A very long-lived atomic state

Has been measured in an astrophysical nebula. Most excited atomic states last much less than a second. The lifetimes of longer-lived states are difficult to measure because collisions between atoms cause deexcitations before the atoms can decay radiatively. With a vacuum far better than any on Earth, outer space is the laboratory of choice for such atomic measurements. That’s why Tomas Brage of Lund University (Lund, Sweden), Philip Judge of the High Altitude Observatory (Boulder, Colorado), and Charles Proffitt of Computer Sciences Corp (Baltimore, Maryland) turned their gaze on the planetary nebula NGC3918 (shown here). Amid that wreckage of a dying star, there is enough energy to excite atoms but a low enough density (a few thousand atoms per cubic centimeter) that collisions are not a problem. Using the Hubble Space Telescope, the scientists looked at the emissions from triply ionized nitrogen atoms and found a lifetime of 2500 seconds for one particular hyperfine-induced transition in which a nuclear spin flip induces an electronic transition. The measurement provides important confirmation of earlier calculations, and thus lends support to theoretical studies of both atoms and large-scale, low-density astrophysical sources. (T. Brage, P. G. Judge, C. R. Proffitt, Phys. Rev. Lett. 89, 281101, 2002 http://dx.doi.org/10.1103/PhysRevLett.89.281101 .).