New laser tech sniffs out toxic gases from afar

Scientists have developed a new way to sniff out tiny amounts of toxic gases - a whiff of nerve gas, for example, or a hint of a chemical spill - from up to one kilometre away.
The technology can discriminate one type of gas from another with greater specificity than most remote sensors - even in complex mixtures of similar chemicals - and under normal atmospheric pressure, something that wasn't thought possible before, researchers said.
The researchers say the technique could be used to test for radioactive byproducts from nuclear accidents or arms control treaty violations, for example, or for remote monitoring of smokestacks or factories for signs of air pollution or chemical weapons.

"You could imagine setting this up around the perimeter of an area where soldiers are living, as a kind of trip wire for nerve gas," said lead author Henry Everitt, an Army scientist and adjunct professor of physics at Duke University.
The technique uses a form of invisible light called terahertz radiation, or T-rays.
Already used to detect tumours and screen airport passengers, T-rays fall between microwaves and infrared radiation on the electromagnetic spectrum.

Zapping a gas molecule with a terahertz beam of just the right energy makes the molecule switch between alternate rotational states, producing a characteristic absorption spectrum "fingerprint," like the lines of a bar code.

The new approach by Ohio State University physicist Frank De Lucia and colleagues works by blasting a cloud of gas with two beams at once.
One is a steady terahertz beam, tuned to the specific rotational transition energy of the gas molecule they're looking for. The second beam comes from a laser, operating in the infrared, which emits light in high-speed pulses.
The researchers directed the two beams onto samples of methyl fluoride, methyl chloride and methyl bromide gases in the lab to determine what combination of laser settings would be required to detect trace amounts of these gases under different weather conditions.

"Terahertz waves will only propagate so far before water vapour in the air absorbs them, which means the approach works a lot better on, say, a cold winter day than a hot summer day," Everitt said.
The researchers say they are able to detect trace gases from up to one kilometre away. But even under ideal weather conditions, the technology is not ready to be deployed in the field just yet.
The study was published in the journal Physical Review.