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Visions of invisible aircraft.

Visions of an invisible aircraft

Research on the chemistry of vision has led to the identification of a set of compounds that absorb electromagnetic waves at radio frequencies. These compounds, called Schiff base salts, when applied to the surface of a military aircraft, may significantly lower the aircraft's tendency to reflect radar signals. A coating of this type would make any object practically invisible to radar.

Chemist Robert R. Birge of Carnegie-Mellon University in Pittsburgh and his colleagues made the discovery accidentally while studying a group of compounds that mimic some of the properties of rhodopsin. Rhodopsin is a chemical substance found in light-sensitive rods in the retina at the back of the eye. Light entering the eye appears to trigger a slight change in the structure of rhodopsin molecules. That alteration sets off a chain of chemical events leading to the transmission of a signal to the brain.

To understand better how rhodopsin molecules can be so sensitive to individual photons of light, researchers have studied simpler molecules that appear to have similar properties. One such analog is the retinyl Schiff base salt known as ATRSBS. One piece of the salt consists of a long chain of carbon atoms, with alternating double and single bonds and a nitrogen atom interrupting the string near one end. The chain carries a positive charge, associated largely with the nitrogen atom. A negatively charged perchlorate "counterion,' made up of four oxygen atoms and a chlorine atom, sits nearby, weakly connected to the chain. The substance itself is a fine black powder physically resembling graphite.


Birge and his group found that the perchlorate counterion prefers to sit in one of two locations near the chain (see diagrams). A single photon easily dislodges the counterion from one location and forces it into the other. A short time later, the molecule relaxes, and the counterion returns to its original position. These results were reported earlier this year in the JOURNAL OF THE AMERICAN CHEMICAL SOCIETY (Vol. 109, No.7).

What Birge noticed was that the energy required to shift the counterion in a Schiff base salt of the right structure is very small. Even radio waves of the right frequency would do the job. Thus, a coating containing a Schiff base salt could absorb radio waves, then dissipate the energy as heat. A mixture of carefully engineered Schiff base salts, each sensitive to a particular radio frequency, would provide a coating that responds to a wide range of radar signals.

The possibility of combining sensitivity to a wide range of radar signals with relatively low weight suggests an attractive alternative to methods now used to reduce a military aircraft's radar signature. Last week, the Department of Defense classified research on the application of Schiff base salts to radar-absorbing coatings. Future research will be conducted in secret. One of the problems that must be solved is the difficulty in dissolving these salts in materials such as polyurethane plastic, which are usually applied to aircraft surfaces.

Ironically, the Carnegie-Mellon research showed that ATRSBS is not a correct or realistic model for what goes on in rhodopsin, says Lionel P. Murray, now at the National Institute of Diabetes and Digestive and Kidney Diseases in Bethesda, Md. Nevertheless, he says, related research has clarified how the chemical signal responsible for vision gets started, although a few issues concerning the details of the process remain to be settled.
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Title Annotation:Schiff base salts may make objects invisible to radar
Publication:Science News
Date:Aug 29, 1987
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