Argon is inert, which means that it doesn't easily react with atoms of other elements.
But "during the very short period when a rubidium atom bangs into an argon atom, [the rubidium] can absorb a photon" from the laser, NIST's Porto explained.
The absorbed photon acts like a strong spring suddenly bridging the two atoms, and this weak link causes the atoms to slow down as they try to fly apart.
But at some point the spring is stretched so far that the link breaks and the photon is released as scattered fluorescent light.
The extra energy required to slow the atoms gets carried away by the escaping photon, so the process ends up removing more energy than the laser puts in, cooling the gas.
In the experiment, described last week in the journal Nature, the rubidium gas fell from 662 degrees Fahrenheit (350 degrees Celsius) to almost 536 degrees Fahrenheit (280 degrees Celsius) within mere seconds.
Much more research needs to be done before the laser-cooling process can be used in real-world applications, study co-author Weitz cautioned.
But NIST's Porto said the work already represents a major departure from traditional cooling of diluted gases, which are currently used for studying quantum effects or preparing gas samples for atomic clocks.
"I think the really amazing thing is that you can even get cooling in this regime, because it's a really dense gas and a very different mechanism," Porto said.
"Traditional cooling powers are so tiny. To cool a physical object by a measurable degree with a laser is amazing."
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