Metastable Helium: A New Determination of the Longest Atomic Excited-State Lifetime

Exited atoms may relax to the ground state by radiative decay, a process which is usually very fast (of order nanoseconds). However, quantum-mechanical selection rules can prevent such rapid decay, in which case these “metastable” states can have lifetimes of order seconds or longer. In this Letter, we determine experimentally the lifetime of the longest-lived neutral atomic state—the first excited state of helium (the $2{}^{3}S_1$ metastable state)—to the highest accuracy yet measured. We use laser cooling and magnetic trapping to isolate a cloud of metastable helium (${\mathrm{He}}^{*}$) atoms from their surrounding environment, and measure the decay rate to the ground $1{}^{1}S_0$ state via extreme ultraviolet (XUV) photon emission. This is the first measurement using a virtually unperturbed ensemble of isolated helium atoms, and yields a value of 7870(510) seconds, in excellent agreement with the predictions of quantum electrodynamic theory.

Figure 1

Energy level diagram for helium. Excited-state decays to the ground state are indicated, along with laser transitions from the metastable $2{}^{3}S_1$ state.

Figure 2

Metastable state lifetimes. Theoretical and experimental determinations of the helium $2{}^{3}S_1$ lifetime with corresponding references (present result circled, only experimental error bars presented). References 17, 18 deviate markedly from other theoretical predictions as they do not account adequately for electron correlations [1].

Figure 3

Experimental schematic. The geometry of the 1083 nm MOT laser beams and the $2{}^{3}P_1$ excitation ($P1$) laser beams is shown relative to the detection system, comprising the channeltron with aluminium filters and shield (quadrupole magnetic field trapping coils not shown).

Figure 4

XUV photon flux averaged over 56 000 experimental cycles. Main figure: count rate for 62.6 nm photons emitted by magnetically trapped $2{}^{3}S_1$ atoms. Error bars are associated with statistical ($1\sigma$) fluctuations in the photon count rate. The fitted exponential is used to derive the integrated flux at point $T_1$. Inset: count rate for 58.4 nm photons emitted by untrapped $2{}^{3}P_1$ atoms. The 1083 nm illumination saturates the excited-state population at point $T_2$. Note the change in the time scale and count rates for the inset.

Figure 5

Helium isoelectronic sequence. The ratio of the experimental to the most recent theoretical $2{}^{3}S_1$ lifetime, along with experimental uncertainties, for the heliumlike isoelectronic sequence. Figure adapted from Ref. 1, with He $(Z=2)=\mathrm{\text{present}}$ experimental value divided by the theoretical value from Ref. 9.