The mystery of unidentified infrared emission bands

Abstract

A family of unidentified infrared emission (UIE) bands has been observed throughout the Universe. The current observed spectral properties of the UIE bands are summarized. These properties are discussed in the frameworks of different models of the chemical carriers of these bands. The UIE carriers represent a large reservoir of carbon in the Universe, and play a significant role in the physical and chemical processes in the interstellar medium and galactic environment. A correct identification of the carrier of the UIE bands is needed to use these bands as probes of galactic evolution.

Buckyball-metal Complexes as Potential Carriers of Astronomical Unidentified Infrared Emission Bands

“…The origin and formation mechanisms of fullerenes in space are still mysterious (Woods 2020;Kwok 2022). Recalling the catalytic roles of metals in the nucleation and growth of various carbon nanostructures, including carbon nanotubes (Esconjauregui et al 2009) and carbon cages under oxygen-and hydrogen-rich conditions (Dunk et al 2013(Dunk et al , 2014, we conjecture that the presence of fullerene-metal complexes can link the chemical pathways and mechanisms of the formation and evolution of various carbonaceous species and thus carbon chemistry in space.…”

“…Although the presence of C 60 in space is nowadays unambiguous, current knowledge is incomplete as neither fluorescence nor thermal excitation mechanisms can explain the observed band intensity ratios of the four C 60 features, while the carriers of many other UIE features remain unknown (Cami et al 2010;Bernard-Salas et al 2012;Berné et al 2013;Otsuka et al 2016;Woods 2020;Kwok 2022). One possible source could be fullerene complexes with other astronomically abundant species.…”

Structure and evolution of interstellar carbonaceous dust. Insights from the laboratory

“…Complex carbon-based molecules and solids make much of interstellar dust. Progress in the study of IS carbonaceous dust and related species is reviewed in several recent works (Tielens, 2008;Kwok, 2016;Herrero et al, 2018;Dartois, 2019;Peeters et al, 2021;Hansen et al, 2022;Kwok 2022).…”

“…The PAH hypothesis has been criticized (Kwok and Zhang, 2011;Kwok and Zhang, 2013;Zhang and Kwok, 2015;Kwok, 2016;Kwok, 2022) because an artificial mixture of PAH structures, sizes and ionization states, with a large number of free parameters, must be used to account for the broad emission features, and because it cannot satisfactorily explain the very small variations in the AIB pattern in regions with widely different UV backgrounds. Besides, individual PAHs have been elusive in space.…”

Raman Scattering by Atomic Hydrogen in Photodissociation Regions: An Alternative to the Polycyclic Aromatic Hydrocarbons Hypothesis

“…The origin of the unidentified infrared bands (UIBs), which peak close to 3.3, 6.3, 7.7, 8.7, and 11.2 μm and are observed under higher colortemperature radiation fields (T > 10 4 K) in reflection and planetary nebulae (NGC 7027, BD+30 3639, NGC 7023, K), H II regions, and some galaxies, was more controversial (Gillett et al 1973;Russell et al 1977Russell et al , 1978Allamandola & Norman 1978;Willner et al 1978). By the mid-1980s, however, a consensus was reached that these bands are carried by complex carbonaceous compounds, most likely polycyclic aromatic hydrocarbons (PAHs; Léger & Puget 1984;Allamandola et al 1985;Kwok 2022). No laboratory spectrum ever confirmed these proposed identifications, leading researchers to speculate that the infrared bands may result from the joint effect of molecules with slightly different composition (Speck et al 2005(Speck et al , 2009Newgard et al 2010;Andrews et al 2015).…”