Simulation of vibrationally resolved absorption spectra of neutral and cationic polyaromatic hydrocarbons
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Simulation of vibrationally resolved absorption spectra of neutral and cationic polyaromatic hydrocarbons Rebecca J. Chadwick1 · Katherine Wickham1 · Nicholas A. Besley1 Received: 12 August 2020 / Accepted: 11 November 2020 © The Author(s) 2020
Abstract The identification of the carriers of the absorption features associated with the diffuse interstellar bands (DIBs) is a longstanding problem in astronomical spectroscopy. Computational simulations can contribute to the assignment of the carriers of DIBs since variations in molecular structure and charge state can be studied more readily than through experimental measurements. Polyaromatic hydrocarbons have been proposed as potential carriers of these bands, and it is shown that simulations based upon density functional theory and time-dependent density functional theory calculations can describe the vibrational structure observed in experiment for neutral and cationic naphthalene and pyrene. The vibrational structure arises from a small number of vibrational modes involving in-plane atomic motions, and the Franck–Condon–Herzberg– Teller approximation improves the predicted spectra in comparison with the Franck–Condon approximation. The study also highlights the challenges for the calculations to enable the assignment in the absence of experimental data, namely prediction of the energy separation between the different electronic states to a sufficient level of accuracy and performing vibrational analysis for higher-lying electronic states. Keywords Polyaromatic hydrocarbons · Vibrational structure · Diffuse interstellar bands
1 Introduction The diffuse interstellar bands (DIBs) are absorption features associated with the interstellar medium [1–3]. DIBs were first reported in 1922 [1], and the identification of the carriers of the DIBs has proved to be elusive, with their assignment being referred to as the longest standing challenge in astronomical spectroscopy [4]. It has long been believed that these bands have a molecular origin, and many potential candidates have been proposed [5–7]. It has been postulated that DIBs are associated with neutral and charged large carbon containing molecules that are present within the interstellar dust [7, 8]. One example is C +60 which has been confirmed as the source of two lines in the DIBs [9, 10]. Polyaromatic Electronic supplementary material The online version of this article (https://doi.org/10.1007/s00214-020-02697-7) contains supplementary material, which is available to authorised users. * Nicholas A. Besley [email protected] 1
School of Chemistry, University of Nottingham, University Park, Nottingham NG7 2RD, UK
hydrocarbons (PAHs) have also been proposed as potential carriers of DIBs [11, 12], and there has been considerable effort toward confirming the association of PAHs with DIBs [13–15]. The unambiguous assignment of lines in the DIBs is extremely challenging and a detailed characterisation of the relevant spectra of potential carriers is a vital component to achieve a defi
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