3smd1Dto3png1Fautoionization resonances of Mg

Abstract

Intense ultranarrow resonances have been observed in the photoionization of Mg from the singly excited $3smd{}^{1}D$ $(m=3,4,5,6)$ states to the doubly excited $3png{}^{1}F$ $(n=5,6,7)$ autoionization states using stepwise laser excitation and time-of-flight mass spectrometry detection. The reported experimental energy resolution at an order of ${10}^{\ensuremath{-}7}$ 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 $3smd{}^{1}\stackrel{\ensuremath{\rightarrow}}{D}3png{}^{1}F$ two-electron transition due to a strong configuration interaction with an overlapping broad $3pnd{}^{1}F$ autoionization series and the $3s\ensuremath{\epsilon}f{}^{1}F$ 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.