A Spectroscopic View on Cosmic PAH Emission

Abstract

Metrics & MoreArticle Recommendations CONSPECTUS: Polycyclic aromatic hydrocarbon molecules (PAHs) are ubiquitously present at high abundances in the Universe. They are detected through their infrared (IR) fluorescence UV pumped by nearby massive stars. Hence, their infrared emission is used to determine the star formation rate in galaxies, one of the key indicators for understanding the evolution of galaxies. Together with fullerenes, PAHs are the largest molecules found in space. They significantly partake in a variety of physical and chemical processes in space, influencing star and planet formation as well as galaxy evolution.Since the IR features from PAHs originate from chemical bonds involving only nearest neighbor atoms, they have only a weak dependence on the size and structure of the molecule, and it is therefore not possible to identify the individual PAH molecules that make up the cosmic PAH family. This strongly hampers the interpretation of their astronomical fingerprints. Despite the lack of identification, constraints can be set on the characteristics of the cosmic PAH family thanks to a joint effort of astronomers, physicists, and chemists. This Account presents the spectroscopic properties of the cosmic PAH emission as well as the intrinsic spectroscopic properties of PAHs and astronomical modeling of the PAH evolution required for the interpretation of the cosmic PAH characteristics. We discuss the observed spectral signatures tracing PAH properties such as charge, size, and structure and highlight the related challenges. We discuss the recent success of anharmonic calculations of PAH infrared absorption and emission spectra and outline the path forward. Finally, we illustrate the importance of models on PAH processing for the interpretation of the astronomical data in terms of the charge balance and PAH destruction. Throughout this Account, we emphasize that huge progress is on the horizon on the astronomical front. Indeed, the world is eagerly awaiting the launch of the James Webb Space Telescope (JWST). With its incredible improvement in spatial resolution, combined with its complete spectral coverage of the PAH infrared emission bands at medium spectral resolution and superb sensitivity, the JWST will revolutionize PAH research. Previous observations could only present spectra averaged over regions with vastly different properties, thus greatly confusing their interpretation. The amazing spatial resolution of JWST will disentangle these different regions. This will allow us to quantify precisely how PAHs are modified by the physical conditions of their host environment and thus trace how PAHs evolve across space. However, this will only be achieved when the necessary (and still missing) fundamental properties of PAHs, outlined in this Account, are known. We strongly encourage you to join this effort.

The mystery of unidentified infrared emission bands

“…Due to the strengths of the UIE bands, the carrier represents a major reservoir of carbon. If the carrier of UIE bands are PAH molecules, they would contain ∼10% of cosmic carbon (Peeters et al 2021). Many current models of the interstellar medium are based the premise that PAH molecules are the dominant factors in photoelectric heating of interstellar gas and in the ionization balance inside molecular clouds.…”

“…The 3.3 µm UIE band is assigned to C-H stretching modes, the 6.2 µm and 7.7 µm features to C-C stretching modes, the 8.6 µm feature to C-H in-plane bending modes, and the 11.3 µm to C-H out-of-plane bending modes of PAH molecules (Allamandola et al 1989). Details of the PAH hypothesis are reviewed by Peeters (2011) and Peeters et al (2021). The application of PAH hypothesis to the study of galaxies is reviewed by Li (2020).…”

“…In part to address the above list of problems, the PAH hypothesis has been revised to incorporate different ionization states and large sizes to increase the absorption cross sections in the visible. The molecular size range has been extended to >1,000 atoms (Peeters et al 2021). The PAH molecules are also modified to include dehydrogenation, superhydrogenation and minor aliphatic side groups (Li and Draine 2012).…”

“…Details of the PAH hypothesis are reviewed by Peeters ( 2011 ) and Peeters et al. ( 2021 ). The application of PAH hypothesis to the study of galaxies is reviewed by Li ( 2020 ).…”

Ab Initio Quantum-Mechanical Predictions of Semiconducting Photocathode Materials

“…Current applications of ab initio material modeling encompass a huge range of scientific fields, from astrochemistry [ 49 ] to photobiology [ 50 ], and from metallurgy [ 51 ] to geology [ 52 ]. The reason for this immense success is certainly related to the versatility of first-principles methods, which require only structural information and chemical composition as input in calculations, as well as to the excellent parallelizability and scalability of corresponding algorithms [ 53 , 54 ], which currently enable the treatment of realistic systems described by up to several thousands of electrons.…”