The autoionization cross section of rubidium atoms was obtained by measuring the total normalized intensities of ejected-electron spectra arising from the decay of the 4p 5 n 1 l 1 n 2 l 2 autoionizing levels. The electron impact energy range from the 4p 6 excitation threshold at 15.31 up to 50 eV was investigated. The cross section reaches the maximum value of (2.9 ± 0.6) × 10 −16 cm 2 at 21.8 eV impact energy. The general behaviours of the cross section and the role of particular autoionizing configurations in its formation were considered on the basis of large-scale configuration interaction calculations of energies, cross sections, autoionization probabilities in 5snl(n 7; l 4) and 4d nl(n 5; l 2) configurations as well as the measured excitation functions for the lowest levels in 5s 2 and 4d5s configurations. The resonance behaviour of the cross section between 15.3 and 18.5 eV impact energy is caused exclusively by the negative-ion resonances present close to the excitation thresholds of the (5s 2 ) 2 P and (4d5s) 4 P autoionizing levels. At higher impact energies, the autoionization cross section is composed of contributions from the high-lying quartet and doublet levels in 4d5s, 5p and 5s5p, 5d, 6s, 6p configurations. From the comparison of the present data with available experimental and calculated ionization cross sections, the 5s + 4p 6 direct ionization cross section of rubidium atoms was determined with the maximum value of (7.2 ± 2.2) × 10 −16 cm 2 at 36 eV. It was also found that the 4p 6 excitation-autoionization is the dominant indirect ionization process contributing over 30% of the total single ionization of rubidium atoms by electron impact in the 15.3-50 eV energy range.

Electron impact excitation of the (4p55s2)2P3/2,1/2 and (4p54d5s)4P1/2,3/2,5/2 autoionizing states in rubidium atoms was studied experimentally by measuring the ejected-electron excitation functions and theoretically by employing a fully relativistic Dirac B-spline R-matrix (close-coupling) model. The experimental data were collected in an impact energy range from the respective excitation thresholds up to 50 eV with an incident electron energy resolution of 0.2 eV and an observation angle of 54.7°. Absolute values of the excitation cross sections were obtained by normalizing to the theoretical predictions. The observed near-threshold resonance structures were also analysed by comparison with theory. For the 2P3/2,1/2 doublet states, a detailed analysis of the R-matrix results reveals that the most intense resonances are related to odd-parity negative-ion states with dominant configurations 4p55s5p2 and 4p54d5s6s. The measured excitation functions for the 2P1/2 and 4PJ states indicate a noticeable cascade population due to the radiative decay from high-lying autoionizing states. A comparative analysis with similar data for other alkali atoms is also presented.

The relativistic distorted-wave and binary-encounter-dipole approximations were employed for calculating the electron-impact single ionization cross sections of the 5s, 4p6, 4s2, 3d10 shells and 4p6 excitation cross section for Rb atom taking into account both configuration interaction and relativistic effects. The capabilities of the most used theoretical approaches in describing the single ionization of Rb atom were considered by comparing the present and other available calculated data with the experimental total ionization and total direct single ionization cross sections over the electron-impact energy range from the 5s threshold to 600 eV. The best agreement within experimental uncertainty was obtained by using the non-relativistic binary-encounter-dipole approximation included in the LANL Atomic Physics Codes package. At present none of the used approximations (plane-wave Born or relativistic distorted wave) can satisfactorily describe the experimental excitation-autoionization cross section in rubidium because the efficient formation of the 4p6 core-excited negative-ion rubidium states at near-threshold impact energies is ignored in calculations.

Ejected-electron energy spectra resulting from decay of the (4p55s2)2P3/2, 1/2 autoionizing states in rubidium atoms have been studied at electron impact energies up to 5 eV above the excitation thresholds. The energy shift of corresponding lines due to the post-collision interaction (PCI) involving the outgoing electrons and the residual ion have been observed for the first time. For both states the energy dependence of the shift possesses a smooth asymptotic behaviour which is satisfactorily described by the classical model of the PCI effect. Similar behaviour of the line shift was reported earlier also for the (5p56s2)2P3/2 autoionizing state in heavier Cs atoms. Both results contrast with the clear oscillatory-like character of the line shift for the (np5[n + 1]s2)2P3/2, 1/2 autoionizing states in the lighter Na (n = 2) and K (n = 3) atoms. The comparative analysis of the data on the PCI effect and excitation cross-sections of the (np5[n + 1]s2)2P3/2, 1/2 autoionizing states in Na (n = 2), K (n = 3), Rb (n = 4) and Cs (n = 5) atoms has shown that the high polarizability of the resonantly excited target and partial structure of resonances are the main factors which determine the dynamics of the PCI shift of lines in ejected-electron spectra of alkali atoms.

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ЕЛЕКТРОННЕ ЗБУДЖЕННЯ КВАЗІМЕТАСТАБІЛЬНОГО СТАНУ (4p 5 5s5p) 4 S 3/2 АТОМА РУБІДІЮ Експериментально досліджено електронне збудження квазіметастабільного стану (4p 5 5s5p) 4 S3/2 атома рубідію в діапазоні енергій зіткнень від порогу до 19 еВ. Функція збудження, яка одержана при моноенергетичності електронного пучка 0.12 еВ, вказує на наявність припорогових резонансів негативного іона рубідію. Здійснено порівняння з експериментальними та теоретичними перерізами збудження квартетного автоіонізаційного стану (4p 5 4d5s) 4 P1/2 атома рубідію та квазіметастабільного стану (3p 5 4s4p) 4 S3/2 атома калію. Максимальне значення перерізу збудження стану (4p 5 5s5p) 4 S3/2 складає σ=3.77×10-18 см 2. Ключові слова: атом, збудження, рубідій, квазіметастабільний стан.