Figure 1

Schematic diagram of microwave RESIS apparatus used for this study. An ion beam is created in a Colutron model-101 ion source. The ions are then collided with a selectively excited 9$F$/10$D$ Rb target where some ions capture an electron. States of $n=15$ and higher are then Stark ionized in the pre-ionizer and deflected with the remaining ${\text{Mg}}^{+}$ ions to be collected in a Faraday cup. Neutral magnesium atoms with $n<15$ enter the first ${\text{CO}}_2$ laser interaction region. Here a Doppler-tuned ${\text{CO}}_2$ laser excites the $n=9$ atoms to $n=17$, depicted as the first arrow in the diagram of the energy levels below the apparatus schematic. Let each circle represent 1000 atoms residing in the energy level they are overlapping, so after the first laser region there is an equal population of 4000 atoms in the $n=9$, $L=6$ and the $n=17$, $L=7$ states. The microwave region can then excite a transition within the $n=17$ upper state, splitting the upper state population equally between the $L=7$ and 8 states. The second laser interaction region then repopulates the initial upper state if a microwave transition has occurred, as shown by the second large arrow and a population of 3000 atoms in the $n=9$, $L=6$ and $n=17$, $L=7$ levels. The excited states are then Stark ionized in the stripper and focused by the lens and the resulting signal ions are detected on a channeltron electron multiplier. A total current of 5000 ions is found if the microwave transition occurs, but only 4000 ions if it does not. The measured signal, the change in ion current caused by the microwaves, is 1000 ions in this case.Reuse & Permissions

Figure 2

The Mg $n=17$ $L=6to7$ microwave transition observed both co- and counterpropagating. The straight average of the fitted line centers will give the Doppler-free transition frequency.Reuse & Permissions

Figure 3

Magnesium polarization plot, where the corrected line centers normalized to $\Delta ⟨r^{-4}⟩$ are plotted vs $\Delta ⟨r^{-6}⟩/\Delta ⟨r^{-4}⟩$. The solid data points are the normalized corrected fine-structure intervals, from Table . The hollow points have been corrected for the stray dc electric field Stark shifts using the fitted value of $B_E$ from Eq. (3). The solid line illustrates the fit to the data, including only the first three terms of Eq. (3).Reuse & Permissions