Furthermore, for several species, mostly Polycyclic aromatic hydrocarbons (PAHs) and fullerenes, NANOCOSMOS has combined experiments and theory, including the development of new theoretical codes, to study their UV / thermal processing and anharmonic infrared spectra.
PAHs stand out in multiple research fields. In Astrophysics they are supposed to contribute to a series of UV-visible absorption bands, the so-called Diffuse Interstellar Bands, as well as the precursors of mid-infrared emission bands. In Astrobiology, they are proposed as playing an intermediary role in the origin of life leading to the RNA world. In Atmospheric and Environmental Sciences, they are studied as pollutants while in Medicine as powerful immune suppressants.
Consequently, NANOCOSMOS has made a huge effort in characterizing the spectral properties of neutral clusters of PAHs and the dependence upon charge both for ionized or protonated species. This characterization implies a correct treatment of long-range forces and charge localization with the implementation of theoretical frameworks like the Density Functional based Tight Binding (DFTB) for neutrals and the Configuration Interaction with the DFTB (DFTB-CI) for neutrals and ions. In summary, these methods allow us to calculate binding energies and ionization potentials in agreement with experimental measurements.
Below, we show a summary table on the experimental and theoretical characterizations and techniques for PAH clusters. We also include the Nanocosmos publication when clicking on the compound.
| Technique | Laboratory / theoretical characterization | Main results |
|---|---|---|
| Two-step laser desorption laser ionization mass spectrometry (AROMA) | Molecular analysis of soot nucleation and growth | Just-nucleated soot is rich in PAHs dominated by medium sizes (18 to 40 C atoms). Conversion of large PAHs into fullerenes through thermal processing. |
| DFTB and DFTB-CI theoretical computations | Most stable structures of pyrene clusters | Computation of IR spectra for neutral and cationic clusters of PAHs, namely pyrene clusters. New insights to detect PAH clusters or small grains in interstellar / circumstellar media. |
| Synchrotron VUV ionization (SOLEIL, DESIRS beamline) | Trapped PAH cations of sizes between 30 and 48 C atoms | We provide tools to tackle the evolution of PAHs in extreme astronomical environments such as H II regions and ionization fronts. |
| IR spectroscopy in transmission mode (14 K to 723 K) with ESPOIRS | Solid pyrene (98%) in KBr pellets | The IR spectrum of condensed pyrene is reported for the first time over a wide range of temperature (14 -723 K). This opens perspectives to generalize these measurements on large PAHs (50-100 C atoms) which are better analogs of the astro-PAHs. |
| Implementation of the AnharmoniCaOs code ((the Cagliari-Orsay model for anharmonic molecular spectra in 2nd order perturbation theory) for PAHs. | Two prototypical PAHs, pyrene and coronene | Theoretical band positions fairly improved relative to harmonic Density Functional Theory (DFT) calculations. First step towards calculating spectra of PAHs using Monte Carlo sampling. |
| Infrared pre-dissociation (IR-PD) spectroscopy and COLDTRAP | Isomeric structure of C7H7+ | Full mid-infrared vibrational spectrum of C7H7+ at 10 K. This technique opens new perspectives for our understanding of the isomerisation paths of C7H7+. |
| iPEPICO spectroscopy and RRKM modeling | Ionized PAHs acenaphthylene, fluorene, cyclopenta[d,e,f]phenanthrene, pyrene, perylene, fluoranthene, dibenzo[a,e]pyrene, dibenzo[a,l]pyrene, coronene and corannulene | Imaging photoelectron photoion coincidence spectroscopy (iPEPICO) was employed to explore the unimolecular dissociation of ionized PAHs. For all the ions, the loss of a hydrogen atom is the primary dissociation channel. |
| Infrared pre-dissociation (IR-PD) spectroscopy and COLDTRAP | 1-methylpyrene cation, C17H11+ | First IR spectrum of the C17H11+ species together with the IRMPD spectrum of the bare cation. Further exploration of anharmonic effects and isomerization while heating the trapped ions. |
| iPEPICO spectroscopy and Synchrotron VUV ionization (SOLEIL, DESIRS beamline) | PAH clusters made of pyrene C16H10 and coronene C24H12 | First experimental measurement of the near-threshold photoionization spectra. The results might suggest these species are ionized in astronomical environments. |