# The Mystery of Unidentified Infrared Emission Bands

Sun Kwok11affiliationmark<EMAIL_ADDRESS>
###### 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.

00footnotetext: Department of Earth, Ocean, and Atmospheric Sciences,
University of British Columbia, Vancouver, Canada; corresponding
<EMAIL_ADDRESS>
Astrophysics and Space Science (in press)

Keywords astrobiology; astrochemistry; ISM: lines and bands; ISM: molecules;
planetary nebulae: general; reflection nebulae; stars: AGB and post-AGB;
galaxies: starburst

##  1 Discovery of the unidentified infrared emission bands

When astronomical infrared observational capabilities first developed in the
1960s, the expectation was that the infrared spectra of emission nebulae would
be dominated by atomic fine-structure lines. The detection of a strong
infrared excess in the planetary nebula NGC 7027 was totally unexpected
(Gillett et al., 1967). The large infrared continuum emission in NGC 7027 and
other planetary nebulae was later identified as thermal emission from
circumstellar solid-state grains. Even more surprising was that on top of this
strong infrared continuum, there is a strong emission feature at 11.3 $\mu$m
that cannot be identified with any known atomic lines (Gillett et al., 1973).
Further ground-based observations found another strong unidentified emission
feature at 3.3 $\mu$m in NGC 7027 (Merrill et al., 1975). Russell et al.
(1977a) found that the strength of the 11.3 $\mu$m feature closely correlates
with the 3.3 $\mu$m feature, suggesting a common origin for the two features.
Since the 3.3 and 11.3 $\mu$m features are too broad to be atomic lines and
show no substructures to qualify as molecular bands, they were believed to
arise from mineral solids such as carbonates (Gillett et al., 1973; Russell et
al., 1977a). This seemed a reasonable interpretation at the time because
silicate minerals have already been found to be common in the circumstellar
envelopes of evolved stars (Woolf and Ney, 1969).

As ground-based observations in the infrared are limited by the availability
of atmospheric windows, there was a spectral gap between 4–8 $\mu$m that
remained unexplored. From spectrophotometric observations obtained from the
Kuiper Airborne Observatory (KAO), additional features at 6.2, 7.7, and 8.6
$\mu$m were discovered (Russell et al., 1977b, 1978). The failure to detect
the expected 7.0 $\mu$m carbonate feature ruled out the mineral hypothesis.
Since the 3.3, 6.2, 7.7, 8.6, and 11.3 $\mu$m features have no counterparts in
the infrared spectra of atoms, they are collectively known as the unidentified
infrared emission (UIE) bands.

The UIE bands are found to be present in planetary nebulae, reflection
nebulae, Hii regions, novae, as well as spatially widely distributed in the
diffuse interstellar medium in the Milky Way Galaxy and in external galaxies.

In addition to their ubiquitous nature, the most intriguing element of the UIE
phenomenon is that it is a manifestation of organic matter in space. Among the
first to suggest that there could be an organic component in the interstellar
medium was Bertram Donn, who proposed polycyclic hydrocarbons as a possible
component of interstellar grains (Donn, 1968). After the discovery of the UIE
bands, Hoyle and Wickramasinghe (1977) suggested that the spectrum of
polysaccharides shows resemblance to the astronomical UIE bands.

The infrared spectra of organic compounds have actually been well studied by
chemists since the 1960s. This connection between astronomical spectra and
laboratory spectroscopy was made by Knacke (1977), who identified the 3.3
$\mu$m UIE band as originating from C$-$H stretching mode of aromatic
compounds. The possible organic origin of the UIE feature were also raised by
Puetter et al. (1979). Sagan and Khare (1979) suggested a connection between
the astronomical UIE bands and the laboratory-synthesized complex organic
polymer tholins (Khare and Sagan, 1973). The organic origin of the UIE bands
was extensively discussed by Duley and Williams (1981), who assigned the 3.3
and 11.3 $\mu$m features to aromatic materials. All these were treated as
speculations at the time and received little attention from the astronomical
community.

##  2 The UIE phenomenon

Fig. 1 : UIE bands in the planetary nebula NGC 7027 as observed by the
Infrared Space Observatory . The observed spectrum is spectrally decomposed
into the UIE bands (in black), plateau features (in orange), and the
underlying dust continuum (in blue). The wavelengths of the UIE bands are
labeled in red. The narrow features (labeled in grey) are atomic lines. Figure
adapted from Kwok and Zhang (2011).

Although most of the discussions of UIE bands have focused on the strong 3.3,
6.2, 7.7, 8.6, and 11.3 $\mu$m bands, the UIE phenomenon is much richer than
these features. There are a number of minor emission features at 12.1, 12.4,
12.7, 13.3, 15.8, 16.4, 17.4, 17.8, and 18.9 $\mu$m, which have been observed
in proto-planetary nebulae (Kwok et al., 1999), reflection nebulae (Sellgren
et al., 2007), and galaxies (Sturm et al., 2000) (Figures 1 and 2). Most
significantly, the UIE features are often accompanied by strong, broad
emission plateaus features at 6–9, 10–15, and 15–20 $\mu$m (Figs. 1 and 2).

Fig. 2 : Spitzer IRS spectrum of NGC 7023 showing the 15–20 $\mu$m broad
emission plateau (the shaded region) centered around 17 $\mu$m and the minor
UIE bands at 15.8, 16.4, 17.4, and 17.8 $\mu$m. The 18.9 $\mu$m feature is
assigned to C60. Figure adapted from Peeters et al. (2012).

###  2.1 Discovery of aliphatic organics

Motivated by Hoyle’s idea that organic matter is common in interstellar space,
Wickramasinghe and Allen (1980) undertook a search for absorption signatures
along the sight to the Galactic Center and detected a strong signal at 3.4
$\mu$m. This feature is most likely due to C$-$H stretch of aliphatic
compounds, which Duley and Williams (1979) had suggested to be observable
through absorption spectroscopy toward infrared sources. This work was
followed by a number of studies using both ground-based and space-based
telescopes (Pendleton et al., 1994; Sandford et al., 1991; Chiar et al., 2000;
Dartois et al., 2004a), leading to the discovery of the aliphatic C$-$H
bending mode at 6.9 $\mu$m. The 3.4 $\mu$m feature has also been detected in
external galaxies (Imanishi, 2000; Imanishi et al., 2010) The strength of the
3.4 $\mu$m feature suggests that at least 15% of all the carbon atoms are in
the form of aliphatic compounds (Dartois, 2011)

The same 3.4 $\mu$m feature was also found in emission in planetary nebulae
showing the 3.3 $\mu$m feature (Jourdain de Muizon et al., 1986), which
implies that the carriers of UIE bands are not purely aromatic. Although the
3.3 $\mu$m band is generally much stronger than the 3.4 $\mu$m band, in some
sources (e.g., proto-planetary nebulae) the two features can be comparable in
strength (Geballe et al., 1992; Goto et al., 2007). Distinct components of the
3.4 $\mu$m feature at 3.40, 3.46, 3.52, and 3.56 $\mu$m have been attributed
to symmetric and anti-symmetric C$-$H stretching modes (Jourdain de Muizon et
al., 1990; Hrivnak et al., 2007).

###  2.2 Band positions and profiles

The observed astronomical UIE spectra can be classified into different groups
based on the band positions and profiles. Peeters et al. (2002) and van
Diedenhoven et al. (2004) classified the spectra into classes A, B, C, and D,
with the 7.7 $\mu$m feature showing the largest variation among the classes.
The 7.7 $\mu$m band shows peak positions at 7.6 and 7.8 $\mu$m, and sometimes
at 8.0, and 8.2 $\mu$m, and these variations are used to designate the
different classes (right panel, Figure 3). The 6.2 $\mu$m feature sometimes
peaks at wavelength as long as 6.29 $\mu$m (left panel, Figure 3). Generally
speaking, Hii regions, reflection nebulae, and galaxies belong to Class A,
Herbig AeBe stars to class B, and post-Asymptotic-Giant-Branch stars to Class
C. This classification scheme is also applied to UIE sources in the Magellanic
Clouds (Sloan et al., 2014).

Fig. 3 : The band position and profiles of the 6.2 $\mu$m (left panel) and
7.7/8.6 $\mu$m (right panel) features in different classes of objects. Class A
objects have band peaks at 6.2 (A), 7.6 (A′) and 8.6 $\mu$m (A′′), Class B
objects have band peaks at 6.25 (B), 7.8 (B′), and 8.8 $\mu$m (B′′). Class C
objects have band peaks at 6.3 (C), broad 8.2 $\mu$m (C′ and C′′). Figure
adapted from Peeters et al. (2002).

###  2.3 Underlying continuum

The UIE bands are often observed in emission on top of a continuum (Figures 1
and 4). This continuum is attributed to thermal emission from micron-size
solid particles (“dust” in astronomical nomenclature). Since this continuum is
often featureless, it is assumed to originate from amorphous carbon grains. In
the diffuse interstellar medium, the strengths of the UIE bands are correlated
with the strength of the dust continuum (Kahanpää et al., 2003). Such
correlations imply that the UIE bands and the dust continuum must share the
same heating source.

Fig. 4 : The UIE bands (expanded in insert box) of the starburst galaxy M82
are observed on top of an infrared continuum with a color temperature of
$\sim$50 K. The narrow lines are atomic lines. Figure adapted from Kwok (2007)

###  2.4 Association with fullerenes

Since the initial discovery of fullerene (C60) in the planetary nebula Tc-1
(Cami et al., 2010), C60 emissions have been detected in reflection nebulae
(Sellgren et al., 2010), planetary nebulae (García-Hernández et al., 2010),
and proto-planetary nebulae (Zhang and Kwok, 2011) where UIE bands are
simultaneously present. Although Tc-1 has no strong UIE features, it does show
the broad 8 and 12 $\mu$m plateaus features. The association between C60 and
the 8 and 12 $\mu$m plateau features (Zhang and Kwok, 2013; Otsuka et al.,
2013) suggests that amorphous carbonaceous solids could be precursors of
fullerenes (García-Hernández et al., 2012; Bernard-Salas et al., 2012).

##  3 Chemical nature of the carriers

Although the carrier of the UIE bands are generally recognized as due to a
carbonaceous compound since the work of Duley and Williams (1981), the exact
chemical composition and structure of the carrier is not yet settled. Some of
the models are discussed below.

###  3.1 Polycyclic aromatic hydrocarbon molecules

The idea that UIE bands originate from polycyclic aromatic hydrocarbon (PAH)
molecules was proposed by Léger and Puget (1984) and Allamandola et al.
(1985). Allamandola et al. (1985) calculated the vibrational spectrum of gas-
phase chrysene and suggest possible matches to the UIE bands. The PAH
hypothesis was developed based on (i) some general similarities between the
infrared spectra of UIE bands with PAH molecules; and (ii) the 12 $\mu$m
excess emission observed in cirrus clouds in the diffuse interstellar medium
can be explained by single-photon excitation of small molecules (Sellgren,
1984, 2001). The physics of PAH molecules as a component of the interstellar
medium was discussed by Omont (1986).

The thesis of the PAH hypothesis is summarized by Tielens (2008) as “These
features are (almost) universally attributed to the IR fluorescence of far-
ultraviolet (FUV)-pumped polycyclic aromatic hydrocarbon (PAH) molecules,
containing 50 C atoms”. The small size of the PAH molecules allow them to be
stochastically excited to temperature of 1,000 K upon absorption of a single
UV photon, and subsequent cascade produces the vibrational emission bands in
the infrared. In the past 30 years, the PAH hypothesis has become extremely
popular and is commonly accepted in the astronomical community as the
explanation of the UIE phenomenon. The 3.3 $\mu$m UIE band is assigned to
C$-$H stretching modes, the 6.2 $\mu$m and 7.7 $\mu$m features to C$-$C
stretching modes, the 8.6 $\mu$m feature to C$-$H in-plane bending modes, and
the 11.3 $\mu$m 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). The spectral properties of PAH
molecules have been extensively studied (Hudgins and Allamandola, 2004;
Salama, 2008) and collected into a comprehensive database (Bauschlicher et
al., 2010; Boersma et al., 2014; Mattioda et al., 2020).

The first serious objections to the PAH hypothesis was given by Donn et al.
(1989). Below is a list of problems with the PAH hypothesis, expanded and
updated from the list of Donn et al. (1989).

  * •

PAH molecules have well-defined sharp absorption bands but the UIE bands are
broad (Figs. 1 and 3). Superpositions of vibrational bands from a large
variety of PAH molecules and artificial broadening profiles have to be
introduced to fit the astronomical spectra (Li & Draine, 2001; Draine & Li,
2007; Cami, 2011).

  * •

Neutral PAH molecules are primarily excited by UV and have little absorption
in the visible (Ciar, 1964; Léger et al., 1989), but UIE features are seen in
proto-planetary nebulae and reflection nebulae with no background UV
radiation.

  * •

The expected strong absorption features of PAH molecules in the UV are not
seen in interstellar extinction curves. The observed PAH to hydrgogen
abundance ratio upper limits range from $10^{-10}$ to $10^{-8}$ (Clayton et
al., 2003; Salama et al., 2011; Gredel et al., 2011), which are much lower
than the PAH/H ratio of $3\times 10^{-7}$ predicted from the strength of the
infrared features (Tielens, 2008).

  * •

The infrared spectra of small PAH molecules are well studied by chemists
(Schlemmer et al., 1994; Cook et al., 1996; Cook and Saykally, 1998) and they
found “No PAH emission spectrum has been able to reproduce the UIE spectrum
w.r.t. either band positions or relative intensities” (Wagner et al., 2000).

  * •

The PAH model predicts that the UIE band ratios are strongly dependent on the
UV background as individual UIE bands are assumed to arise from different
neutral or ionized PAHs (Hudgins and Allamandola, 2004; Draine et al., 2021).
But the shapes and peak wavelengths of UIE bands are found to be independent
of temperature of exciting star, ranging from thousands to tens-of-thousands
of degrees (Uchida et al., 2000) (Fig. 5). An analysis of 820 UIE spectra in
the diffuse interstellar medium shows no variation of the UIE band ratios in
regions of widely different UV backgrounds (Chan et al., 2001).

  * •

Presence of aliphatic features in UIE sources. In the PAH hypothesis, the 3.4
$\mu$m feature is interpreted as superhydrogenation of PAH molecules (Schutte
et al., 1993). It has also been suggested as arising from hot bands from
anharmonic aromatic C$-$H stretch, which shifts the 3.3 $\mu$m feature to a
longer wavelength (Barker et al., 1987). However, theoretical calculations
including anharmonicity show a simultaneous increase of the width of the 3.3
$\mu$m feature, which was not observed (van Diedenhoven et al., 2004).
Furthermore, the expected strong overtone bands at the 1.6–1.8 $\mu$m region
is not detected, making the anharmonicity explanation unlikely (Goto et al.,
2003).

Fig. 5 : Normalized Infrared Space Observatory spectra of three reflection
nebulae showing that the UIE bands have similar peak wavelengths, spectral
shapes, and continuum levels independent of the temperatures of the
illuminating stars (vdB 17, 11,000 K; vdB 59, 19,000 K; vdB 133 6,800 K).
Figure adapted from Uchida et al. (2000).

An independent way to establish the presence of PAH molecules in space is to
search for them through their rotational transitions. However, due to the
absence of permanent dipole moment of these molecules, this is not as feasible
as for other classes of molecules. The simplest ring molecule benzene is
detected via its infrared transitions in the proto-planetary nebula AFGL 618
(Cernicharo et al., 2001). Two double-ring molecules with nitrile functional
groups (1- and 2-cyanonaphthalene, $c-$C6H5CN) have been detected in the
molecular cloud TMC-1 (McGuire et al., 2021). Two pure hydrocarbon cyclic
molecules: cyclopentadiene ($c-$C5H6, a 5-ring molecule) and indene($c-$C9H8,
a bicyclic molecule with both a 5- and 6-membered ring) have been detected in
TMC-1 (Cernicharo et al., 2021; Burkhardt et al., 2021).

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). Heteroatoms (defined as those
atoms other than C or H) such as N are also introduced to explain the 6.2
$\mu$m band (Hudgins and Allamandola, 1999).

In order to fit the astronomical observations, the PAH model appeals to a
mixture of PAH of different sizes, structures (compact, linear, branched) and
ionization states, as well as artificial broad intrinsic line profiles
(Allamandola et al., 1999; Cami, 2011). The number of free parameters is so
large that the routines used by the PAH model to fit the astronomical UIE
spectra have been shown to be able to fit any artificial spectra (Zhang and
Kwok, 2015).

###  3.2 Other hydrocarbons

In addition to pure carbon materials such as graphite (hybridization state
$sp^{2}$) and diamond (hybridization state $sp^{3}$), different amorphous
C$-$H alloys can be created when H is introduced (Cataldo, 2004; Jones et al.,
2013). These amorphous solids consist of varying degrees of aromatic to
aliphatic ratios, number of aromatic rings, different lengths of aliphatic
chains, and arranged in different geometric structures. A schematic of
possible structures of amorphous carbonaceous solids is shown in Figure 6. The
lower left corner of the triangle represents graphite (C rings on a plane with
no H), the top corner represents diamonds (C arranged in tetrahedral forms),
PAHs (aromatic rings arranged on a plane) are on the bottom edge, and various
forms of amorphous hydrogenated carbon can exist in the interior of the
triangle. Theoretical calculation on the infrared spectra of such amorphous
solids suggest that they can be good candidates as carrier of UIE bands
(Jones, 2012a, b). Since these amorphous carbonaceous solids have absorption
bands in the visible, they can be easily excited by visible light from stars.

Fig. 6 : Phase diagram of amorphous carbonaceous (pure C and H) compounds. The
lower right corner of the triangle represents pure H, lower left corner pure
graphite-like ($sp^{2}$) materials, and the upper corner pure diamond-like
($sp^{3}$). Areas inside the triangle represent various H/C ratios and
$sp^{2}/sp^{3}$ mixed hybridization states. Figure adapted from Robertson
(2002).

The possibility that the UIE bands could be due to some form of hydrogenated
amorphous carbon (HAC) was made by Buss et al. (1990), and HAC was proposed to
be a major constituent of interstellar dust by Jones et al. (1990). Further
work on HAC was described in Duley (1993) and Duley (2000). The infrared
properties of HAC were studied in the laboratory by Scott et al. (1997), Duley
et al. (2005), and Gadallah et al. (2012).

By subjecting methane gas in a vacuum to microwave radiation, Sakata et al.
(1983) were able to collect carbonaceous composite particles on a substrate
after rapid cooling, which they name quenched carbonaceous composite (QCC).
The infrared spectra of QCC show strong resemblance to the astronomical UIE
bands (Sakata et al., 1984, 1987).

By accreting C, C2, and other Cn molecules, carbon nanoparticles with sizes
less than 100 nm can be created in the laboratory. The infrared spectra of
these particles show similarities to the astronomical UIE spectra (Hu et al.,
2006; Hu and Duley, 2008).

###  3.3 Amorphous hydrocarbons with heteroatoms

Mixed aromatic/aliphatic structures are natural products of combustion. The
first nucleation products of soot particles formed in flames have structures
consisting of islands of aromatic rings linked by chains (Colangeli et al.,
1997; Chung and Violi, 2011). In natural interstellar or circumstellar
environments, other elements such as O, N, and S are abundantly present and
can be expected to be incorporated into any carbonaceous compounds condensed
from gas phase. Based on the similarities between the astronomical UIE spectra
and the infrared spectra of coal, Papoular et al. (1989) suggested that coal
as the carriers of the UIE bands. This idea was later extended to kerogen-like
materials (Papoular, 2001). Petroleum fractions as a carrier of UIE bands are
discussed by Cataldo et al. (2002). Coal, kerogen, and petroleom are natural
products that contain heteroatoms in their structures. The model of mixed
aromatic/aliphatic organic nanoparticles (MAON) considers a carbonaceous
compound containing aromatic rings of different sizes and aliphatic chains of
different lengths and orientations arranged in a 3-D amorphous structure,
mixed with heavy elements (Kwok and Zhang, 2013). Fig. 7 shows a schematic
structure of MAON, illustrating the complexity of the chemical structure, as
well as the kinds of functional groups (including heteroatoms) that may be
contained in such structures.

Fig. 7 : The MAON structure is characterized by a highly disorganized
arrangement of small units of aromatic rings linked by aliphatic chains. This
structure contains 169 C atoms (in black) and 225 H atoms (in white).
Impurities such as O (in red), N (in blue), and S (in yellow) are also
present. A typical MAON particle may consist of multiple structures similar to
this one.

By introducing nitrogen gas into the QCC experiment, a solid condensate
quenched nitrogen-included carbonaceous composite (QNCC) is collected. The
infrared spectra of QNCC show resemblance of the UIE spectra of novae (Endo et
al., 2021).

##  4 Other synthesized substances

Since the early work on tholins (Khare and Sagan, 1973), various carbonaceous
compounds have been synthesized in the laboratory by subjecting simple
hydrocarbons to different forms of energy injections. The techniques used
include laser ablation of graphite (Scott and Duley, 1996; Mennella et al.,
1999; Jäger et al., 2008), laser pyrolysis of gases (Herlin et al., 1998), arc
discharge (Mennella et al., 2003), microwave irradiation (Sakata et al., 1984;
Wada et al., 2009; Godard et al., 2011), UV photolysis (Dartois, 2004b), and
flame synthesis (Carpentier et al., 2012). Chemical analysis of these
laboratory synthesized carbonaceous nanoparticles show that they consist of
networks of chains and rings (Hu et al., 2006) and their spectra show varying
degrees of resemblance to the astronomical UIE spectra.

##  5 Atomic origin

The first proposal for an atomic origin of the UIE bands was made by Holmlid
(2000) who suggests that the astronomical bands are the result of electronic
deexcitation of Rydberg matter states with principal quantum numbers
$n=40-200$. Rydberg matter is proposed to be a new phase of matter equivalent
to that of liquid and solid which could exist in space in large quantities. In
support of this hypothesis, Holmlid (2000) shows that the Rydberg matter model
can fit the observed UIE spectra with fewer parameters than the PAH model.
This idea was extended by Zagury (2021) who assigns the 3.3, 6.2, and 11.3
$\mu$m UIE bands to hydrogen recombination line series $n$=6, 8, and 11,
respectively, and the 15$-$20 $\mu$m plateau emission band to the continua of
hydrogen transitions to $n=13$ and $n=14$. However, the hydrogen hypothesis is
contradicted by the fact that the UIE phenomenon is observed only in carbon-
rich sources (Cohen et al., 1986, 1989).

##  6 Corresponding vibrational modes of the UIE features

Since the UIE phenomenon consists of a family of emission features, a
successful theory must be able to consistently explain the positions,
profiles, relative strengths of all the entire family. The ideal candidate for
the carrier of UIE bands should consistently produce the entire family of UIE
bands without appealing to special conditions. PAH molecules show spectral
features near 3.3 and 11.3 $\mu$m but do not have clear counterparts for the
6.2, 7.7, and 8.6 $\mu$m UIE bands. In the words of Cook and Saykally (1998):
“In order to reproduce the narrow 6.2 and 11.2 $\mu$m UIR bands, the carriers
must consistently exhibit bands at these positions with a consistency similar
to that which is observed with the 3.3 $\mu$m emission. In addition, the
carriers of the UIRs must, in general, exhibit an absence of strong bands in
the gap between the 6.2 and 7.7 $\mu$m UIR features. The PAHs used in these
model spectra simply do not meet these criteria; hence they do not reproduce
the details of the UIR spectra.”

One of the advantages of the HAC/MAON-type of models is that they naturally
produce broad infrared features without appealing to artificial fittings
(Dischler et al., 1983; Guillois et al., 1996). The exact positions of the
bands depend on the composition of the material (C to H ratio, fraction of
impurities), aromatic to aliphatic ratios, and geometric factors, but their
resemblance to the astronomical UIE bands is unmistakable (Fig. 9, Kwok,
2016). However, there is no specific vibrational mode assignments to these
observed bands and the molecular dynamical origin of the strong bands observed
in laboratory specimens (e.g., those observed in Dischler et al., 1983; Herlin
et al., 1998) are not identified. In order to advance the MAON hypothesis
further, we need to perform quantum chemistry calculations of large varieties
of MAONs to identify the major vibrational modes as well as to see whether
some of these modes will converge to resemble the observed astronomical UIE
bands (Sadjadi et al., 2016).

###  6.1 The 3.3 $\mu$m band

The 3.3 $\mu$m UIE feature was first identified as aromatic C$-$H stretch by
Knacke (1977). This is a prominent band and shows a consistent emission
profile in many astronomical sources (Figure 8). The measured central
wavelength of the feature is 3.2887$\pm$0.0009 $\mu$m (Tokunaga and Bernstein,
2021). The invariance in feature profile, independent of source excitation
conditions and evolutionary history, puts severe constraints on the chemical
composition of the carrier and excitation mechanism. The central wavelengths
of the C$-$H stretching mode of PAH molecules lie shortward of the observed
wavelength of the 3.3 $\mu$m band (Sakata et al., 1990; Kwok and Zhang, 2013).
Joblin et al. (1995) consider the effects of anharmonic couplings and suggest
that the astronomical 3.3 $\mu$m feature can be consistent with PAH molecules
at high temperatures. But the observed peak wavelengths of the 3.3 $\mu$m
feature lie within a very narrow range, even when observed in circumstellar or
interstellar environments under very different temperature conditions. This
makes it unlikely that red shift by high temperatures be the general cause of
this wavelength discrepancy.

Fig. 8 : Profiles of the 3.3 $\mu$m feature observed in NGC 7027 (plotted in
black in all panels) and other astronomical sources (plotted in red and green
in panels b-f, as well as in black dashed line in panel b). The sharp lines at
3.207, 3.234, and 3.297 $\mu$m are the [Ca IV], H2 1$-$0 O(5), and H
Pfund-$\delta$ lines, respectively. Figure adapted from Tokunaga and Bernstein
(2021). Fig. 9 : A comparison of the absorption band strengths between neutral
(top curve) and ionized (bottom curve) PAH molecules. The UIE bands in the
6$-$9 $\mu$m region are attributed to ionized PAH molecules in the PAH
hypothesis. Figure adapted from Hudgins and Allamandola (2004).

Ground-based high-spectral resolution observations of the 3.3 $\mu$m of
HD44179 (the Red Rectangle) suggest that the 3.3 $\mu$m feature can be
decomposed into two components, one at 3.28 $\mu$m and another at 3.30 $\mu$m
(Song et al., 2003). The existence of these two components has been attributed
to “bay” and “non-bay” hydrogen sites of the PAH units, based on the detection
of two bands within the range of 3.33$-$3.21 $\mu$m in the experimental gas
phase infrared spectra of some small PAH molecules with bay-type hydrogens in
their molecular structures. The aromatic C$-$H stretching mode of PAH has
slightly different frequencies depending on how many of the edge H atoms are
in “bay” or “non-bay” configurations (Bauschlicher et al,, 2009) and it is
possible to separate these components by imaging spectroscopic observations
(Candian et al., 2012). However, the peak wavelengths of the “bay” and “non-
bay”modes are at 3.17–3.27 and 3.26–3.27 $\mu$m, respectively, both shortward
of the astronomical wavelengths of the 3.28 and 3.30 $\mu$m components derived
by Candian et al. (2012) by fitting the asymmetric profile of the 3.3 $\mu$m
feature.

The aromatic C$-$H stretch at 3.3 $\mu$m and aliphatic C$-$H stretch at 3.4
$\mu$m is commonly seen in many carbonaceous compounds (Cataldo et al., 2020).
There are also olefinic C$-$H stretching modes in this wavelength region. For
example, Chiar et al. (2013) interpret the 3.28 $\mu$m component as the
stretching mode of olefinic C$-$H bonds in amorphous hydrocarbons. The
possibility that the 3.30 $\mu$m subfeature is due to olefinic C$-$H stretch
is discussed by Sadjadi et al. (2017).

###  6.2 The 6.2, 7.7, and 8.6 $\mu$m bands

Based on a comparison between the astronomical UIE spectra with the laboratory
spectrum of coronene, Léger and Puget (1984) assigns the 6.2 $\mu$m band to
C–C stretch and the 8.6 $\mu$m to C–H in-plane bending mode. However, the
origin of the 7.7 $\mu$m feature is not clear (Allamandola et al., 1985). From
the laboratory spectrum of coronene, the 7.4 and 6.4 $\mu$m features are
suggested to be the combination of aromatic C$-$C stretch and C$-$H in plane
bending modes (Langhoff, 1996). In later discussions under the PAH hypothesis,
infrared bands between 6.1 and 6.5 $\mu$m are suggested to be due to pure
aromatic C$-$C stretching modes, bands from 6.5 to 8.5 $\mu$m are due to
coupled C–C stretching and C–H in-plane bending modes, and bands between 8.3
to 8.9 $\mu$m are due to C–H in-plane bending modes (Peeters, 2011). Since the
peak wavelengths of PAH features shift with size of the molecule, the
wavelength difference between 6.2 and 7.7 $\mu$m features have been used to
infer the size of the PAH molecules (Hudgins and Allamandola, 1999;
Bauschlicher et al., 2008; Bauschlicher et al,, 2009).

However, all C–C stretching modes are very weak in comparison to the C–H modes
in neutral PAH molecules. In order to explain the prominence of the features,
PAH ions are suggested to be responsible for the UIE features in the 6$-$9
$\mu$m region (Hudgins and Allamandola, 2004) (Figure 9). The need by the PAH
hypothesis to simultaneously rely on neutral PAHs to explain the 3.3 and 11.3
$\mu$m features and PAH ions to explain the 6.2, 7.7, and 8.6 $\mu$m features
poses difficulties in explaining the near universal presence of all these
features in astronomical sources of very different UV backgrounds (Fig. 5).

As the molecular structure gets more complex (e.g. in MAONs), many of the
vibrational modes in the 6–10 $\mu$m region become coupled and cannot be
identified as a single vibrational mode (Sadjadi et al., 2016).

###  6.3 The 11.3 $\mu$m band

Soon after the discovery of the UIE bands, the 11.3 $\mu$m feature was
identified as C–H out-of-plane bending mode of aromatic compounds (Duley and
Williams, 1981). The observed peak position of the 11.3 $\mu$m feature in
astronomical sources is well defined and does not vary much in wavelength. The
11.3 $\mu$m feature is also detected in absorption (Bregman et al., 2000), and
its wavelengths and profiles closely resemble those seen in emission. The 11.3
$\mu$m feature has a distinctive asymmetric profile, having a steep decline in
the short wavelength side and a gradual extended wing in the long wavelength
side (van Diedenhoven et al., 2004). Such asymmetric profiles are difficult to
explain by gas-phase molecular emissions. In order to explain the observed
profile of the 11.3 $\mu$m feature in the PAH hypothesis, PAH molecules of
different mass are needed: high-mass PAHs will produce the short-wavelength
side and low-mass PAHs the long-wavelength side (Candian and Sarre, 2015).

Furthermore, the peak positions of the C$-$H out-of-plane bending modes of PAH
molecules can be quite different due to mode coupling, molecular structure,
and charge states of the molecules (Hony et al., 2001). Because of the large
wavelength variations of the out-of-plane bending modes of PAH molecules, it
is difficult to assign or match the observed astronomical feature to specific
PAH molecules. Ring deformation vibrational mode of small carbonaceous
molecules such as ethylene oxide ($c$-C2H4O) has also been proposed to explain
the narrow 11.3 $\mu$m feature (Bernstein and Lynch, 2009).

Sadjadi et al. (2015) show that a mixture of pure PAH molecules, even
including units of different sizes, geometry and charged states, is unable to
fit the astronomical spectra. In order to fit the astronomical profile of the
11.3 $\mu$m feature, oxygen-containing molecules are needed.

###  6.4 Plateau features

The 8 and 12 $\mu$m plateau features were first detected in proto-planetary
nebulae by IRAS LRS and KAO observations (Kwok et al., 1989; Buss et al.,
1990). Li and Draine (2012) assign these two plateau features as “wings of C–C
and C–H bands” of PAH molecular vibrations. If this is the case, then similar
“wings” should be seen around the 3.3 $\mu$m feature but it is not generally
observed. Alternatively, these plateau features have been identified as
superpositions of in-plane and out-of-plane bending modes emitted by a mixture
of aliphatic side groups attached to aromatic rings (Kwok et al., 2001).

The 15–20 $\mu$m plateau feature has been detected in young stellar objects,
compact HII regions, and planetary nebulae, and is especially strong in some
proto-planetary nebulae (Zhang et al., 2010). The wavelength region of this
broad feature suggests that it could arise from C–C skeleton vibrational modes
(Van Kerckhoven et al., 2000).

##  7 Synthesis pathways

The circumstellar envelopes of evolved stars (from AGB stars through proto-
planetary nebulae to planetary nebulae) are ideal laboratories to test models
of molecular synthesis. The sequential formation of molecules from C2, C3, CN,
to HCN, HC3N, HC5N, C2H2, to C6H6 suggests a bottom-up scenario of molecular
synthesis. PAH molecules can be the products of molecular synthesis (Frenklach
& Feigelson, 1989). The linking of islands of aromatic rings by aliphatic
chains can lead to formation of MAONs. Fullerenes could be results of bottom-
up processes (e.g., from SiC grains, Bernal et al., 2019) or top-down process
through the break up of MAONs (García-Hernández et al., 2012; Bernard-Salas et
al., 2012; Micelotta et al., 2012).

The chemical time scales of molecular synthesis in the circumstellar
environment is constrained by the dynamical time scales of expanding
circumstellar envelopes. Gas-phase molecules and inorganic mineral solids are
known to be forming in the circumstellar envelopes of AGB stars and planetary
nebulae under low density and over very short time scales (Kwok, 2004). In
proto-planetary nebulae, UIE bands are observed to emerge over time scales of
$\sim$103 years (Kwok et al., 1999). Infrared spectroscopic observations of
novae following their outburst show that UIE bands emerge over time scales of
weeks (Helton et al., 2011). In nova V705 Cas, the UIE bands, including a
strong 3.4 $\mu$m feature, were seen within one year after outburst (Evans et
al., 2005). These observations suggest that the abiotic synthesis of complex
organics is extremely efficient in the circumstellar environment.

After their synthesis in circumstellar envelopes and their ejection into the
interstellar medium, there is also a question of whether the carriers of UIE
bands (whether they are PAH molecules or MAONs) can survive their journeys
through the interstellar medium. The aliphatic side groups in MAONs could be
broken off by interstellar background radiation. If multiple units of MAONs
aggregate together they may resemble the macroscopic carbonaceous solids
similar to the insoluble organic matter (IOM) observed in meteorites (Cody et
al., 2011). Such solids are sturdy and are less likely to be destroyed by
interstellar processes. Isotope anomalies in IOM suggest that it is probably
of interstellar origin (Busemann et al., 2006).

##  8 Size and energetics

One of the key questions about the carriers of the UIE bands is whether they
are free-flying gas-phase molecules, nanoparticles, or solids. Whereas the
spectral properties of molecules (in particular PAH molecules) are relatively
simple and well studied, the properties of nanoparticles consisting of
$10^{2}-10^{3}$ heavy atoms cannot be easily extrapolated from bulk materials
because of quantum surface effects.

The astronomical UIE bands in planetary nebulae, Hii regions, and galaxies are
observed to lie on top of an infrared continuum of color temperature $\sim$102
K (Fig. 4). In the diffuse interstellar medium, the color temperature of the
infrared continuum is even lower. Since the 3.3 $\mu$m band lies shortward (on
the Wien’s side) of the peak of the infrared continuum, it is unlikely that
the 3.3 $\mu$m band is excited thermally. Instead, stochastic heating by a
single photon, exciting the carrier temporarily to a high temperature, is
suggested to be the excitation mechanism (Sellgren, 1984; Omont, 1986).

An alternate model of chemical heating was proposed by Duley and Williams
(2011), who suggest that this process can heat carbonaceous solid particles of
sizes from 5$-$100 nm to emit the 3.3 $\mu$m band. The release of H2 molecules
from chemical heating is also consistent with the correlation between observed
2.18 $\mu$m H2 emission with the 3.3 $\mu$m band.

If stochastic heating is indeed the excitation mechanism of UIE bands, then
the sizes of the carrier must be limited to molecules or nanoparticles. If
chemical heating can work, then the size limit can be extended upwards. In
either case, we still have to work out the relationship between the UIE
carriers and the micron-size grains that contribute to the underlying
continuum (section 2.3).

##  9 The need for a correct identification of the origin of the UIE bands

Since the UIE bands are commonly used as a tracer of galactic evolution, it is
pertinent that we correctly identify the chemical nature of the carrier of the
bands. The UIE bands are detected in galaxies with redshifts $>$4 (Riechers et
al., 2014; Armus et al., 2020) and are particularly prominent in
ultraluminuous infrared galaxies. The power emitted in the UIE bands can be as
high as 20% of the total energy output of these galaxies (Smith et al., 2007).
Given the nearly invariance of the band positions, the UIE bands have been
used as redshift indicators (Elbax et al., 2005). In nearby galaxies, the
distribution of the UIE bands can be mapped. The AKARI satellite has mapped
the distribution of the 3.3 and 3.4 $\mu$m bands, showing emission from
aromatic and aliphatic species are present in the halo of M82, as far as 2 kpc
from the galactic center (Yamagishi et al., 2012).

Under the assumption that UIE bands are PAH molecules excited by UV photons,
the UIE bands have been used as tracers of star formation in galaxies (see
e.g., Smith et al., 2007; Galliano et al., 2008; Wu et al., 2010; Li, 2020) as
well as the star formation activities as a fraction of the total infrared
luminosities of galaxies (Peeters et al., 2004; Spoon et al., 2004). Under
similar assumptions, the UIE bands have also been used as tracers of elemental
and chemical evolution of galaxies (Genzel et al., 1998).

Based on the assumption that the 11.3 $\mu$m feature originates from neutral
and large PAHs, and the 6.2, 7.7, and 8.6 $\mu$m features originate from PAH
ions, the strength ratios between the 11.3 $\mu$m feature and 6 to 9 $\mu$m
UIE features have been used to estimate the properties of the background
radiation fields (Galliano et al., 2008; Berné et al., 2009). These results
are used to determine the star formation rates of galaxies (Calzetti, 2011).
The physical validity of these studies, however, depends on a correct
interpretation of the origin of the UIE bands.

##  10 Conclusions

The family of astronomical UIE bands is a rich spectral phenomenon which
carrier is yet to be unambiguously identified. It is likely to be a
carbonaceous substance whose structure is more complicated than it is often
assumed. Although the PAH hypothesis has been popular in the astronomical
community, its definition has been evolving from the original chemical
definition of planar, pure carbon and hydrogen ring molecules, to a collection
of molecules including “PAHs with side groups, heterosubstituted PAHs, fully
or partially (de)hydrogenated PAHs, and PAH clusters.” (Peeters et al., 2021).
This revision of definition is moving the PAH hypothesis closer to other
models of the UIE bands.

The debate between the PAH and HAC/MAON models is more than a question of
semantics as there are fundamental differences between these two models: Do
the UIE carriers contain pure rings or mixed rings and chains? Are their
geometry 2-D or 3-D? Are they pure C$-$H compounds or contain impurities? Are
their structure regular or amorphous? Are the carriers gas-phase free-flying
molecules or solids? If answers to these questions are toward the latter, we
have to call these carriers by their proper chemical terminology.

Since the UIE phenomenon is seen throughout the Universe, even during its
early epochs, a correct identification of the carrier is of great importance.
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 $\sim$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. Whether the carriers of the UIE
bands are a collection of free-flying gas-phase PAH molecules or complex
organic solid particles has significantly different implications on our
understanding of cosmic chemical synthesis, energy exchange between stars and
the interstellar medium, and galactic chemical enrichment. Further
computational and experimental studies of the vibrational properties of
amorphous carbonaceous compounds are needed to solve the UIE mystery.

Acknowledgements I thank SeyedAbdolreza Sadjadi for producing Figure 7. I also
thank Larry Bernstein, Franco Cataldo, Anibal García-Hernández, Louis
d’Hendecourt, Walt Duley, Roger Knacke, Takashi Onaka, Ray Russell, Alan
Tokunaga, and Yong Zhang for helpful discussions, as well as the two anonymous
referees who provided helpful comments leading to an improvement the paper.
This work is supported by a grant from the Natural and Engineering Research
Council of Canada.

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