The infrared bands of polycyclic aromatic hydrocarbons in the 1.6–1.7 μm wavelength region

¹ School of Engineering Sciences in Chemistry, Biotechnology and Health, Department of Theoretical Chemistry and Biology, Royal Institute of Technology, 10691 Stockholm, Sweden
e-mail: taochen@kth.se
² Hefei National Laboratory for Physical Science at the Microscale, Department of Chemical Physics, School of Chemistry and Materials Science, University of Science and Technology of China, Hefei 230026 Anhui, PR China
e-mail: luo@kth.se
³ Department of Physics and Astronomy, University of Missouri, Columbia, MO 65211, USA
e-mail: lia@missouri.edu

Received: 14 July 2019
Accepted: 13 October 2019

Abstract

Context. The 3.3 μm aromatic C–H stretching band of polycyclic aromatic hydrocarbon (PAH) molecules seen in a wide variety of astrophysical regions is often accompanied by a series of weak satellite bands at ∼3.4–3.6 μm. One of these sources, IRAS 21282+5050, a planetary nebula, also exhibits a weak band at ∼1.68 μm. While the satellite features at ∼3.4–3.6 μm are often attributed to the anharmonicities of PAHs, it is not clear whether overtones or combination bands dominate the 1.68 μm feature.

Aims. In this work, we examine the anharmonic spectra of eight PAH molecules, including anthracene, tetracene, pentacene, phenanthrene, chrysene, benz[a]anthracene, pyrene, and perylene, to explore the origin of the infrared bands in the 1.6–1.7 μm wavelength region.

Methods. Density functional theory (DFT) in combination with the vibrational second-order perturbation theory (VPT2) was used to compute the anharmonic spectra of PAHs. To simulate the vibrational excitation process of PAHs, the Wang–Landau random walk technique was employed.

Results. All the dominant bands in the 1.6–1.7 μm wavelength range and in the 3.1–3.5 μm C–H stretching region are calculated and tabulated. It is demonstrated that combination bands dominate the 1.6–1.7 μm region, while overtones are rare and weak in this region. We also calculate the intensity ratios of the 3.1–3.5 μm C–H stretching features to the bands in the 1.6–1.7 μm region, I_{3.1 − 3.5}/I_{1.6 − 1.7}, for both ground and vibrationally excited states. On average, we obtain ⟨I_{3.1 − 3.5}/I_{1.6 − 1.7}⟩≈12.6 and ⟨I_{3.1 − 3.5}/I_{1.6 − 1.7}⟩≈17.6 for PAHs at ground states and at vibrationally excited states, respectively.