Simulation of UIB spectra with IR emission from CHONS molecules

TL;DR

This study develops a computational method to simulate the IR emission spectra of various CHONS molecules, aiming to identify carriers of Unidentified Infrared Bands (UIBs) by matching observed spectral features.

Contribution

It introduces a new simulation procedure for IR emission spectra of candidate molecules, applied to over 100 structures inspired by kerogens, to identify potential UIB carriers.

Findings

Simulated spectra match main UIB features within computational errors.

Identified four classes of molecules contributing to different UIBs.

Adjustable weights can mimic observed variations in sky spectra.

Abstract

The present work purports to identify candidate carriers of the UIBs. This requires a procedure for the computation of the emission spectrum of any given candidate. The procedure used here consists in exciting the carrier into a state of internal vibration, waiting until the system has reached dynamic equilibrium and, then, monitoring the time variations of the overall electric dipole moment associated with this vibration. The emission spectrum is shown to be simply related to the FT of these variations. This procedure was applied to more than 100 different chemical structures, inspired by the exhaustive experimental and theoretical analyses of Kerogens, the terrestrial sedimentary matter, which is known to be mainly composed of C, H, O, N and S atoms. From this data base, 21 structures were extracted, which fall in 4 classes, each of which contributes preferentially to one of the main UIBs. Summing their adequately weighted spectra delivers an emission spectrum which indeed exhibits the main UIB features (allowing for computational errors inherent in the chemical simulation code). By changing the weights, it is possible to change the relative band intensities so as to mimic the corresponding (moderate) changes observed in the sky. The defects of the present simulation are discussed, and directions for improvement are explored.