Aromatic hydrocarbons, diamonds, and fullerenes in interstellar space: puzzles to be solved by laboratory and theoretical astrochemistry

New research is presented, and previous research is reviewed, on the emissi on and absorption of interstellar aromatic hydrocarbons. Emission from arom atic hydrocarbons dominates the mid-infrared emission of many galaxies, inc luding our own Milky Way galaxy. Only recently have aromatic hydrocarbons b een observed in absorption in the interstellar medium, along lines of sight with high column densities of interstellar gas and dust. Much work on inte rstellar aromatics has been carried out, with astronomical observations and laboratory and theoretical astrochemistry. In many cases, the predictions of laboratory and theoretical work are confirmed by astronomical observatio ns but, in other cases, clear discrepancies exist that provide problems to be solved by a combination of astronomical observations, laboratory studies , and theoretical studies. The emphasis of this paper will be on current ou tstanding puzzles concerning aromatic hydrocarbons that require further lab oratory and theoretical astrochemistry to resolve. This paper will also tou ch on related topics where laboratory and theoretical astrochemistry studie s are needed to explain astrophysical observations, such as a possible abso rption feature due to interstellar 'diamonds' and the search for fullerenes in space. (C) 2001 Elsevier Science B.V. All rights reserved.