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Anharmonicity strongly influences the absorption and emission spectra ofpolycyclic aromatic hydrocarbon (PAH) molecules. Here, ion-dip spectroscopyexperiments together with detailed anharmonic computations reveal the presenceof fundamental, overtone, as well as 2- and 3-quanta combination bandtransitions in the far- and mid-infrared absorption spectrum of phenylacetyleneand its singly deuterated isotopologue. Strong absorption features in the400-900 cm$^{\rm -1}$ range originate from CH(D) in-plane and out-of-plane(OOP) wags and bends, as well as bending motions including the C$\equiv$C andCH bonds of the acetylene substituent and the aromatic ring. Forphenylacetylene, every absorption feature is assigned either directly orindirectly to a single or multiple vibrational mode(s). The measured spectrumis dense, broad, and structureless in many regions but well characterized bycomputations. Upon deuteration, large isotopic shifts are observed. Atfrequencies above 1500 cm$^{\rm -1}$ for d$_1$-phenylacetylene, a one-to-onematch is seen when comparing computations and experiment with all featuresassigned to combination bands and overtones. The C$\equiv$C stretch observed inphenylacetylene is not observed in d$_1$-phenylacetylene due to a computed40-fold drop in intensity. Overall, a careful treatment of anharmonicity thatincludes 2- and 3-quanta modes is found to be crucial to understand the richdetails of the infrared spectrum of phenylacetylene. Based on theseobservations it can be expected that such an all-inclusive anharmonic treatmentwill also be key for unraveling the infrared spectra of PAHs in general.

The Infrared Absorption Spectrum of Phenylacetylene and its Deuterated Isotopologue in the Mid- to Far-IR