Infrared spectroscopy of gas-phase hydrogenated and methylated pyrenes: from laboratory spectra to the simulated 3.4 $\mu$m emission band

Problem
Aromatic Infrared Bands (AIBs) are prominent emission features in the interstellar medium, primarily attributed to Polycyclic Aromatic Hydrocarbons (PAHs). While the 3.3 µm band is well-established as the signature of aromatic C-H stretching, the 3.4–3.6 µm range contains satellite features (notably at 3.40, 3.46, 3.51, and 3.57 µm) and a broad plateau. These are generally assigned to aliphatic C-H stretching vibrations in hydrogenated (H-PAHs) and methylated (Me-PAHs) PAHs. However, the specific molecular carriers of these features, particularly the prominent 3.403 µm component, remain debated. Furthermore, existing theoretical and experimental data often lack the necessary anharmonic corrections required to accurately simulate the emission spectra of hot PAHs as they cool following UV excitation, limiting direct comparison with high-resolution observations from the James Webb Space Telescope (JWST).

Methodology
The authors conducted a comprehensive study combining high-temperature laboratory spectroscopy, theoretical calculations, and astrophysical modeling:

1. Experimental Spectroscopy: Mid-infrared absorption spectra (1.4–25 µm) were recorded for gas-phase pyrene, methylated pyrene (1-methylpyrene), and hydrogenated pyrenes (dihydro-, tetrahydro-, and hexahydropyrene) at temperatures ranging from 373 K to 673 K. Measurements were performed using a dedicated high-temperature cell on the ESPOIRS setup at IRAP.
2. Theoretical Calculations: Density Functional Theory (DFT) calculations were performed using the B3LYP-D3 functional with the N-07D basis set to obtain semi-quartic force fields. The AnharmoniCaOs code was used to compute anharmonic absorption spectra at 0 K for the ground vibrational state, supporting the assignment of experimental bands.
3. Anharmonicity Extraction: An interactive tool (cosmicPAHmfit) was used to perform multi-component fits of the experimental bands. This allowed the derivation of empirical anharmonicity laws describing the evolution of band positions and widths as a function of temperature. Rotational broadening was estimated and subtracted to isolate vibrational effects.
4. Emission Modeling: A Monte Carlo kinetic code was employed to simulate the cooling cascades of PAHs following UV-photon absorption (up to the dissociation limit, ~6 eV). The model incorporated the derived empirical anharmonicity parameters to generate synthetic emission spectra that account for temperature-dependent band shifts and broadening.
5. Astrophysical Comparison: The resulting synthetic spectra were compared with JWST observations of the Orion Bar (PDRs4All program) and ground-based data of NGC 1333.

Key Contributions

• New Experimental Data: Provided the first state-of-the-art gas-phase infrared spectra for hydrogenated and methylated pyrenes at relevant astrophysical temperatures, covering the critical 3.3–3.6 µm region.
• Empirical Anharmonicity Laws: Derived temperature-dependent parameters for band positions and widths for these aliphatic-containing species, addressing a gap in existing databases which lacked data for PAHs with aliphatic C-H bonds.
• Database Integration: All experimental spectra, anharmonicity parameters, and theoretical calculations have been made publicly available through the CosmicPAH-IRDB and the Cagliari-Toulouse Theoretical spectral database.
• Refined Carrier Identification: Proposed a specific molecular carrier for the 3.403 µm emission feature based on a rigorous comparison of simulated and observed spectra.

Results

• Spectral Characteristics: The study confirmed that hydrogenation significantly alters the C-H stretching region. While methylated pyrene contributes primarily to the 3.3 µm band and a weak plateau, heavily hydrogenated species (specifically 6H-pyrene) dominate the 3.4 µm region.
• Temperature Effects: As temperature increases, aliphatic bands generally redshift and broaden. The study observed that 2H- and 4H-pyrene are thermally unstable at higher temperatures, reverting to pyrene, whereas 6H-pyrene exhibits greater thermal resistance.
• Carrier Identification: The synthetic emission spectrum of 1,2,3,6,7,8-hexahydropyrene (6H-pyrene), excited by ~4 eV UV photons, provides a detailed match to the observed 3.403 µm component in the Orion Bar. The model successfully reproduces the band profile and position without requiring unphysical parameters.
• Methylated Pyrene: While 1-methylpyrene contributes to the underlying emission plateau and the 3.3 µm band, its lack of a strong, distinct infrared band in the 3.4–3.6 µm range complicates its detection as a specific carrier for the satellite features compared to 6H-pyrene.
• Model Accuracy: The inclusion of empirical anharmonicity functions significantly improved the agreement between simulated and observed spectra compared to models using only laboratory spectra measured at a single temperature (e.g., 523 K).

Significance
The paper claims that identifying 6H-pyrene as the carrier of the 3.403 µm band reshapes the interpretation of the 3.4 µm feature in astronomical observations. It suggests that the 3.403 µm component is a robust tracer of aliphatic content in shielded regions where fragile aliphatic bonds can survive mild UV excitation (~4 eV). Consequently, the authors argue that using the 3.4-to-3.3 µm band ratio as a simple tracer for the overall degree of superhydrogenation in PAH populations is an approximation, as the 3.395 µm component (and the 3.3 µm band) may originate from different carriers or excitation conditions. This work provides a more accurate framework for interpreting JWST data across various environments, from photodissociation regions to protoplanetary disks, by linking specific molecular structures to observed spectral features through rigorous laboratory and theoretical constraints.