激光可以嗅出有毒气体

Scientists have developed a 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 kilometer away. The new 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.

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 tumors 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.

Terahertz sensors have been used for decades to identify trace gases in the dry, low-pressure conditions of interstellar space or in controlled conditions in the lab, where they are capable of unambiguous identification and ultra-sensitive, part-per-trillion detection.