ELECTRONIC SPECTRA OF PERI-HEXABENZOCORONENE AND OVALENE ISOLATED IN SOLID PARA-HYDROGEN

Weber, Isabelle

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https://hdl.handle.net/2142/122612 Copy

Description

Title

ELECTRONIC SPECTRA OF PERI-HEXABENZOCORONENE AND OVALENE ISOLATED IN SOLID PARA-HYDROGEN

Author(s)

Weber, Isabelle

Contributor(s)

• Lee, Yuan-Pern
• Witek, Henryk A.
• Langner, Johanna

Issue Date

2023-06-23

Keyword(s)

Astronomy

Abstract

Diffuse interstellar bands (DIB), narrow absorption features observed from the near IR to the UV, have drawn a lot of interest since their first discovery in 1922. Polycyclic aromatic hydrocarbons (PAH) and their cationic, protonated and hydrogenated derivatives are considered particularly promising candidates for the DIB carriers, but laboratory spectra of these unstable species suitable for comparison to astronomical observations are scarce. para-Hydrogen (para-H₂) matrix isolation spectroscopy has frequently been employed to record the IR spectra of PAH derivatives. The obtained spectra exhibit only small shifts in line positions due to small interactions with the matrix host, in line with the ‘softness’ of the quantum solid para-H₂. However, electronic spectra of PAH isolated in solid para-H₂ have rarely been reported. Ovalene (C₃₂H₁₄) and peri-hexabenzocoronene (HBC, C₄₂H₁₈) have both been discussed as potential DIB carriers and, therefore, their electronic absorption spectra have been studied in the gas-phase and in rare gas matrices. To assess the properties of para-H₂ as a matrix host for electronic spectroscopy, we present the fluorescence excitation and dispersed fluorescence spectra of these two large PAH isolated in solid para-H₂. We located the 0⁰ bands of the S₁–S₀ transitions at 21049 cm−¹and about 22075 cm−¹for ovalene and HBC, respectively. The recorded excitation spectra in general show a good agreement with previously reported absorption spectra indicating a matrix shift below 100 cm−¹due to the para-H₂ matrix host, consistent with our earlier experiments on several smaller PAH. We complemented our experimental work with Franck-Condon Herzberg-Teller simulations on the basis of optimized geometries and vibrational frequencies obtained from (TD-)DFT calculations to derive a first assignment of individual vibrational modes to the observed absorption and emission bands associated with the electronic S₀–S₁ transition. For ovalene, we find that a consideration of the closely lying S₂ state is required to reproduce the complexity of the experimental excitation spectrum by the simulation.

Publisher

International Symposium on Molecular Spectroscopy

Type of Resource

Text

Language

eng

Handle URL

https://hdl.handle.net/2142/122612

DOI

https://doi.org/10.15278/isms.2023.7121

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