This project is comprised of theoretical studies of the excitation, alteration, and destruction of the class of large organic molecules known as polycyclic aromatic hydrocarbons (PAHs). These molecules are detected by their characteristic spectra in observations of the interstellar medium (ISM) in our own Milky Way and other galaxies. Logically, they would have been present in the original material that formed our Solar System. But the fragility of PAH molecules implies that they would not have been expected to survive in Solar System bodies, except possibly in comets. Detecting PAHs in comets would establish an important chemical link between the ISM and the Solar System. PAHs have not been unambiguously detected in the observed infrared spectra of comets, in part because the expected infrared emission spectra and destruction rates of PAHs in cometary environments are not adequately understood. This is the problem that the proposal aims to address. The Principal Investigator and a supported graduate student will study the photoexcitation, photoionization, and photodissociation of PAH molecules and their ions in cometary comae illuminated by the Sun. They will calculate, using numerical statistical mechanics techniques, the emission spectra and destruction rates of 23 representative specific PAH molecules and their ions, as well as three nitrogen-substituted PAHs appropriate to nitrogen-rich comets and generic "astro-PAH" models approximating the interstellar species. The goal is to create a web-based "library" of the emission spectra and photodissociation and photoionization rates of PAH molecules as a function of distance from the Sun. By comparing observed cometary spectra the model spectra from the library, researchers will be able to constrain PAH production and destruction rates and interpret observational detections and non-detections.
