Infrared Spectroscopy of Symmetric and Spherical Top Molecules for Space Observation, Volume 2

6Adsorption on a Graphite Substrate

The theoretical models presented in Volumes 1 and 2 of this set are applied to the construction of far infrared (orientational spectrum) and near-infrared spectra in the frequency ranges of vibrational modes of the symmetric top NH₃ molecule adsorbed on a graphite substrate. These vibrational modes can be subjected to frequency shifts and/or degeneracy lifting when the molecule is in this anisotropic environment. This can be used to determine the characteristics of this environment by comparing the spectrum of the nano-perturbed molecule to that of the isolated molecule.

6.1. Introduction

Observations in various cold regions of the interstellar medium, where the temperature nears -250°C (about 20 K), revealed the presence of several hundred molecules, spanning from the simplest ones, composed of several atoms, to complex molecules, among which are many organic molecules. These regions are also characterized by a high density of atoms and molecules (n ≥10³ cm^-3). Moreover, the signature at 2,175 Å in the observed extinction spectra of the interstellar medium is attributed to the presence of carbon grains that are small, rather spherical solids (graphite or diamond) or amorphous of dimension ~ 200 Å or of polycyclic aromatic hydrocarbon (PAH) molecules [DRA 88, MAT 89].

Chapter 5 of Volume 1 and Chapter 6 of Volume 2 [DAH 17, DAH 19a] presented the adsorption of diatomic and triatomic molecules on a graphite substrate (1000). This substrate ...