Abstract
Since the 1970s, astronomers have detected a set of distinct infrared emission bands across diverse astronomical environments, which are widely recognized as signature features of polycyclic aromatic hydrocarbon (PAH). This paper reviews the formation and evolution mechanisms of such interstellar PAHs, mainly covering bottom-up molecular growth pathways such as the hydrogen abstraction-acetylene addition mechanism. It systematically delineates the fundamental vibrational modes corresponding to the characteristic bands at 3.3 μm, 6.2 μm, 7.7 μm, 8.6 μm, 11.2 μm, and 15-20 μm, while elaborating on the effects of molecular edge geometry and heteroatom doping on the peak positions and intensities of these spectral features. Diagnostic methodologies based on band ratio analyses for inferring the ionization states and molecular sizes of PAHs are synthesized, alongside ongoing debates regarding the non-uniqueness of the spectral carriers and the physical-chemical origins of weak far-infrared bands. Finally, the unparalleled advantages of the James Webb Space Telescope (JWST) in infrared imaging spectroscopy are emphasized, providing novel perspectives for advancing our understanding of interstellar organic chemistry.
0 Introduction
Since the 1970s, astronomers have detected a group of infrared emission bands of unknown origin in various celestial objects. These bands are collectively named"Unidentified Infrared Emission"(UIE). The observational study of this series of bands began with a landmark discovery in the early 1970s: in 1973, Gillett F C et al. were the first to capture anomalous infrared radiation signals at 8.6 μm and 11.2 μm in planetary nebulae, marking the beginning of UIE spectral band research [1]; four years later, Russell R W et al. further reported several core UIE bands such as 3.3 μm, 6.2 μm and 7.7 μm, and initially constructed the basic observation framework of UIE bands [2] in 1977. With the iterative upgrade of astronomical observation technology, the detection range of such characteristic bands has also been expanded to the whole domain. Subsequently, they were widely observed not only in planetary nebulae, but also in various types of celestial environments such as H II region, reflection nebulae, molecular clouds, and galactic centers. This universal detection result across celestial types directly confirms that UIE band carriers have extremely high distribution in the universe [3].
The attribution of the UIE band carrier was a subject of long-standing debate, with various carbon-rich materials, such as graphite microparticles, hydrogenated amorphous carbon, and nanodiamond, proposed as possible sources. In the late 1970s and early 1980s, research first indicated that the radiation of this characteristic band might be related to the vibrations of aromatic functional groups on the surface of carbonaceous materials, thus giving the band the alternative name"Aromatic Infrared Band"(AIB). Between 1984 and 1985, Leger A, Puget JL(1984), and Allamandola LJ et al.(1985) almost simultaneously proposed a key hypothesis, suggesting that PAH molecules might be the core carrier of the UIE band [4-5]. On this basis, the research team led by Allamandola L J conducted systematic experimental verification and theoretical deduction through a series of follow-up studies, confirming that PAH molecules in a highly vibrationally excited state can reproduce the major characteristic bands of UIEs at 3.3 μm, 6.2 μm, 7.7 μm, 8.6 μm, 11.3 μm and other wavelengths, which provides core support for the PAH carrier hypothesis [5-6].
If the carriers of the UIE bands are indeed PAH molecules, then it can be inferred that a considerable proportion of carbon in the universe is stored in the molecular form of PAHs. This conclusion not only means that PAHs are crucial carbon reservoirs in the interstellar medium, playing a key role in regulating the interstellar carbon cycle, but their unique optical and vibrational properties also make them a core medium for interstellar energy transfer. After absorbing ultraviolet photons from stars, PAHs release energy again as infrared radiation through vibrational relaxation, thus achieving efficient energy conversion between ultraviolet absorption and infrared re-radiation, maintaining the energy balance of the interstellar medium.
Over the past few decades, groundbreaking progress has been made in the study of the infrared spectral properties of PAH molecules. Researchers have simulated the spectral responses of PAHs of different sizes, ionization states, edge structures, and heteroatom doping in an interstellar environment through numerous high-precision laboratory spectroscopic measurements. Simultaneously, combined with quantum chemical theoretical calculations, they have systematically elucidated the correspondence between PAH molecular vibrational modes and infrared spectral bands. These results provide solid experimental and theoretical support for PAH carrier models. At the space observation level, the launch of the Infrared Space Observatory(ISO) and the Spitzer Space Telescope(Spitzer) has greatly expanded the observational dimensions of the UIE spectral bands. High-resolution observations of the two in the 3-20 μm band have not only verified the universality of classic strong emission bands such as the 3.3μm and 6.2μm bands, but also further revealed a number of previously undiscovered weak emission bands, as well as broad radiation plateaus with a wide coverage in the 6-9μm and 11-13μm bands. These findings have notably refined the overall observational framework of UIE bands and laid a solid observational foundation for deepening the physicochemical connotations of the PAH model.
However, such high-sensitivity space observations also revealed the essential complexity of UIE band carriers: in different celestial environments, the outline morphology and characteristic peak positions of UIE bands will show systematic differences with the intensity of interstellar radiation field, environmental physicochemical conditions and the stage of celestial evolution. This observational phenomenon clearly suggests that its carrier is not a single molecular species, but more likely to be composed of a series of carbonaceous molecular groups with diverse structures and complex compositions. Since the PAH carrier hypothesis was proposed, questions about its applicability and model optimization and modification have never stopped. In 2011, Kwok S and Zhang Y made an important modification to the traditional pure PAH carrier hypothesis. They pointed out that the true carrier of UIE bands may not be a single pure PAH molecule, but a new type of organic nanosolid with both aromatic ring and aliphatic chain structures-Mixed Aromatic-Aliphatic Organic Nanoparticles(MAON) [7], which provides a new perspective for solving the essence of UIE carriers.
This paper integrates the cutting-edge achievements in interstellar organic chemistry and infrared astronomical observation in recent years, and systematically reviews them from the following five core dimensions:(1) the formation pathway and multi-stage evolution mechanism of PAH molecules in interstellar space;(2) the molecular structural diversity of PAHs and their corresponding typical infrared spectral characteristics;(3) the diagnostic significance of PAH characteristic band ratios and their astronomical applications in the inversion of interstellar medium ionization, molecular size, and environmental physical conditions;(4) the core challenges and key controversies faced by PAH carrier models; and(5) the technological advantages and breakthroughs of the new generation of infrared observation equipment, represented by JWST, in promoting PAH and interstellar organic molecule research. Through the systematic review and in-depth analysis of the above content, this paper aims to comprehensively present the current cognitive progress, core disagreements, and future development directions in the field of interstellar PAH research, and provide a comprehensive academic reference framework for subsequent exploration in this field.
1 PAH in interstellar space
The origin of PAHs in interstellar space is generally considered to fall into two core pathways: a bottom-up molecular assembly pathway, where small hydrocarbon molecules gradually aggregate into large aromatic molecules; and a top-down fragmentation pathway, where large-scale carbonaceous solid particles break down to generate PAH-like molecules. In some scenarios, a hybrid mode involving both pathways may also exist. The formation efficiency and molecular structure evolution of PAHs are highly dependent on the physicochemical conditions of the interstellar environment(such as temperature, radiation field intensity, and molecular abundance). The following section will focus on a detailed explanation of the bottom-up reaction pathway.
As the core structural unit of all PAH molecules, the interstellar synthesis of the benzene ring(C6H6) is of fundamental significance to the origin of PAHs. ISO first detected characteristic signals of benzene and related unsaturated hydrocarbons in the peristellar envelope of the carbon star CRL 618 [8], providing direct observational evidence for the existence of interstellar aromatic rings. Subsequently, Woods PM et al.(2002) constructed a targeted chemical reaction network based on the physical parameters of the celestial source. They not only identified the characteristic absorption peak of benzene molecules in its infrared spectrum, but also confirmed that the strong shock wave and high temperature(greater than or equal to 1000 K) conditions in this circumstellar environment can drive small hydrocarbon species to generate benzene rings through rapid free radical reactions [9]. This discovery marks a strong evidence supporting the key transformation of interstellar carbon chain molecules into aromatic skeletons.
After the benzene ring skeleton is constructed, the subsequent growth process of PAH can usually be attributed to the combined effect of various"hydrogen extraction-addition-cyclization" free radical reactions. The core reaction mechanism is the hydrogen-abstraction/aCetylene-Addition(HACA) mechanism: first, hydrogen is extracted to form active sites, and then acetylene is continuously added and cyclized, realizing the ring expansion and size growth of the aromatic skeleton [10]. In addition to the HACA mechanism, there are several reaction channels that have both accelerating and auxiliary effects: for example, the Diels-Alder(DA) cycloaddition reaction can mediate the cyclization process of aromatic rings at characteristic structural sites of PAH(such as bay areas). The contribution of this reaction under high temperature gas phase conditions is usually considered to be very limited, while under special conditions such as low temperature systems or surface catalysis, it is more likely to play a supplementary role [11]. In addition, the hydrogen abstraction–vinylacetylene addition(HAVA) [12] and methyl addition–cyclization(MAC) pathways [13] may increase the growth rate and affect the ordered growth of aromatic lamellae by introducing larger addition units or cyclization couplings involving alkyl radicals. These pathways may compete in parallel in different environments, and their relative importance still needs to be quantitatively constrained by combining observation and experiment.
Most of the above-mentioned classic formation mechanisms are based on the study of high-temperature circumstellar environments, while whether PAHs can be formed under low-temperature interstellar conditions has become an emerging research frontier in this field in recent years. The isotopic analysis of the sample returned by the Japanese Hayabusa2 probe from the asteroid Ryugu showed that the carbon isotopic fractionation characteristics of some small molecule PAHs inside it are highly consistent with the chemical environment of cold molecular clouds with less than 10 K. Based on this, Zeichner S S et al.(2023) proposed that some PAHs may be generated directly in cold interstellar clouds through low-temperature chemical pathways such as ion-molecule reactions [14]. This result shows that the formation of interstellar PAHs is likely to have a"dual source”: on the one hand, large-sized PAHs are formed in a hot high-energy environment through rapid free radical reactions, and on the other hand, small molecule PAHs are slowly synthesized in cold molecular clouds. These small molecule PAHs originating from cold environments can enter the high-energy region with the evolution of the nebula, and then further grow into large-sized aromatic molecules through mechanisms such as HACA and HAVA. It should be emphasized that the efficiency and universality of PAH in situ formation in low-temperature molecular clouds are still controversial. Current understanding is mainly based on theoretical simulations and indirect observational clues, and more direct astronomical observations and experimental constraints are still needed.
2 Structure and infrared spectral characteristics of PAH
2.1 Aromatic and Aliphatic Structures
PAH molecules have a fused aromatic ring as their core framework, with carbon atoms forming a delocalized π-electron conjugated system through sp² hybridization. This structural feature endows PAHs with extremely high chemical and thermal stability. The typical infrared vibrational modes of the aromatic ring structure correspond to a series of characteristic spectral bands.
Theoretical calculations and experimental results show that when sp3 hybridized aliphatic carbons(such as methyl and methylene groups) are introduced into PAH molecules, they exhibit unique infrared vibrational characteristics, often accompanied by a broader infrared"emission plateau"(such as near wavelengths of 8 μm, 12 μm, and 17 μm). Observations have revealed that in the UIE spectra of numerous celestial objects, the aromatic 3.3 μm feature band is frequently accompanied by the aliphatic 3.4 μm feature band, and a broad emission plateau at 12 μm is also detected in the vicinity of the strong aromatic 11.2 μm C-H out-of-plane bending band. This observational phenomenon implies that the carriers of UIE spectra may contain both aromatic ring and aliphatic structural moieties, rather than being purely PAH molecules with a homogeneous structure.
Based on the above observational evidence and structural inferences, Kwok S and Zhang Y.(In 2011) proposed that the carrier of UIE spectrum may be a type of mixed aromatic-aliphatic organic nanoparticles MAON) consisting of aliphatic chains connecting aromatic ring units [7]. The structure of such nanoparticles contains both sp2 hybridized aromatic ring carbon atoms and sp3 hybridized aliphatic chain carbon atoms; similar to coal, it can simultaneously produce characteristic infrared radiation of aromatic and aliphatic groups. This can explain, to some extent, the observation phenomenon of the coexistence of aromatic and aliphatic bands in UIE spectrum. Figure 1 is a schematic diagram of the possible structure of MAON particles. It should be noted that although MAON has potential value as a candidate carrier for UIE in research related to the origin of life, its existence still lacks direct and exclusive evidence at the laboratory and astronomical observation levels, and its applicable scope and contribution ratio still need further verification.
Fig.
1
A schematic diagram of a possible MAON particle structure. The diagram shows a MAON particle composed of multiple aromatic ring units (black) connected by aliphatic carbon chains, containing hydrogen atoms (white), and doped with small amounts of heteroatoms such as oxygen (red), nitrogen (blue), and sulfur (yellow). This structure combines the planar characteristics of aromatic rings with the structural features of aliphatic side chains, and is thus able to contribute both the narrow aromatic characteristic bands at 3.3μm, 6.2μm, 7.7μm, 8.6μm, 11.2μm and others, as well as the aliphatic characteristic bands and broad emission plateaus at 3.4μm, 6.9μm, 7.25μm and other wavelengths. It therefore provides a plausible structural explanation for the coexistence of radiative emissions from UIE characteristic bands and broad emission plateaus. The model illustrated in this figure is derived from Ref. [7] and subsequent relevant studies [15], and intuitively demonstrates the inherent complexity of the aromatic-aliphatic hybrid structure in the MAON model.
2.2 PAH infrared radiation characteristics
PAH molecules exhibit a set of highly characteristic infrared emission bands in various astronomical observations. This band combination is considered a"fingerprint" for identifying the existence of interstellar PAHs. The most representative band is the 3.3 μm aromatic C–H stretching vibration band(corresponding to approximately 3040 cm-1). This band has the shortest wavelength and highest energy among the characteristic PAH bands. Following closely behind is the 6.2 μm aromatic C=C skeleton stretching vibration band, which is the core identifier of the PAH skeleton structure. The 7.7 μm"mixed band" is actually composed of multiple adjacent vibrational peaks superimposed, originating from the coupled vibrations of aromatic C=C stretching and C–H bending. Its peak position varies significantly due to the influence of the interstellar environment(such as radiation field intensity and molecular ionization state), and it is distributed in the range of 7.6–7.8μm. The 8.6 μm band corresponds to the aromatic C–H in-plane bending vibration, and its intensity is usually slightly lower than that of the 7.7 μm mixed band.
11.2 μm band is a typical representative of aromatic C–H out-of-plane bending vibrations, mainly originating from the vibration of C–H bonds with only a single hydrogen atom connected to the edge of the benzene ring. In addition, several weaker aromatic C–H out-of-plane bending bands exist in the 10–15 μm band, each corresponding to a specific ring-edge hydrogen substitution mode: the 11.0 μm band(coexisting with the 11.2 μm band, indicating a"single hydrogen" substituted ring edge structure), the 12.0 μm band(two adjacent hydrogen atoms, i.e., a"double hydrogen" substituted edge), the 12.7 μm band(a" trihydrogen" substituted edge), and the 13.6 μm and 14.2 μm bands(a"tetrahydrogen" substituted edge). The relative intensity ratio of these bands can serve as a key indicator for diagnosing the regularity of the PAH ring edge structure. If the band corresponding to the"double hydrogen"/"trihydrogen" substituted edge is dominant, it indicates a smooth and complete ring edge; if the band corresponding to the"single hydrogen" substituted edge has a higher intensity, it indicates the presence of irregular gaps or structural defects in the ring edge.
Compared with aromatic characteristic bands, aliphatic C–H vibrations exhibit distinct infrared spectral features: the 3.4 μm band shows a series of fine component peaks(e.g., 3.38 μm, 3.40 μm, 3.46 μm), corresponding to the C–H stretching vibrations of CH3, CH2 and other groups in different bonding environments [16]; the 6.9 μm band and 7.25 μm band are assigned to the scissoring bending vibration of the CH2 group and the asymmetric bending vibration of the CH3 group, respectively. Such aliphatic spectral bands often act as companion features to the 11–13 μm aromatic C–H out-of-plane bending band and have been widely observed in the unidentified infrared emission(UIE) spectra of numerous celestial objects.
Figure 2 shows a typical example of an interstellar PAH infrared emission spectrum and its band decomposition results. The figure shows the ISO observation spectrum of the planetary nebula NGC 7027. The dark red curve represents the observed spectrum, in which a series of narrow feature bands are superimposed on a wide plateau of 6–9 μm and smooth continuous radiation. This overall morphology of "narrow bands, wide plateau, and continuous spectrum" radiation is a direct observation result. The colored curves are obtained via parametric decomposition and fitting of the observed spectra: the solid black curve corresponds to the narrow aromatic characteristic bands of PAHs at 3.3 μm, 6.2 μm, 7.7 μm, 11.2 μm, etc.; the blue curve represents the continuous thermal dust emission background; the orange region denotes the broad emission plateau in the ranges of 6–9 μm and 11–13 μm, where the radiation intensity accounts for as high as 36% of the total infrared radiation [7], indicating that aliphatic structures may exist in large quantities even in the strong ultraviolet radiation environment of planetary nebulae. These coexistent emission features suggest that the carriers of the UIE spectra may possess structural characteristics of both aromatic rings and aliphatic side chains/linking chains, in which the aromatic backbones are responsible for the discrete narrow bands, while the aliphatic side chains/linking chains correspond to the formation of the broad emission plateaus. The mainstream view in the academic community holds that such carriers originate from various PAH populations and their structural modifications, a conclusion that shares certain common ground with the MAON-related hypothesis; the core difference between the two lies in that the former attributes the origin of UIE to the joint contribution of various molecular populations, whereas the latter emphasizes that it derives from a single molecular system with complex structures.
Fig.
2
Infrared emission spectrum (ISO/SWS observation) and composition decomposition diagram of planetary nebula NGC 7027. The data in this figure are taken from reference [7]: the dark red solid line and the black dashed line are the measured and fitted spectra, respectively. The compositional components of the spectrum are intuitively presented via multicolor decomposition in the figure: the solid black curve corresponds to a series of aromatic PAH characteristic bands at 3.3 μm, 6.2μm, 7.7μm, 8.6μm, 11.2μm, 12.7μm, etc., which originate from the characteristic vibrations of C-H and C=C bonds in aromatic rings; the orange curve represents the broad emission "plateau" or band complex, whose emission is mainly contributed by the vibrations of PAH molecules with a small number of aliphatic side chains or more complex organic nanoparticles (e.g., MAON); the blue curve is a smooth continuous thermal emission background, derived from the thermal radiation of dust grains.
3 PAH spectroscopy
The rich infrared characteristic bands of PAHs not only reveal diverse astronomical phenomena but also constitute a"spectral fingerprint" for decoding the physicochemical properties of the interstellar medium. By analyzing the relative intensity, peak shift, and band ratio of each characteristic band, researchers can infer the key physicochemical states of PAH-related molecules, including core parameters such as ionization distribution, molecular size distribution, and ring edge configuration. The following section will explain the physical meaning of the core spectral diagnostic indicators and their application value in the inversion of interstellar environment characteristics.
3.1 Diagnosis of Ionization Degree
The ionization state of PAHs is a key factor regulating their infrared band intensity distribution: PAH cations(PAH+) significantly enhance the relative intensity of bands related to the C=C skeleton vibration of aromatic rings, while neutral PAHs emphasize the radiative contribution of aromatic C–H vibration bands. Specifically, when PAHs are predominantly ionized, the relative intensities of the 6.2 μm and 7.7 μm bands(both corresponding to aromatic C=C stretching vibrations) are typically significantly increased; while the radiation of the 3.3 μm aromatic C–H stretching band and the 11.2 μm aromatic C–H out-of-plane bending band is more pronounced in environments where neutral PAHs dominate.
Astronomical observations provide direct evidence for the above pattern: in interstellar regions dominated by strong ionizing radiation fields, the relative intensity of the 6–8 μm band(primarily the C=C vibration band) is usually higher than that of the 11.2 μm band(C–H vibration band). This characteristic is considered a hallmark criterion for the widespread ionization of PAHs. Conversely, in well-shielded, neutral molecular cloud environments with mild radiation fields, the 11.2 μm and 3.3 μm bands dominate, indicating that PAHs mainly exist in a neutral state. For example, in the observations of the bright nebula M17, a distinct gradient in band ratios is evident in its spatial distribution: the 11.2 μm/7.7 μm ratio is significantly low in the vicinity of the ionization front(reflecting a high degree of ionization); as one moves deeper into the molecular cloud, this ratio gradually increases, clearly depicting the spatial evolutionary trajectory of PAHs from ionized to neutral states [17].
The 11.2 μm/7.7 μm and 11.2 μm/6.2 μm band intensity ratios are the diagnostic indicators for PAH ionization states widely adopted in the academic community: a higher ratio indicates a higher proportion of neutral PAHs and a lower overall ionization degree of the system, while a lower ratio points to a higher proportion of PAH cations and a stronger ionizing radiation field in the environment. This diagnostic criterion has been fully validated by various types of observational data, and its reliability is well demonstrated by both the multi-source statistical analyses from the Spitzer Space Telescope and the high spatial resolution observations of the Galactic nuclear region by the JWST. This indicator has also been extensively applied to distinguish the radiation field intensity and ionization characteristics in different astrophysical environments such as star-forming regions and active galactic nuclei [18-19].
3.2 Molecular size diagnosis
The relative intensity distribution of the infrared characteristic bands of PAHs is the core clue to reveal their molecular size characteristics. After absorbing the same amount of energy, small-sized PAHs are more likely to excite high-energy vibrational modes through vibrational relaxation due to the vibrational energy level density of states effect, and then radiate energy in a shorter wavelength band; while large-sized PAHs tend to excite low-energy vibrational modes, and the radiation contribution in the longer wavelength band is higher [20]. Based on this rule, 11.2 The μm/3.3 μm band intensity ratio has become a widely used diagnostic indicator for the average molecular size of PAHs: the higher the ratio, the more dominant the large-sized PAHs; the lower the ratio, the more enriched the small-sized PAHs.
Observations of the reflection nebula NGC 7023 provide direct evidence for the aforementioned diagnostic logic: in regions close to the ultraviolet light source, the 3.3 μm/11.2 μm ratio rises significantly, indicating an enrichment of small-sized PAHs; as the distance from the light source increases, this ratio gradually decreases, reflecting that large-sized PAHs dominate infrared emission [21]. This phenomenon is highly consistent with theoretical expectations—intense ultraviolet radiation fields can induce photodissociation and fragmentation of large-sized PAHs, generating small molecular fragments. Quantum chemical calculations by Bauschlicher Jr C W et al.(2008) reveal that the peak position difference between the two key sub-peaks(~7.6 μm and~7.8 μm) of the 7.7 μm composite band reflects the size information of PAH molecules: small ionized PAHs containing dozens of carbon atoms tend to produce emission around 7.6 μm, whereas the emission of large ionized PAHs(PAH) is concentrated around 7.8 μm [22]. In addition, the excitation of characteristic bands in the 15-20 μm far-infrared range(e.g., 16.4 μm, 17.4 μm, 17.8 μm, etc.) relies on the low-energy vibrational modes of large-sized PAHs and can only be detected in environments dominated by large PAHs. The successful detection of such bands provides direct observational evidence for the existence of large-sized interstellar PAHs.
Observational data show that there is a significant synergistic correlation between the average molecular size of PAHs and their ionization state: in a strong radiation field, the remaining small-sized PAHs are mostly in a neutral state, while large-sized PAHs are more easily ionized into cations. Statistical analysis of thousands of H II regions in multiple galaxies by JWST shows that PAH abundance is significantly negatively correlated with environmental ionization parameters: the higher the ionization parameters, the lower the relative abundance of PAHs [40].
This method typically selects a set of band ratios more sensitive to ionization(e.g., I11.2/I7.7) and a set sensitive to average size(e.g., I11.2/I3.3) to construct a two-dimensional plane: in model calculations/spectral library predictions, as the proportion of positive charges increases, the 7.7 μm band(dominated by C=C vibrations) is relatively enhanced while the 11.2 μm band(dominated by neutral C–H out-of-plane bending) is relatively weakened, leading to an overall decrease in I11.2/I7.7; under given excitation conditions, large-sized PAHs tend to exhibit a higher I11.2/I3.3 ratio. The grids and trajectories in the figure mainly represent model expectations under a set of established spectral model assumptions. Projecting the observed band ratio data points onto this grid allows semi-quantitative constraints to be placed on the ionization degree and average size of the PAH population within the model framework.
Fig.
3
Schematic diagram of the PAH molecular size-ionization diagnostic grid. The horizontal axis represents I11.2/I3.3(molecular size index), and the vertical axis represents I11.2/I7.7(Degree of ionization index). Different symbols represent PAH samples with different degrees of ionization(neutral indicates 100% neutral, N67C33 indicates 67% neutral and 33% cation, N50C50 indicates 50% neutral and 50% cation, N33C67 indicates 33% neutral and 67% cation, cation indicates 100% cation), and the color scale indicates the change in the number of carbon atoms. The curve grid connects points with similar numbers of carbon atoms, and the curve connects points with the same ionization state, showing the synergistic change law between molecular size and degree of ionization. The overall trend shows that the PAH molecular size increases progressively along the horizontal direction(with the increase in I11.2/I3.3), and the PAH ionization degree intensifies along the vertical direction(with the decrease in I11.2/I7.7). Neutral PAHs(represented by circles) are concentrated in the region with higher ratio values, while cationic PAHs(represented by pentagons) are mainly distributed in the region with lower ratio values, which manifests the sensitive response of band ratios to the molecular ionization degree and size of PAHs.
It should be noted that the applicability of this diagnostic grid is mainly limited to cases where the emission is dominated by UIE features, and the signal-to-noise ratio(SNR) and spectral resolution are sufficient to reliably resolve the 3.3 μm, 7.7 μm and 11.2 μm bands(e.g., photodissociation regions, reflection nebulae, and some galactic disk regions). Its potential limitations lie in the following: in addition to being affected by ionization and molecular size, band ratios are jointly modulated by factors such as the excitation energy distribution, hydrogenation degree and edge structures, heteroatom doping, continuum subtraction and extinction correction, as well as multiphase mixing caused by line-of-sight integration. Therefore, this method is more suitable for comparing the relative trends and evolutionary trajectories of different regions/sources, and it is not appropriate to draw overly precise and unique conclusions on physical parameters solely based on this grid.
Compared with the traditional two-dimensional grid method based on the ratio of a few diagnostics, machine learning has been used in recent years to process higher-dimensional PAH spectral information to alleviate the problem of multiple solutions in the"structure-band" mapping. Wang Zhao's team built a model based on the PAH theoretical spectral library and molecular descriptors, realized the rapid prediction of PAH infrared spectra, and quantitatively identified key factors affecting the band correlation(such as molecular size, charge state and edge hydrogen configuration) through interpretable correlation analysis, thus providing a new quantitative framework for inferring the PAH population structure from the observation of multi-band information [23-24].
3.3 Structural Morphology Diagnosis
The ring-edge geometry of PAH molecules leaves characteristic spectral imprints in the infrared spectrum. Among them, the most diagnostically valuable are the out-of-plane bending vibrational bands of the aromatic ring C–H bonds. The relative intensity and peak position of these bands can be used to reveal the spatial arrangement of hydrogen atoms on the ring edge to a certain extent. Existing studies have confirmed that there is a relatively stable empirical correspondence between the adjacency state of the ring edge hydrogen and the characteristic spectral bands, but the two are not strictly one-to-one correspondences:(1) When there is an isolated single hydrogen("solo-H") at the edge of the PAH, it will excite a characteristic spectral band set in the 11.0-11.3 μm range;(2) If it is an adjacent double hydrogen("duo-H"), it often forms a characteristic emission band near 12.0 μm;(3) A continuous triple hydrogen("trio-H") is often associated with a spectral band near 12.7 μm;(4) The vibration of a continuous quadruple hydrogen("quartet-H") produces emission at 13.5 μm, and its spectral band can extend to the long-wavelength region of 14.2 μm [25]. Therefore, different PAH molecules will exhibit the above-mentioned differential intensity ratios of C-H out-of-plane bending vibration bands due to the difference in the adjacency configuration of the ring edge hydrogen.
As shown in Figure 4, the outer geometry of the PAH molecule greatly influences the adjacency type of its peripheral C-H groups, thereby dominating the emission characteristics of the out-of-plane bending vibration of C-H in the 10-15 μm band. Specifically, PAHs with long-range straight zigzag edges are more likely to form solo-H sites, thus dominating the radiation intensity of the 11.2 μm band. In contrast, PAHs with irregular edges such as armchair edges or bay regions are more likely to generate duo-H and trio-H sites, thus significantly enhancing the radiation contribution of the 12.0-12.7 μm composite band [3].
Fig.
4
Schematic structural diagrams of PAH molecules. The left one is C42H16, whose outer edge contains both zigzag and armchair segments; the right one is C20H12, illustrating the boundaries with typical bay and non-bay regions. The corresponding edge structures are marked with blue arrows. The presence of different edge structures and bay regions alters the adjacency types(solo/duo/trio) of peripheral C-H bonds, thereby affecting the peak positions and intensities of the CH out-of-plane bending bands in the 10-15μm region.
Observational evidence further validated the structure-spectrum correspondence mentioned above: in the near-stellar region of the reflection nebula NGC 7023, the 12.7 μm/11.2 μm characteristic intensity ratio of the PAH infrared spectrum decreased significantly, which may indicate that the proportion of single-H on the ring edge increased significantly [26]. Its physical essence is the photochemical erosion of PAH molecules by strong ultraviolet radiation. High-energy photons will erode the regular edges of PAH, making its edge structure tend to be fragmented and incomplete; while in the region far from the star and with a weaker radiation field, the ratio increased significantly, indicating that the PAH ring edge retains a higher degree of structural regularity and the proportion of duo-H and trio-H sites is higher. Therefore, by analyzing the spatial distribution differences in the intensity ratio of C–H out-of-plane bending vibration bands, it is possible to track the structural evolution of PAH molecules under different radiation environments: strong radiation fields drive the molecule's corner stripping and increase the proportion of isolated C–H(solo-H); while in the low-temperature, high-density and well-shielded interstellar environment, PAH can maintain more regular edges with multiple hydrogen adjacencies(duo-H/trio-H bands are dominant).
In addition to the role of the ring-edge hydrogen configuration in regulating molecular size and ionization state, the incorporation of heteroatoms in the PAH molecular framework can also cause subtle peak shifts in the infrared characteristic bands. Among them, the spectral response of polycyclic aromatic nitrogen hydrocarbons(PANHs) formed by nitrogen atom doping is the most typical. Existing theoretical and experimental studies have confirmed that the introduction of nitrogen heterocycles will change the electron cloud distribution of the C=C bond in the aromatic ring, which will lead to a recognizable shift of the characteristic peak of the 6.2 µm aromatic C=C stretching vibration: depending on the substitution site and doping ratio of nitrogen atoms in the aromatic ring, this band may be blue-shifted to 6.0 µm in the short-wave direction or red-shifted to the 6.3-6.4 μm range in the long-wave direction. In astronomical observations, the 6.2 μm band of some planetary nebulae shows an abnormally broadened profile. This phenomenon is thought to be related to the mixed contribution of PANHs components in the PAH molecule, providing indirect observational clues for the existence of interstellar polycyclic aromatic hydrocarbons [27]. In addition to PANHs, sulfur-containing aromatic hydrocarbons(PASHs) have also been proposed in recent years as one of the material fates of the interstellar medium's"sulfur-deficient" problem. Related theoretical studies have quantitatively assessed the C-S vibrational band intensity and key band characteristics of typical sulfur-containing PAHs, providing observational criteria that can be further verified for testing the existence and contribution of PASHs using high-sensitivity mid-infrared spectroscopy such as JWST/MIRI [28]. It should be noted that the peak position shift and spectral bandwidth expansion caused by heteroatom doping are usually not exclusive spectroscopic fingerprints, and still need to be combined with other spectral band characteristics, abundance constraints and astronomical environment information for comprehensive judgment.
Furthermore, when a five-membered ring unit(including a pure carbon five-membered ring or a five-membered heterocyclic ring containing heteroatoms) is embedded in the PAH molecular framework, its infrared spectrum may also excite atypical weak emission bands such as 6.0 µm or 6.6 µm. The generation of such bands may be related to the unique ring strain and vibrational modes of the five-membered ring [29-30]. However, it should be noted that the spectral shift and weak emission effect caused by heteroatom doping and five-membered ring embedding are usually weak in magnitude and easily masked by the radiation of strong aromatic characteristic bands; at present, the relevant conclusions mainly rely on theoretical deductions from quantum chemical calculations and laboratory simulated spectra, and no direct and exclusive astronomical observation evidence has been obtained. Its actual proportion and spectral contribution in the interstellar PAH population still need to be verified by higher resolution infrared observations. The formation of five-membered rings may be related to the origin of fullerenes, and this connection has attracted much attention from the academic community in recent years.
4 Challenges and controversies surrounding the PAH model
Although the PAH model has made significant progress in explaining interstellar infrared emission phenomena, its theoretical framework still has several unresolved core issues, which have long been the subject of continuous debate and in-depth discussion in the academic community.
PAH is not the only reasonable explanation for UIE bands. MAON has both aromatic ring skeleton and aliphatic side chain, with a typical size on the order of tens of atoms. It can simultaneously reproduce the narrow aromatic emission band of 3.3 µm and the wide aliphatic emission plateau near 12 µm [31]. This shows that the successful cases of fitting UIE bands with hundreds of PAH mixtures in the past cannot directly prove that the support must be a free PAH molecule. The support of UIE is likely a group of carbon molecules with diverse structures. PAH is the core component, but it may also be accompanied by aliphatic chains, heterocycles and other additional structures. Since the infrared spectrum alone cannot effectively distinguish pure PAH from other aromatic carbon materials, the PAH hypothesis still lacks unique and exclusive spectroscopic verification evidence. It should also be noted that for alternative models such as MAON, no characteristic spectroscopic fingerprints that can be exclusively verified have been found in astronomical observations. From free PAHs to PAHs with aliphatic side chains, or those modified by hyperhydrogenation or substitution, and then to more complex solid organic molecules(such as HAC/MAON), these interstellar organic molecules either constitute a continuous evolutionary lineage or are ubiquitous in astronomical environments and exhibit evolutionary correlations.
To date, the scientific community has not confirmed the existence of any specific PAH molecule capable of reconstructing the UIE spectrum in interstellar space. In contrast, some small-molecule aromatic derivatives(such as cyano-containing polycyclic aromatic hydrocarbons) have been detected in cold molecular clouds using radio rotation spectroscopy; however, large-sized PAHs, due to their high molecular symmetry and near-zero dipole moments, exhibit extremely weak rotational transition signals, making direct observation extremely difficult. Previous studies on bowl-shaped PAHs... Multiple deep radio spectral searches for PAHs(such as C20H10) have yielded no substantial results, only providing a strict upper limit for the interstellar abundance of such PAHs [32-33]. Recent searches using the Chinese FAST telescope for large PAHs with more than 130 carbon atoms have also failed to observe the expected large PAH"comb" characteristic spectral lines, and the derived upper limit of abundance is even lower than the theoretical model prediction [34]. This result may suggest that the actual abundance of large PAHs in the interstellar medium is less than the model prediction, but since the observational analysis still relies on a series of simplified assumptions, the universality of this conclusion still needs to be carefully evaluated.
These challenges to the PAH model have become the core driving force for the development of the field: current research should actively explore the specific observational criteria for PAHs, such as infrared polarization signals, characteristic transition lines in the far-infrared or radio bands, etc. Once the unique spectroscopic fingerprint of PAH molecules is captured in celestial bodies, it will provide decisive support for the PAH model; before that, the academic community still needs to maintain an open research attitude, relying on more sensitive observation equipment and laboratory simulations to continuously test and improve this classic theoretical framework.
5 New-generation observation equipment in PAH research
With the commissioning of the new generation of infrared observation equipment, especially the launch of JWST, interstellar PAH research has achieved key improvements in spectroscopic resolution and detection sensitivity. The high spectral resolution of JWST/MIRI has further decomposed the previously mixed feature bands: the 3.3 µm aromatic C–H stretching vibration band presents a more complex fine structure; the 3.4 µm aliphatic side chain related emission also shows multiple narrow peaks superimposed on a wide platform, pointing to the contribution of different bonding environments such as CH3 and CH2 [35]. At the same time, the high sensitivity of JWST has expanded the detectability of weak spectral bands(such as weak emission at 5.25 µm and 5.70 µm) and detected a feature signal suspected to be related to PAH functional groups near 4.4 µm. Although the signal at 4.36-4.43 µm corresponding to the aromatic C–D stretching vibration has not yet been confirmed, JWST has significantly enriched the spectral information that can be used for carrier confinement [36]. Yang Xuejuan et al. pointed out through calculation that aromatic C–D(about 4.4 µm) and aliphatic C–D(about 4.6-4.7 µm) have potential detectability under suitable excitation and abundance conditions. This provides an important observational approach for constraining the destination of interstellar" deuterium-deficient" materials and studying PAH isotope chemistry [37-38]. In the 1-5 µm near-infrared band, JWST/ NIRSpec began to touch on possible signs of PAH overtone and combination frequency vibrations(such as the 1.68 µm overtone signal corresponding to the 3.29 µm main peak), but it still needs further verification [36]. In the 15-20 µm range, MIRI’s ability to detect weak spectral bands was significantly enhanced. A very weak 16.4 µm emission peak was detected in the Orion Bar, and a suspected C60 fullerene signal was reported near 17.4 µm [35]. High-sensitivity mid-and far-infrared observations will provide a more direct window for exploring PAH skeleton vibrations(such as drumhead vibrations).
In the study of spatial distribution and evolution characteristics, JWST's high spatial resolution imaging capability enables direct analysis and characterization of the fine structure of PAH emission. Relying on the narrowband observation methods of NIRCam and MIRI, researchers can obtain two-dimensional spatial distribution maps of characteristic spectral bands such as 3.3 µm and 11.3 µm(resolution is about 0.1″~0.3″), and then accurately track the spatial distribution evolution of PAH at the scale of galaxies and interstellar medium. The observation results of the PHANGS-JWST survey project on nearby galaxies show that the 3.3 µm PAH emission radiation is more concentrated in the starburst ring and molecular cloud edge region, while it is significantly weakened or even almost disappeared in the H II region. This observation feature confirms the dissociation and destruction effect of strong ultraviolet radiation on PAH [39]. In typical photodissociation regions(PDRs) of the Milky Way(e.g., the Orion Bar), the JWST has also clearly resolved the systematic evolutionary trend of the PAH characteristic spectral line profiles with distance from the ionization front: on the side close to the ionization source, the spectral lines exhibit characteristic morphologies induced by intense radiation, while in the process of extending toward the molecular region, the spectral line morphologies gradually transition to the stable characteristics typical of low-radiation environments—this is prominently manifested by the broadening and redshift of the 3.3 µm main emission band, as well as the significant shifts in the profile and peak position of the 11.2 µm band. Meanwhile, the intensity ratio of 3.4 µm/3.3 µm shows a distinct gradient distribution within the radiation layer. This characteristic indicates that as the intensity of ionizing radiation decreases from high to low, the PAH molecular population likely undergoes a photochemical evolutionary process, transitioning from components with aliphatic side chains(or hydrogen-rich substituents) to a state dominated by pure aromatic components with more stable aromatic skeletons [35]. Overall, the JWST has provided more stringent observational constraints for relevant research in terms of spectroscopic deconvolution, weak emission band detection, and spatially resolved tracing, and has advanced the identification of UIE carriers and the investigation of their evolutionary mechanisms to a testable and refined research stage.
China is also actively promoting the research and development and construction of a new generation of space astronomical observation platforms. For example, the mission statement of the China Space Station Telescope(CSST) [40] clearly states that the device is equipped with a high-performance terahertz spectrometer, and the observation band covers the near-infrared to terahertz band(including the far-infrared and submillimeter-wave extensions), which can provide a large field of view and a large area of spectral observation and physical constraints for the target sky area. Its large field of view survey observation capability and the mid-infrared high-resolution spectroscopic observation of JWST form a good complement to each other in terms of band and observation scale. The collaborative observation of the two will provide more comprehensive observation data and scientific research support for the frontier research directions of astrochemistry, such as interstellar PAH chemical evolution, interstellar isotope distribution, and terahertz band interstellar molecular spectral line detection.
6 Conclusion
As a leading candidate carrier for the UIE, the systematic study of PAHs provides crucial support for resolving the complex landscape of interstellar organic chemistry. Regarding formation mechanisms, the bottom-up HACA mechanism may dominate PAH synthesis in high-temperature circumstellar environments, while the top-down carbonaceous particle fragmentation pathway and ion-molecule reactions in low-temperature molecular clouds expand the environmental diversity of their origin. At the spectral level, PAH infrared radiation exhibits a significant structure-spectrum correlation, and the diagnostic system based on characteristic band ratios demonstrates strong astronomical application value in retrieving key attributes such as PAH ionization state distribution, molecular size gradient, and ring edge structure morphology. The controversy surrounding PAHs as UIE carriers and the lack of direct observational evidence for large-scale PAHs remain major bottlenecks hindering the theoretical development of this field.
The launch of JWST has injected groundbreaking momentum into this field. Its high-resolution spectroscopy successfully resolved the multi-component fine structure of the 3.3 μm aromatic C–H stretching band and achieved effective detection of the 16.4 μm far-infrared weak emission band. High spatial resolution imaging also clearly revealed the spatial distribution and evolution of PAHs at the galactic scale, providing direct observational evidence for the PAH radiation field response mechanism. Looking ahead, research should focus on the core challenge of exclusive spectroscopic evidence for the identity of UIE carriers, clarifying whether PAHs are their dominant carriers through more sensitive far-infrared or radio observations. Simultaneously, it is necessary to clarify the multi-scale coupling mechanisms between PAHs and star formation and galaxy evolution, and promote the deep integration of theoretical models and machine learning-driven spectral analysis, thereby deepening human understanding of the evolution of the cosmic carbon cycle and the interstellar origin of life precursors.