Aromatic Infrared Bands


A substantial fraction (~80%) of the about 130 molecules identified in interstellar and circumstellar regions and solar system bodies, as well as a significative component of interstellar dust grains are carbon-based. The realization that some unidentified, ubiquitous molecular spectral features are probably related to carbon in space gave rise, in the past few years, to intensive investigation on this subject. Among these, we does include the Aromatic Infrared Bands (AIBs), emission bands seen in the infrared near e.g. 3.3, 6.2, 7.7, 8.6 and 11.3 microns, observed along a large number of interstellar sight-lines covering a wide range of excitation conditions in numerous galactic objects as well as in external galaxies.

AIBs are generally ascribed to large free-flying molecules in the diffuse ISM, and have been hypothesized to originate in transitions involving the vibrations of CH and CC bonds in Policyclic Aromatic Hydrocarbons (PAHs). Molecules of this family, or other large carbon-bearing species, were subsequently proposed as carriers of also at least some of the DIBs. They were also supposed to contribute to the UV-bump of the interstellar extinction curve. n the so-called ``strong'' PAH hypothesis, these molecules are supposed to absorb Vis/UV radiation (hence producing DIBs), undergo internal energy conversion among the vibrational levels and subsequently relax by IR emission in their IR active modes of vibration (hence producing AIBs). Despite intensive research, no specific interstellar PAH has been identified yet.

  1. A detailed model of the interstellar photophysics of these elusive molecules may help to interpret observations and hopefully permit their eventual identification. We have undertaken (see references at the end of this chapter) a long-term project, which spans from theoretical to laboratory to observational work aimed at comparing optical synthetic spectra with available high-resolution DIBs observations obtained in the past decade, as well as with new ad hoc ones;
  2. comparing the IR synthetic spectra with the wealth of spectroscopic data collected in the past years by ISO, in particular in the far-IR spectral range. This comparison, which is part of the collaboration with Dr. Christine Joblin of the Centre d’Etudes Spatiales des Rayonnements (Toulouse), wishes to provide indications for the strategic planning for the future ESA Herschel mission. It will also provide a precious guide for targeted laboratory experiments on selected species.

This line of research is based on the use of a computational Monte-Carlo model which simulates the photophysics of an isolated PAH molecule in the ISM. This computer code uses available experimental or quantum-chemical results to derive the rotational profiles of absorption bands (directly comparable to astronomical observations of DIBs) and the detailed IR emission spectra following the UV/Vis excitation. This yields many independent, simultaneous constraints on DIB spectral profiles, positions and equivalent widths, as well as on the absolute IR emission fluxes. While none of these pieces of information is sufficient per se for a specific molecular identification, when considered together they provide an unprecedented, unambiguous spectral fingerprint.






Selected synthetic rotational profiles of DIBs