Photoluminescence Spectroscopy: New Technique for Detecting Explosives

Luminescence is the emission of absorbed light by a substance. When a material is exposed to electromagnetic radiation of sufficiently short wavelength, the emission of electrons is observed.

The phenomenon was originally described as the photoelectric effect, since there is a measurable photo current; but nowadays the terms photoionization or photoemission are commonly used.

It occurs when an electron returns to the electronic ground state from an excited state and loses its excess energy as a photon. The electronic states of most organic molecules can be divided into singlet states and triplet states;

Singlet state: All electrons in the molecule are spin-paired.
Triplet state: One set of electron spins is unpaired.

A molecule in the singlet state emits photons continuously when exposed to electromagnetic radiation and stops emitting when the source stops transmitting. But a molecule in the excited triplet state decays by using the process of intersystem crossing. This means that it decays in steps emitting photons of lower intensity each time it decays.

This process generally also continues after the source has stopped emitting. This phenomenon is called photoluminescence and the study of the emitted light is called photoluminescence spectroscopy. However, it may not always use intersystem crossing to return to the ground state. It could lose energy by directly emission of a photon.

A team of University of Florida researchers has invented a way to rapidly detect traces of TNT (TriNitroToluene) or other hidden explosives simply by shining a light on any potentially contaminated object, from a speck of dust in the air to the surface of a suitcase.

"We have to find explosives quickly, inexpensively and, particularly, reliably," said Rolf Hummel, a University of Florida Professor Emeritus of Materials Science and Engineering who heads the lab where the method was invented.

The development provides instantaneous results, gives no false positives, can be used remotely and is portable―attributes he says will make it indispensable at all levels of law enforcement, from local police to homeland security.

Using photoluminescence to reveal the presence of TNT is similar to how 'black light' uses ultraviolet radiation to make white clothes glow, but in this case the black light is a laser, Hummel said. "Once you shine a laser at the sample, the laser then re-emits (it) at specific wavelengths that are different for each material―it's a kind of a fingerprint."

The researchers discovered that TNT's fingerprint is a sharp, distinct photo luminescent peak at a specific wavelength within the electromagnetic spectrum. The electromagnetic spectrum encompasses the entire range of light and sound waves, from long-wavelength radio waves to short-wavelength gamma rays.

The peak occurs just outside the longer-wavelength, or red portion of the spectrum that includes visible light. TNT shares this characteristic peak with other explosive materials, such as plastic explosives and nitroglycerin, but not with safe materials.

The key to this common attribute, Hummel said, lies in the explosives' chemical makeup―they all contain at least two 'nitro groups', molecules made up of one nitrogen atom bound to two oxygen atoms. The peak is a narrow spectral line and would be easy to miss if you don't know where in the spectrum to search.

The University of Florida's discovery of TNT's signal was prompted by a request from the U.S. Army Research Office that challenged universities to find a way to make inexpensive, quick, and reliable explosive-detection systems.

Out of curiosity, one of Hummel's graduate students tested TNT in the lab's photoluminescence spectrometer. With its high resolution, the machine scanned across the entire light spectrum and caught the explosive's elusive signal. "That's why we detected it the first time," Hummel said.

"This is a very complex phenomenon," said Chuck Schau, a scientist at Raytheon Missile System's Radiation Technology Laboratory who also was part of the experiments on explosive detection using photoluminescence but initially did not observe the TNT peak discovered by the UF team.

Raytheon is now interested in following up on this discovery, he said. That development may include a future for this detection technology that goes beyond airport lines and into uncovering dangerous materials on a much larger scale―though that technology may still be years away.

"If I see a ship approaching, I'd like to know if it's packed with explosives," Schau said. "It's in the field of remote detection that this is exciting. This really looks like it may give us a leg up on that."

Collecting samples for explosives is familiar to anyone who has recently passed through an airport: a swab brushed across an object, such as a suitcase, clothing or even a person, or puffs of air blasted across a filter that can trap tiny amounts of airborne explosives.

The main advantage of photoluminescence-based explosive detection is that it can be remotely applied, and requires neither time-consuming and expensive machines nor trained dogs, said Hummel, who has applied for a patent on the technique.

"My major aim is that I would like to help to make a contribution towards secure life, airports and transportation," he said. "Just shine a laser on a car, ship or person and see if that specific wavelength comes back―that's my goal."