Lifetime measurement of the Mg i intercombination line

The lifetime of the 3${\mathit{s}}^2$ ${}_{}{}^1{}_{}{}^{}$ ${\mathit{S}}_0$–3s3p ${}_{}{}^3{}_{}{}^{}$ ${\mathit{P}}_1$ 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 ${}_{}{}^3{}_{}{}^{}$ ${\mathit{P}}_1$ 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 ${}_{}{}^3{}_{}{}^{}$ ${\mathit{P}}_{1\mathrm{-}}^3$ ${\mathit{P}}_0$ (at 601 GHz) with and without laser optical pumping. Due to the decay of the ${}_{}{}^3{}_{}{}^{}$ ${\mathit{P}}_1$ state, a population difference between ${}_{}{}^3{}_{}{}^{}$ ${\mathit{P}}_1$ and ${}_{}{}^3{}_{}{}^{}$ ${\mathit{P}}_0$ levels arises before the interaction of the microwave radiation with the atomic beam allowing the observation of the ${}_{}{}^3{}_{}{}^{}$ ${\mathit{P}}_{1\mathrm{-}}^3$ ${\mathit{P}}_0$ magnetic dipole transition; its intensity is then related to the ${}_{}{}^3{}_{}{}^{}$ ${\mathit{P}}_1$ decay time constant. Afterwards, the ${}_{}{}^3{}_{}{}^{}$ ${\mathit{P}}_1$ state is entirely depopulated through optical pumping just outside the beam exit. The microwave-induced transition intensity in this case is independent of the ${}_{}{}^3{}_{}{}^{}$ ${\mathit{P}}_1$ 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