- Author: David Bradley
Triatomic
Researchers in Austria have observed rotational transitions of the nonlinear triatomic molecular anion NH2- - the amide ion - using terahertz spectroscopy in a cryogenic radio frequency ion trap. The finding could point the way to detecting nitrogen-containing molecules in space.
Astrophysicists first observed a spectral line in observational data from the Herschel Space Telescope in 2014 and suggested that it might be due to the amide ion. The observation would represented the first proof of the existence of this moiety in space. However, Roland Wester from the Institute of Ion Physics and Applied Physics at the University of Innsbruck, Austria, and his group have now shown that this assumption is incorrect.
Inbetweeners
They measured two previously unknown frequencies in the laboratory for the first time using terahertz spectroscopy - the region between microwave and infrared wavelengths. Using this region allows the rotation of very small molecules to be studied, explains Wester, adding that for larger molecules, vibrations of whole molecular groups can be determined. It is important to know the precise chemical composition of the interstellar medium (ISM) because it is here that gigantic clouds of dust and gas are drawn into clumps by gravity that ultimately lead to the birth of stars. Radio telescopes usually reveal the spectral lines of material in the ISM.
The new work by Wester's group was funded by the European Research Council ERC and involved confining chemical moieties in ion traps and exciting them terahertz radiation. "The amide ion consists of a nitrogen atom and two hydrogen atoms, looks just like water and behaves very similar in terms of quantum mechanics," explains team member Olga Lakhmanskaya. "For the first time, we directly measured the elementary excitation of the rotation of this molecule," she adds. The team also collaborated closely with theoretician Viatcheslav Kokoouline of the University of Central Florida, USA, who was a visiting professor at the University of Innsbruck at the time.
Amide no-go
The Innsbruck team has now been able to show that the spectral line measured in 2014 cannot be produced by amide ions by comparison with the data obtained from the Herschel Space Telescope. "We were able to show, with our measurements, that this tentative assignment is not correct," stresses Wester. In the Universe one can find various nitrogen molecules such as ammonia but, according to the Innsbruck experiments, it remains to be shown that the amide ion is also present. The second spectral line determined by the physicists however could assist in searching for this species in space. "We hope that in the future, with new telescopes, this line can be observed leading to its detection in space." Wester’s team now hopes to apply the new method to molecules with four or five atoms, where vibrations and rotations are much more complex than with a triatomic
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