Blackbody radiation

Scientists shed new light on blackbody radiation

by Susan Griffith

Two friends comparing research notes suddenly realized in 1991 that ambient blackbody radiation -- a light source prevalent everywhere, including interstellar space -- can cause unimolecular dissociation, or fragmentation of individual molecules in gaseous form. This revitalized a theory rejected by the science community more than eight decades ago.

Robert Dunbar, professor of chemistry at CWRU, and Terrance B. McMahon, professor of chemistry at the University of Waterloo in Ontario, described their discovery in the article "Activation of Unimolecular Reactions by Ambient Blackbody Radiation," published in the January 9 issue of Science.

Friends for more than 25 years, Dunbar and McMahon shared research findings on how molecules react and begin to fall apart when they are energized by colliding with other molecules or by the introduction of an artificial light source, such as lasers.

In the past 28 years at CWRU, Dunbar has worked with ions in gaseous form, studying their reactions and behaviors in varying light levels, using mass spectroscopy methods to track their reactions. McMahon was doing similar work in Ontario.

The two researchers found that some molecules still fragment, even when a molecule is suspended in a low-pressure environment where it will not collide with other molecules and no artificial light source is present. Since the low pressures, lack of light sources, and low concentration of molecules could not account for the molecules' fragmentation, the researchers began to wonder what caused this.

"We had an inspiration that it had to be infrared blackbody radiation," recalls Dunbar, who was giving a talk at Waterloo when the two notched their revelation.

The theory that radiation causes the dissociation of a molecule as it gains energy from light and starts to break apart was proposed as early as 1919 by Jean Baptiste Perrin, a French Nobel laureate.

Irving Langmuir, an American Nobel laureate in chemistry, discredited the idea. He said that the radiation which a molecule must absorb in order to produce the internal energy to cause it to fragment fell within the ultraviolet light range, and there was not enough of such light to produce observed reactions.

Later other scientists, such as Lindemann and Christiansen, independently proposed that the dissociation resulted from collisions between two molecules. This explanation was rapidly accepted for the experiments of the time.

The radiation theory was ultimately rejected because scientists did not have the instruments to create collision-free, low-pressure conditions under which radiation-induced unimolecular dissociation could be observed.

"The radiation hypothesis came to be considered little more than a historical curiosity," the researchers wrote in their Science paper.

Dunbar and McMahon revitalized Perrin's radiation theory, using improved Fourier-transform ion cyclotron instrumentation. This instrumentation allowed for the introduction of gas-phased ions into a special trap called the Fourier Transform Ion Cyclotron Resonance (FT-ICR) Ion Trap, approximately a one-inch cube, where the unimolecular reaction was observed over an unlimited amount of time.

The instrumentation was originally designed for studying bimolecular reactions, Dunbar said. A weak electric and strong magnetic field is created within the trap and allows for the observation of these gas-phase ions over a long period of time.

"This same condition was exactly that required for the first observations of blackbody radiation driven unimolecular decompositions," said the researchers.

This discovery will not immediately produce a new device or cure for a disease, Dunbar said. But learning more about unimolecular dissociation of trapped ions will offer the kinds of fundamental information about the strengths of interactions that can feed into all sorts of research other people are doing.

He said this discovery has importance for the study of the upper atmosphere. Because pressures in the upper atmosphere are still too great and cause decomposition, this research has particular importance for how molecules act in the extreme low pressure environments in interstellar space, in which perhaps only one molecule may exist in a cubic meter of space.

These studies of the upper atmosphere and interstellar space are key because the lower pressures, lack of artificial light sources, and lower concentration of molecules will help researchers better identify the effects of blackbody radiation.

Other scientists are using this research in studying proteins, enzymes, and DNA, Dunbar added.