Polycyclic aromatic hydrocarbon (PAH) molecules are an important component of the interstellar medium (ISM) of galaxies locking up some 10% of the elemental carbon and understanding their characteristics and their dependence on local physical conditions is crucial in astrophysics.
As our knowledge on interstellar PAHs results mainly from IR observations, much effort has been expended over the last three decades on determining IR spectra of PAHs and their dependence on the molecular characteristics. This has led to much insight in the characteristics of PAHs in space, including the importance of the PAH charge state and of the molecular edge structure. Conversely, this has been used to determine the physical conditions in the regions from which the PAH emission originates. However, due to the involatile nature of PAHs, low-temperature and high-resolution spectra are difficult to measure in laboratory, especially for astronomically relevant PAHs (with number of carbon atoms more than 50). Therefore, current study of the IR spectrum of PAHs have led to an ever-increasing reliance on computational quantum chemistry.
Previous computations were typically performed within the framework of Density Functional (DFT) Theory, in which the double harmonic approximation is used for vibrational IR intensities. A scaling factor has to be implemented to down-scale the calculated harmonic frequencies to match with experimental features. As a first-order approximation, this approach has been successful in reproducing the overall IR spectrum of certain individual PAHs roughly, but serious problems persist when comparing it with high-resolution experiments. Specifically, the IR features in the C-H stretching region are poorly modeled using the harmonic level analysis, due to the effects of mode couplings and resonances that are not described at the harmonic level.
Yet, the Near Infrared Spectrograph (NIRSpec) and The Mid-Infrared Instrument (MIRI) instruments on board of the James Webb Space Telescope (JWST) - to be launched in 2021 - will open up the mid-IR spectral range to systematic studies of PAHs at high spectral resolution and this has great potential as a probe of the characteristics of the emitting PAHs and the physical conditions in the emission zones. In order to make progress in this area and to fully exploit the data to be expected from JWST, I propose to perform theoretical study of the anharmonic IR spectra of astronomically relevant PAHs.