Anharmonicity in the mid-infrared spectra of polycyclic aromatic hydrocarbons: molecular beam spectroscopy and calculations

Abstract

Aims. In this work we determine the effects of anharmonicity on the mid-infrared spectra of the linear polycyclic aromatic hydrocarbons (PAHs) naphthalene, anthracene, tetracene and pentacene recorded using the free electron laser FELIX. Methods. Comparison of experimental spectra obtained under supersonic jet conditions with theoretically predicted spectra was used to show if anharmonicity explicitly needs to be taken into account. Results. Anharmonic spectra obtained using second-order vibrational perturbation theory agree on average within 0.5% of the experimental frequencies. Importantly, they confirm the presence of combination bands with appreciable intensity in the 5-6 µm region. These combination bands contain a significant fraction of the IR absorption, which scales linearly with the size of the PAH. Detection and assignment of the combination bands are a preliminary indication of the accuracy of far-IR modes in our anharmonic theoretical spectra. Detailed analysis of the periphery-sensitive CH out-of-plane band of naphthalene reveals that there is still room for improvement of the VPT2 approach. In addition, the implications of our findings for the analysis of the aromatic infrared bands are discussed.

Infrared action spectroscopy of doubly charged PAHs and their contribution to the aromatic infrared bands

“…Taking into account the fact that the absolute intensities of the fundamental bands are generally predicted lower in anharmonic calculations than harmonic (see tables 2, 3), about 45 % and 25 % of the absolute integrated intensity arises entirely from combination bands of singly and doubly charged phen and anth, respectively. The significant differences observed in the relative integrated intensities in each of the spectral regions and the presence of combination bands suggest that anharmonic calculations are necessary to predict accurate vibrational spectra for the family of comparatively small PAH cations as discussed here and also in previous work (Maltseva et al 2015;Lemmens et al 2019). Thus, including these anharmonic effects has consequences on the interpretation of relative UIR band intensities observed in the ISM.…”

High-resolution infrared spectroscopy of naphthalene and acenaphthene dimers

“…The major advantage of this region is that it is much less affected by Fermi-resonances, since the density of vibrational states is lower and resonances are thus less likely to occur. Therefore, the harmonic calculations here are more reliable than in the 3 μm region [49]. Theoretical studies of Bauschlicher and coworkers have shown that a parallel configuration of the monomer units would lead to a redshift [39].…”

Modeling the infrared cascade spectra of small PAHs: the 11.2 μm band

“…In order to draw more firm conclusions, and extrapolate to larger PAHs a bigger data set would be required. Unfortunately, anharmonic calculations of PAHs are not straightforward and are hampered by some unresolved issues [27,28,31]. Computational power remains a roadblock for calculating anharmonic spectra of larger PAH species.…”

“…functional/basis set combination, with the convergence parameters recommended for anharmonic calculations [27]. Comparison of the theoretical anharmonic zero-Kelvin spectra with measured ion-dip spectra for a variety of PAHs shows excellent agreement (to within 0.3%) in peak frequency without the need for correction factors [28][29][30][31]. Anthracene (C 14 H 10 ) -a molecule with a strong solo out-of-plane bending mode -is used as a stand-in for the analysis of these parameters.…”

“…Nowadays, anharmonic effects can be accounted for computationally using the vibrational self-consistent field method [22], for example, or through vibrational second-order perturbation treatment (VPT2) [23,24]. VPT2 in particular has shown great success in reproducing the experimental spectra of isolated, cold, gas-phase IR spectra of PAHs [25][26][27][28][29][30][31]. Anharmonic treatment of a vibrational spectrum is computationally expensive, which limits the size of computable PAHs down to only ∼ 30 carbon atoms, which is smaller than the commonly accepted size of an interstellar PAH (50-100 carbon atoms) [32].…”