimolecular dissociation of excited PAHs, such as denydrogenation and fragmentation. In those cases the quantum statistical approach was required.

Type
Section II: The Overidentified Infrared Emission Features
Copyright
Copyright © Kluwer 1989 
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