Molecular Spectroscopy of Interstellar Polycyclic Aromatic Hydrocarbons

Molecular spectroscopy of interstellar PAHs examines vibrational and electronic transitions that underlie the aromatic infrared bands (AIBs) and the so-called unidentified infrared emission features observed across the cosmos. By combining space-based observations with laboratory gas-phase and matrix-isolation experiments, researchers characterise specific PAH species, their ionisation, fragmentation, hydrogenation and radiative cooling pathways. These studies reveal how PAHs absorb ultraviolet photons, redistribute energy via vibronic coupling and recurrent fluorescence, and emit characteristic mid-IR signatures. Identifying individual molecules, such as cyanonaphthalene, has resolved longstanding abundance anomalies in molecular clouds and has informed models of carbon chemistry in photodissociation regions. Advances in high-resolution spectroscopy and quantum chemical calculations now enable the deconvolution of overlapping aliphatic and aromatic CH modes, improving estimates of molecular size distributions, charge states and aliphatic fractions. The field intersects astrochemistry, molecular physics and observational astronomy to elucidate the lifecycle of carbonaceous material from diffuse clouds to protoplanetary discs, with implications for interstellar heating, the formation of H₂ and the origins of complex organic compounds in space.

Research from Nature Portfolio

Studies have measured the unimolecular dissociation and radiative cooling rates of the 1-cyanonaphthalene cation in conditions analogous to cold interstellar clouds. These experiments demonstrate that recurrent fluorescence efficiently stabilises small PAH cations, resolving the unexpectedly high abundance of cyanonaphthalene in dark clouds. In parallel, quantum chemistry and ion-trapping investigations of coronene cations have revealed stepwise hydrogenation pathways that produce “magic” numbers of attached H atoms. This site-specific sequence explains the prevalence of partially hydrogenated PAHs in space and suggests key roles for PAH hydrogenation in cosmic H₂ formation and molecular survival under ultraviolet irradiation.

Molecular Spectroscopy of Interstellar Polycyclic Aromatic Hydrocarbons publication trend

The graph below shows the total number of articles in molecular spectroscopy of interstellar polycyclic aromatic hydrocarbons across all publications each year (not limited to Nature Index journals).

Technical terms

Polycyclic aromatic hydrocarbons (PAHs): Carbon-based molecules composed of fused aromatic rings found throughout the interstellar medium.

Aromatic infrared bands (AIBs): Strong mid-infrared emission features attributed to vibrational modes of PAHs.

Photodissociation region (PDR): Interstellar zones where far-ultraviolet photons regulate the chemistry and heating of gas and dust.

Recurrent fluorescence: A radiative relaxation process in which thermally populated electronic excited states emit photons, stabilising excited molecules.

Vibronic coupling: Interaction between electronic and vibrational states that enhances transition probabilities and affects spectral line shapes.

References

1. Efficient stabilization of cyanonaphthalene by fast radiative cooling and implications for the resilience of small PAHs in interstellar clouds. Nature Communications (2023).
2. The sequence to hydrogenate coronene cations: A journey guided by magic numbers. Scientific Reports (2016).
3. Battle of the CH motions: aliphatic versus aromatic contributions to astronomical PAH emission and exploration of the aliphatic, aromatic, and ethynyl CH stretches. Monthly Notices of the Royal Astronomical Society (2024).
4. JWST: Deuterated PAHs, PAH Nitriles, and PAH Overtone and Combination Bands. I. Program Description and First Look. The Astrophysical Journal (2023).