Probing the diversity of Astrophysically relevant Carbon and HYdrogen NanOparticles – PACHYNO

In order to explain specific infrared (IR) emission bands observed in the interstellar medium (ISM), and often referred to as the aromatic infrared bands (AIBs), the presence of polycyclic aromatic hydrocarbons (PAHs) was suggested about 30 years ago. The AIB carriers are also suspected to strongly contribute to the broad absorption feature known as the ultraviolet (UV) bump present in the interstellar extinction curve. Despite a large number of experimental and theoretical studies, the exact nature of the AIB carriers has remained elusive. Most previous experimental and theoretical studies focused on the electronic and vibrational spectroscopy of a limited set of relatively small and planar PAHs. The PACHYNO project is dedicated to the study of isolated carbon/hydrogen-based nanoparticles in the size domain of 20 to 200 atoms as potential AIB carriers. The project relies on the best use of state-of-the-art computational and experimental approaches and their synergy, without any a priori selection of molecular species. Experimentally, astrophysical analogues will be produced using a low-pressure flame resulting in a distribution of carbon/hydrogen nanoparticles from which spectroscopic signatures will be measured. In the theoretical part, a systematic and automatic exploration of molecular structures and their spectroscopic responses will be investigated.

The large variety of nanostructures arising from the rich allotropy of carbon and the presence of hydrogen will be computationally explored by means of atomistic simulations using an approach combining the AIREBO reactive force field and the density-functional theory-based tight-binding (DFTB) approach. The diversity of hydrocarbons will be explored by varying the size of the nanostructures, the relative amounts of carbon and hydrogen, and external parameters such as temperature. Systematic characterization will be achieved by mapping these structures with order parameters that account for the different aspects of structural organization and chemical bonding. New order parameters derived from cluster physics will be developed to describe the carbonaceous nanoparticles.

Using DFTB and time-dependent DFTB, spectroscopic properties of the simulated nanoparticles will be quantified from the IR to the UV wavelengths in connection with experimental and astronomical data. The simulations will be performed over a large statistical sampling. The joint characterization of the hydrocarbon nanostructures in terms of structural, chemical, and spectroscopic signatures will be rationalized by mapping out the relevant correlations between these different properties. Our ambition is to provide some general rules allowing spectral features to be related to structural and chemical properties such as size, C/H ratio or via the order parameters.

Experimental measurements of IR emission and electronic UV-visible electronic spectra will be performed using an existing experimental setup developed at ISMO. Large hydrocarbon compounds of up to few nanometers will be generated using a low-pressure flame that can produce well-controlled and reproducible distributions of species. Comparison between experimental and theoretical results will give information about the structural organization of the nanoparticles under various experimental conditions. Finally, comparison with astrophysical spectra will advance our understanding of interstellar carbonaceous matter and provide a solid background to improve dust models in astrophysics. Such progress will contribute to the scientific impact of future space missions such as the James Webb Space Telescope (JWST) to be launched in 2018.