The infrared bands of polycyclic aromatic hydrocarbons in the 1.6-1.7 {\mu}m wavelength region

TL;DR

This study uses advanced computational methods to analyze the infrared spectral features of PAHs, revealing that combination bands, not overtones, mainly cause the 1.6-1.7 μm bands observed in space.

Contribution

The paper provides the first detailed anharmonic spectral analysis of PAHs in the 1.6-1.7 μm range, clarifying the origin of these bands and their relation to the well-known 3.3 μm features.

Findings

Combination bands dominate the 1.6-1.7 μm region.

Overtones are rare and weak in this spectral region.

Intensity ratios between 3.1-3.5 μm and 1.6-1.7 μm bands are quantified.

Abstract

Context. The 3.3 $\mu$m 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 $\mu$m. One of these sources, IRAS 21282+5050, a planetary nebula, also exhibits a weak band at ~1.68 $\mu$m. While the satellite features at ~3.4-3.6 $\mu$m are often attributed to the anharmonicities of PAHs, it is not clear whether overtones or combination bands dominate the 1.68 $\mu$m 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 $\mu$m waveelngth region. Methods. Density Functional Theory (DFT) in combination with the vibrational second-order perturbation theory (VPT2) is utilized for computing the anharmonic spectra of PAHs. To simulate the vibrational excitation process of PAHs, the Wang-Landau random walk technique is employed. Results. All the dominant bands in the 1.6-1.7 $\mu$m wavelength range and in the 3.1-3.5 $\mu$m C-H stretching region are calculated and tabulated. It is demonstrated that combination bands dominate the 1.6-1.7 $\mu$m region, while overtones are rare and weak in this region. We also calculate the intensity ratios of the 3.1-3.5 $\mu$m C-H stretching features to the bands in the 1.6-1.7 $\mu$m region, $I_{3.1-3.5}/I_{1.6-1.7}$, for both ground and vibrationally excited states. On average, we obtain $\langle I_{3.1-3.5}/I_{1.6-1.7} \rangle$ $\approx$ 12.6 and $\langle I_{3.1-3.5}/I_{1.6-1.7} \rangle$ $\approx$ 17.6 for PAHs at ground states and at vibrationally excited states, respectively.