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, they are thought to be a mixture of various polyaromatic hydrocarbons, each containing about fifty carbon atoms.

Now research by physicist Hans Piest at University of Nijmegen, The Netherlands, has provided new experimental evidence that may help solve an almost 30-year-old problem in astronomy.

Nobody had yet succeeded in measuring the spectrum of the 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 in space is so rarefied that a spectrum can scarcely be measured.

Piest found a way of measuring the spectrum indirectly, making 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 Dutch physicist synthesized 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 it with laser light, using a different wavelength for each bombardment.

This laser light 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.

Piest 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.

NASA scientists 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 remains as to exactly which polyaromatics are found in space -- but the answers should be forthcoming.


[Contact: Hans Piest]

19-Apr-2002