LIFETIME MEASUREMENT OF THE MG-I INTERCOMBINATION LINE

The lifetime of the 3s2 1S0-3s3p 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 3P1 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 3P1-3P0 (at 601 GHz) with and without laser optical pumping. Due to the decay of the 3P1 state, a population difference between 3P1 and 3P0 levels arises before the interaction of the microwave radiation with the atomic beam allowing the observation of the 3P1-3P0 magnetic dipole transition; its intensity is then related to the 3P1 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 3P1 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 tau(3P1) = 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.