3smd ¹D to 3png ¹F autoionization resonances of Mg

Intense ultranarrow resonances have been observed in the photoionization of Mg from the singly excited 3smd ¹D (m=3,4,5,6) states to the doubly excited 3png ¹F (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 10⁵ for the synchrotron radiation light sources. The ultra-narrow 3png ¹F resonances acquire a significant strength for the normally weak 3smd ¹D→3png ¹F two-electron transition due to a strong configuration interaction with an overlapping broad 3pnd ¹F autoionization series and the 3sεf ¹F 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.