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 D-1 (m=3,4,5,6) states to the doubly excited 3png F-1 (n=5,6,7) 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 F-1 resonances acquire a significant strength for the normally weak 3smd D-1 --> 3png F-1 two-electron transition due to a strong configuration interaction with an overlapping broad 3pnd F-1 autoionization series and the 3s epsilonf F-1 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.