The autoionization branching ratios from Eu 4f76p1/26d [J] autoionizing states to its 4f76s+ (9So), 4f76s+ (7So), and 4f75d+ (9Do) final ionic states are investigated with the combination of the three-step laser excitation and the velocity-map imaging technique. These different autoionizing states are excited via 4f76s6d 8DJ [J = 5/2, 7/2, and 9/2] intermediate states, respectively. The experimental photoelectron images are obtained, from which energy distributions of ejected electrons are achieved with the mathematical transformation. Furthermore, the energy dependence of the branching ratio is investigated within the autoionization resonance, by which population inversion is observed as an important characteristic. The J-dependence is also studied systematically. The validity of the well-known isolated core excitation technique used for obtaining the autoionization spectrum is also studied.

To explore the dynamic properties of Eu 4f76p1/2ns autoionization process, the autoionization branching ratios of ions and the angular distributions of ejected electrons from the Eu 4f76p1/2ns (n=7, 9) autoionizing states are systematically investigated with the combination of the three-step isolated-core excitation (ICE) and the velocity-map imaging techniques The Eu 4f76sns Rydberg states are populated via a two-step laser excitation, from which the Eu 4f76p1/2ns autoionizing states are excited by the wavelength of the third laser around the Eu 6s+6p1/2+ ionic resonance in order to obtain autoionization spectra and the velocity-map images of ejected electrons from the Eu 4f76p1/2ns autoionizing states. Once the velocity-map images have been measured, both the energy distribution and angular distribution of ejected electrons can be acquired. Moreover, the spectra of the branching ratios and the anisotropic parameters within the autoionization resonances are also measured to observe their energy dependence and the relation with the autoionization spectra. Comparisons of the observed spectra of 4f76p1/2ns autoionizing states with n = 7, 8, and 9 manifest that the ICE technique is more suitable for the higher-n members of autoionization series. It is found that the Eu atoms in the 4f76p1/2ns (n = 8, 9) autoionizing states mainly decay into 4f75d+(9D) ionic state, leading to the population inversion between 4f75d+(9D) and 4f76s+ (7S) or 4f76s+ (9S) ionic states, which is significant for developing the autoionization laser. The angular distributions of the ejected electrons from the Eu 4f76p1/2ns autoionizing states show simple patterns at the energy points corresponding to the peaks of autoionization spectra, and have complicated patterns in the energy regions off the peaks of autoionization spectra, especially in the regions corresponding to the sharp increase or decrease in the autoionization spectra. The above phenomena can be explained with the strength of configuration interaction among different autoionization series converging to different ionic states, which is fluctuated within the energy region of autoionization spectra. In addition, within the autoionization resonance both the spectra of branching ratios and anisotropic parameters vary irregularly, and no obvious correlation with the spectra of 4f76p1/2ns autoionizing states can be found.