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Science  14 Aug 2009:
Vol. 325, Issue 5942, pp. 795
DOI: 10.1126/science.325_795a

The natural lifetime of an atomic or molecular excited state is typically measured in millionths, billionths, or trillionths of a second. Traditionally, the challenge in achieving precision has therefore been developing a laser source and detection system with sufficiently fine time resolution. Hodgman et al. tackle a challenge at the opposite extreme: precise measurement of the longest atomic excited state lifetime. Promotion of one of the two electrons in helium from the 1s to the 2s orbital, concomitant with a spin flip, produces the 23S1 state, which persists for more than 2 hours before relaxing by emission of a photon in the extreme ultraviolet regime. The lifetime is especially long in this case because both orbital angular momentum and spin considerations render direct relaxation highly improbable in quantum mechanical terms. In measuring the comparatively long duration of such a state, the challenges are twofold: collisional interference by other atoms in the system must be compensated for, and the rare photon-emission events must be detected with great efficiency and matched to a well-determined number of excited atoms. The authors addressed the first of these challenges by isolating the excited helium atoms in a magnetically confined trap. To accurately quantify emission events, they switched from a previously implemented absolute detection scheme to a relative detection mode, comparing the number of photons emerging from atoms in the 23S1 state to those emerging from atoms prepared in a shorter-lived (and thus more easily calibrated) state at somewhat higher energy. The extracted lifetime of 7870 s compares well with theoretical predictions.

Phys. Rev. Lett. 103, 53002 (2009).

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