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