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

Abstract

The profile of the 11.2 μm feature of the infrared (IR) cascade emission spectra of polycyclic aromatic hydrocarbon (PAH) molecules is investigated using a vibrational anharmonic method. Several factors are found to affect the profile including: the energy of the initially absorbed ultraviolet (UV) photon, the density of vibrational states, the anharmonic nature of the vibrational modes, the relative intensities of the vibrational modes, the rotational temperature of the molecule, and blending with nearby features. Each of these factors is explored independently and influence either the red or blue wing of the 11.2 μm feature. The majority impact solely the red wing, with the only factor altering the blue wing being the rotational temperature.

Anharmonicity and the IR Emission Spectrum of Neutral Interstellar PAH Molecules

“…Highly symmetric, compact PAHs, such as pyrene, develop rather simple emission spectra in the CH stretching region, dominated by a single peak. This emission band also develops a low-frequency red-shaded wing: the telltale signature of the effects of anharmonicity in highly excited species . As discussed in section , at low temperatures, asymmetric, noncompact PAHs show significant absorption over a wide frequency range .…”

“…This shift becomes more pronounced when the excitation increases as more spectator states become populated and the population of these spectator states climbs higher on their vibrational ladder. This effect is already apparent in the frequency shift of the CH stretching mode with internal energy (Figure ), and it is more pronounced for the relatively isolated CH out-of-plane bending modes . During the radiative decay, the excited species cascade down in energy and the resulting profile centers on the low-temperature absorption peak but with a pronounced red-shaded wing.…”

“…The detailed profile of the vibrational bands depends on the internal excitation of the emitting species. The energy cascade during the emission process will partially obscure this dependence but subtle variations remain noticeable, and this has been discussed in detail for the out-of-plane bending modes . Here, we discuss this aspect for the aromatic CH stretching mode.…”

“…The details of the simulated PAH cascade emission model have been described in detail previously. ,, A second-order vibrational perturbation treatment (VPT2) is applied to the harmonic vibrational modes of the PAHs in order to obtain the anharmonic constants, x (see ref for their derivation), which in turn are used to calculate the perturbed vibrational energies of mode ν given by where ω represents the harmonic frequencies, and n represents the number of quanta in the vibrational modes. Equation is valid for an asymmetric top.…”

“…These theoretical absorption spectra are then converted into emission spectra using simple emission models. ,− The validity of this procedure is not well established because during radiative vibrational relaxation the anharmonicity and resonances (i.e., nearly degenerate intermode perturbations) can have profound effects on the peak position and profiles of the molecular vibrational modes. Recent experimental − and quantum chemical − studies have focused on quantifying these effects on the absorption spectra, and sophisticated simulations have been developed to incorporate them into emission models. − …”