Anharmonicity and deuteration in the IR absorption and emission spectrum of phenylacetylene

Abstract

Anharmonic cascade emission simulations, herein evinced by full reproduction and deep insights into recent emission spectroscopy experiments of phenylacetylene, are integral to the future successful analysis of JWST observational spectra. Experimental infrared absorption experiments conducted in this study reveal a complex spectrum dominated by quantum effects that are uncovered by anharmonic computational analysis. From this work, it becomes clear that phenylacetylene exhibits strong resonance coupling between fundamental and two-quanta combination modes as well as giving indication for coupling with higher-order, three-quanta combination bands. This study benchmarks the development of advanced computational methods that will be extended to larger systems of astronomical relevance and those including isotopic substitution and side group functionalization with groups such as acetylene. The astrophysical implications of these results, including the potential for detection of acetylenic C–H stretches in space, are discussed in the vein of the impact polycyclic aromatic hydrocarbons have on astronomical infrared emission bands.

GRAPHICAL ABSTRACT

KEYWORDS:

• Infrared spectroscopy
• astrochemistry
• anharmonicity
• emission spectroscopy
• polycyclic aromatic hydrocarbons

Acknowledgements

V.J.E. acknowledges an appointment to the NASA Postdoctoral Programme at NASA Ames Research Centre, administered by the Oak Ridge Associated Universities through a contract with NASA. C.B. is grateful for an appointment at NASA Ames Research Centre through the San José State University Research Foundation (80NSSC22M0107). C.B. acknowledges support from the Internal Scientist Funding Model (ISFM) Laboratory Astrophysics Directed Work Package at NASA Ames. Computer time from the Pleiades cluster of the NASA Advanced Supercomputer (NAS) is gratefully acknowledged. R.C.F. acknowledges support from NASA grant NNH22ZHA004C and the Mississippi Centre for Supercomputing Research supported in part from NSF Grant OIA-1757220. Studies of interstellar PAHs at Leiden Observatory are supported through a NWO Spinoza grant. The HFML-FELIX Laboratory is supported by the project CALIPSOplus under the Grant Agreement 730872 from the EU Framework Programme for Research and Innovation HORIZON 2020. We gratefully acknowledge the Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO) and thank the FELIX staff.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Significance statement

The experimental infrared absorption spectrum of phenylacetylene is complicated due to the presence of strong Fermi resonances, particularly in the acetylene CH stretch fundamental region. Anharmonic computations are required to assign the experimental spectrum because higher-order vibrational transitions (combination bands and overtones) gain strength due to strong coupling. In a similar vein, an anharmonic cascade emission simulation matches the experimental band position with no scaling factors included. Additionally, the anharmonic computations capture the presence of small features missed by the harmonic model; an integral result given the high-fidelity JWST data will resolve new, low-intensity emission features.

Additional information

Funding

This work was supported by Ames Research Center: [grant no 80NSSC22M0107].