Lifetime measurement of the Mg i intercombination line

The lifetime of the 3s² ¹{S}₀$--3s3p $³P₁ intersystem transition of magnesium has been measured by different authors; however, a noticeable discrepancy appears mostly between two groups of measurements. We have measured the ³{P}₁$ decay time constant in a metastable atomic beam with a combined laser-microwave excitation technique, based on the 1.5-m downstream detection of the fine-structure transition $³P_1-³{P}₀$ (at 601 GHz) with and without laser optical pumping. Due to the decay of the $³P₁ state, a population difference between ³{P}₁$ and $³P₀ levels arises before the interaction of the microwave radiation with the atomic beam allowing the observation of the ³{P}_{1{{-}}}³P₀ magnetic dipole transition; its intensity is then related to the ³{P}₁$ decay time constant. Afterwards, the $³P₁ state is entirely depopulated through optical pumping just outside the beam exit. The microwave-induced transition intensity in this case is independent of the ³{P}₁$ decay constant, with all of the other parameters of the experiment being unchanged. The ratio between the microwave-induced transition probabilities in the two cases is sensibly dependent on the $³P₁ lifetime, allowing a determination of its value unaffected by the knowledge of absolute parameters such as atomic flux and detection efficiency, whose evaluation at a sufficient degree of precision may be unreliable. The result of our measurement is {τ}(³{P}₁$)=5.1±{}0.7 ms, which is in good agreement with a group of experimental data previously reported in the literature and with theoretical predictions.