激光可以嗅出有毒气体

Scientists have developed a way to sniff¹ out tiny amounts of toxic² 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 discriminate³ one type of gas from another with greater specificity than most remote sensors⁴ -- even in complex mixtures of similar chemicals -- and under normal atmospheric⁵ 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 violations⁶, 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 perimeter⁷ 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 infrared⁸ radiation on the electromagnetic spectrum⁹.   Zapping a gas molecule¹⁰ with a terahertz beam of just the right energy makes the molecule switch between alternate rotational¹¹ 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.