Molecular phonons and their absorption/emission spectra from the far IR to microwaves

TL;DR

This paper investigates the vibrational phonon spectra of large carbon-rich molecules, exploring their contribution to infrared and microwave emission in space, and deriving rules for their spectral distributions and detectability.

Contribution

It introduces new rules for phonon spectral distributions in molecules, extending understanding of their emission spectra into the millimeter range and assessing their astrophysical detectability.

Findings

Phonon spectra extend from ~20 to over 10^4 μm depending on molecular size.

Spectral energy distributions computed up to 4000 μm for molecules up to 50 Å long.

Maximum phonon wavelength in 2D structures scales roughly with the square of their size.

Abstract

Together with their fingerprint modes, molecules carry coherent vibrations of all their atoms (phonons). Phonon spectra extend from $\sim$20 to more than $10^{4}\,\mu$m, depending on molecular size. These spectra are discrete but large assemblies of molecules of the same family, differing only by minor structural details, will produce continua. As such assemblies are expected to exist in regions where dust accumulates, they are bound to contribute to the observed continua underlying the Unidentified Infrared Bands and the 21-mum band of planetary nebulae as well as to the diffuse galactic emission surveyed by the Planck astronomical satellite and other means. The purpose of this work is to determine, for carbon-rich molecules, the intensity of such continua and their extent into the millimetric range, and to evaluate their detectability in this range. The rules governing the spectral distributions of phonons are derived and shown to differ from those which obtain in the solid state. Their application allow the extinction cross-section per H atom, and its maximum wavelength, to be determined as a function of molecular size and dimensionality. Chemical modeling of more than 15 large molecules illustrate these results. It is found that the maximum phonon wavelength of a 2D structure increases roughly as the square of its larger dimension. Spectral energy distributions were computed as far as 4000 mum, for molecules up to 50 A{\deg} in length.