Experimental Determination of the Helium $2{}^{3}P_1–1{}^{1}S_0$ Transition Rate

We present the first experimental determination of the $2{}^{3}P_1–1{}^{1}S_0$ transition rate in helium and compare this measurement with theoretical quantum-electrodynamic predictions. The experiment exploits the very long ($\sim 1\mathrm{\text{minute}}$) confinement times obtained for atoms magneto-optically trapped in an apparatus used to create a Bose-Einstein condensate of metastable ($2{}^{3}S_1$) helium. The $2{}^{3}P_1–1{}^{1}S_0$ transition rate is measured directly from the decay rate of the cold atomic cloud following 1083 nm laser excitation from the $2{}^{3}S_1$ to the $2{}^{3}P_1$ state, and from accurate knowledge of the $2{}^{3}P_1$ population. The value obtained is $177\pm 8{\mathrm{s}}^{-1}$, which agrees very well with theoretical predictions, and has an accuracy that compares favorably with measurements for the same transition in heliumlike ions higher in the isoelectronic sequence.

Figure 1

Energy level diagram for helium showing the inverse decay rates ($\tau$) for the helium $n=2$ triplet manifold to the ground state.

Figure 2

(a) Historical progress of theoretical determinations for the helium $2{}^{3}P_1–1{}^{1}S_0$ decay rate (references shown), together with the experimental value (and uncertainty) from the present work. (b) Ratio of the experimental to the most recent theoretical decay rates, along with experimental uncertainties, for the $2{}^{3}P_1–1{}^{1}S_0$ transition in the heliumlike isoelectronic sequence. References (experimental, theoretical): He $Z=2$, present work, [13]; C $Z=6$ and N $Z=7$ [14, 15]; O $Z=8$ [15, 16]; F $Z=9$ [6, 16]; Mg $Z=12$ and Al $Z=13$ [6, 17]; Si $Z=14$ [15, 18]; S $Z=16$ [6, 18].

Figure 3

Ion signal (data points with fitted exponential curve) recorded when the $P_1$ light illuminates the cloud of trapped atoms with a 20% duty cycle ($P_1$ population fraction 3.6%). The inset shows the case when the $P_1$ light is absent. $t=0$ occurs $\sim 16\mathrm{s}$ after the MOT loading is switched off.

Figure 4

Measured decay time versus $P_1$ population (each data point is an average over 10 decay curves).