The mysterious spectral bands in the infrared of interstellar gas
clouds in deep space originate from organic compounds. Research by
the Nijmegen physicist Hans Piest confirms this. He has provided new
experimental evidence for this almost 30-year-old problem in astronomy.

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,

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.