The energy structure and decay channels of the 4p⁶-shell excited states in Sr

Abstract

The ejected-electron spectra arising from the decay of the 4p nln l n l 5 ¢ ¢   autoionizing states in Sr atoms have been studied precisely at the incident-electron energies close to excitation and ionization thresholds of the 4p 6 subshell. The excitation behaviors for 58 lines observed between 12 and 21eV ejected-electron kinetic energy have been investigated. Also, the ab initio calculations of excitation energies, autoionization probabilities and electron-impact excitation cross sections of the states 4p nln l n l 5 ¢ ¢   (nl = 4d, 5s, 5p; n l ¢ ¢ = 4d, 5s, 5p; n l   = 5s, 6s, 7s, 8s, 9s, 5p, 6p, 5d, 6d, 7d, 8d, 4f, 5g) have been performed by employing the large-scale configuration-interaction method in the basis of the solutions of Dirac-Fock-Slater equations. The obtained experimental and theoretical data have been used for the accurate identification of the 60 lines in ejected-electron spectra and the 68 lines observed earlier in photoabsorption spectra. The excitation and decay processes for 105 classified states in the 4p 5 5s nl 2 , 4p 5 4d nl 2 and 4p 5 5snln l ¢ ¢ configurations have been considered in detail. In particular, most of the states lying below the ionization threshold of the 4p 6 subshell at 26.92eV possess up to four decay channels with formation of Sr + in 5s 1 2 , 4d 3 2,5 2 and 5p 1 2,3 2 states. Two-step autoionization and two-electron Auger transitions with formation of Sr 2+ in the 4p 6 S 1 0 ground state are the main decay paths for high-lying autoionizing states. The excitation threshold of the 4p 6 subshell in Sr has been established at 20.98±0.05eV.

Theoretical study of 4p5nln'l'LSJstates of Sr ion excited by electron impact on Sr atom

“…The same basis set was used to calculate electron-impact excitation cross sections, and to describe the final state in the ionization of Sr atoms. The autoionization probabilities of Sr atom were calculated for the transitions to the 4p 6 nl (nl = 5s, 6s, 5p, 6p, 4d, 5d, 4f), 4p 5 5s 2 , 4p 5 4d 2 , 4p 5 4d5s, 4p 5 5s5p, 4p 5 4d5p, 4p 5 5s6s, 4p 5 5p 2 and 4p 5 4d5d configurations of Sr + to include energetically allowed decay channels for the levels with energy greater than 26.92 eV [10].…”

“…The dipole allowed transitions populate core-excited states of alkaline earth ions with the total angular momenta J = 1/2, 3/2. Electron-impact excitation and ionization processes are more powerful tools allowing to obtain states with all possible values of the total angular momentum J resulting in registration of more lines in the Auger decay spectrum [2][3][4][5][6][7][8][9][10]. Theoretical investigation of those spectra is complicated as the correlation effects play a significant role both for creation ways and decay channels of states.…”

Electron-impact ionization of the 4p6 subshell in strontium

“…Excitation and ionization of the 4p 6 subvalence shell in strontium lead to autoionization and Auger effects, which make a significant contribution, respectively, to single and double ionization of this atom [6][7][8]. The electron-impact excitation of the 4p 6 subshell was previously studied experimentally by measuring the spectra of ejected electrons in a wide range of impact energies and theoretically using the FAC code [9,10]. In particular, the excitation cross sections, decay channels, and spectroscopic classification of the 4p 5 nln ′ l ′ n ′′ l ′′ autoionizing states were obtained.…”

“…Reaction (2) can be assigned as an ionization-excitation process. Finally, reactions (3) and ( 4) represent the population of the ionic states 4p 5 nln ′ l ′ due to the decay of high-lying atomic 4p 6 -core excited autoionizing states (EA process [10]). As theoretical estimates show [9,10], at an impact energy of 40 eV, reaction (1) is almost three times more efficient than reactions (2) and (3).…”

“…However, the study of ejected-electron spectra in a wide range of impact energies, including in the near-threshold region, makes it possible to distinguish atomic and Auger lines due to the higher excitation thresholds of the latter. An accurate study of excitation dynamics of lines and their identification with the multichannel decay of atomic states 4p 5 nln ′ l ′ n ′′ l ′′ [10] showed that out of the 83 lines observed in total in the electron spectra [19,21,23], only 21 lines located between 10.2 and 14.5 eV can be unambiguously attributed to Auger decay (5). Of these, 13 lines were assigned as the Rydberg series limits in the 4p 6 photoabsorption spectra [13,14].…”