$3smd{}^{1}D$ to $3png{}^{1}F$ autoionization resonances of Mg

Intense ultranarrow resonances have been observed in the photoionization of Mg from the singly excited $3smd{}^{1}D$ $\mathrm{}(m=3,4,5,6\mathrm{})\mathrm{}$ states to the doubly excited $3png{}^{1}F$ $\mathrm{}(n=5,6,7\mathrm{})\mathrm{}$ autoionization states using stepwise laser excitation and time-of-flight mass spectrometry detection. The reported experimental energy resolution at an order of ${10}^{-7}$ of the photon energy represents a considerable improvement over the best photon energy resolution of about one part in ${10}^5$ for the synchrotron radiation light sources. The ultra-narrow $3png{}^{1}F$ resonances acquire a significant strength for the normally weak $3smd{}^1\overrightarrow{D}3png{}^{1}F$ two-electron transition due to a strong configuration interaction with an overlapping broad $3pnd{}^{1}F$ autoionization series and the $3s\epsilon f{}^{1}F$ continuum. The measured line positions and transition widths are in close agreement with the theoretical results derived from a B-spline-based configuration-interaction calculation.