Measurement of the $3s3p{{}^{3}P}_1$ lifetime in magnesium using a magneto-optical trap

We demonstrate an accurate method for measuring the lifetime of long-lived metastable magnetic states using a magneto-optical trap (MOT). Through optical pumping, the metastable $\left(3s3p\right){{}^{3}P}_1$ level is populated in a standard MOT. During the optical pumping process, a fraction of the population is captured in the magnetic quadrupole field of the MOT. When the metastable atoms decay to the $\left(3s^2\right){{}^{1}S}_0$ ground state they are recaptured into the MOT. In this system no alternative cascading transition is possible. The lifetime of the metastable level is measured directly as an exponential load time of the MOT. We have experimentally tested our method by measuring the lifetime of the $\left(3s3p\right){{}^{3}P}_1$ of ${}^{24}\text{M}\text{g}$. This lifetime has been measured numerous times previously, but with quite different results. Using our method we find the $\left(3s3p\right){{}^{3}P}_1$ lifetime to be $\left(4.4\pm 0.2\right)\text{ms}$. Theoretical values point toward a lower value for the lifetime.

Figure 1

Energy levels relevant for the lifetime measurement of the metastable $\left(3s3p\right){{}^{3}P}_1$ state of the 24 magnesium isotope. The involved levels are labeled $|a⟩,|b⟩$, and $|c⟩$ respectively.

Figure 2

(Color online) The number of atoms in the excited $\left(3s3p\right){{}^{1}P}_1$ $\left({\rho }_{aa}\right)$ and $\left(3s3p\right){{}^{3}P}_1$ $\left({\rho }_{cc}\right)$ states during the pulsed experiment. Time is in units of $\tau$. The signal monitored in our experiment is proportional to the fluorescence determined by $\left({\rho }_{aa}\right)$.

Figure 3

(Color online) Typical MOT fluorescence measurement, here normalized to the uv power level. The interval marked with dashed lines show the 457 nm pulse. As the 457 nm light is turned on additional background photons are observed. After a load period of about 10 ms the signal is slowly decays due to the 4–5 s MOT lifetime. Lower two graphs: the red curve is a fit to Eq. (3), giving 4.4 ms. The lower part shows the residuals of the fit.