Each molecule has specific wavelengths at which it can either absorb or emit light. This forms the fingerprint of a substance. With this fingerprint, astronomers can demonstrate the presence of a substance in a distant star or cloud. In a wide range of lines of sight, in the almost empty interstellar space, bright infrared emission is observed, the spectrum of which has become commonly known as the "Unidentified Infrared Bands". The most widely accepted hypothesis is that complex organic compounds cause the bands. Put more precisely it is thought to be a mixture of various polyaromatic hydrocarbons, each containing about fifty carbon atoms. Nobody had yet succeeded in measuring the spectrum of these complex molecules under conditions comparable to the cold gas situation in deep space where these spectra are found. In deep space the molecules are so far apart that they no longer collide with each other. Collisions dramatically influence the spectrum. It is difficult to create a collision-free situation in the laboratory. Furthermore, the substance is so rarefied that a spectrum can scarcely be measured. Hans Piest found a way of measuring the spectrum indirectly. For this he made use of a special laser from the Institute for Plasma Physics (FOM) in Rijnhuizen. It is a free-electron laser which can produce every desired wavelength between 5 and 250 microns. There are only a few examples of this type of laser in the world. The physicist synthesised polyaromatic hydrocarbons and bound each of these molecules to a noble gas atom. This can only be done at a temperature just above absolute zero. The bonding energy of noble gas atoms is so small that it scarcely affects the spectrum. In order to investigate which wavelengths this complex can absorb he bombarded its with laser light, using a different wavelength for each bombardment. The light from this laser is sufficient to disassociate the weakly bound noble gas molecule from the organic compound. A sensitive mass spectrometer was able to determine whether the organic substance was produced as a function of the infrared wavelength. The physicist used various noble gas atoms and repeatedly obtained the same spectrum. This strongly indicates that the noble gas did not disrupt the spectrum. The spectra measured strongly agreed with previously disputed measurements from NASA. They had directly determined the very weak absorption spectrum of various sorts of polyaromatic hydrocarbons frozen in noble gas ice. These measurements were controversial because the influence of the noble gas ice was difficult to estimate. Now the question still remains as to exactly which polyaromatics are found in space.
Further information can be obtained from Hans Piest (Department of Molecular and Laser Physics, University of Nijmegen), tel +31 (0)24 3652179, fax +31 (0)24 3653311, e-mail: J.A.Piest@fz-Rossendorf.de The defence of the doctoral thesis will take place on 13 May 2002. Mr Piest's supervisor is Prof. G. Meijer. From May onwards Hans Piest will be working at the Rossendorf Research Centre in Germany.