Near-threshold excitation of the λ132.2 nm resonance line of the Tl+ ion by electron impact


Background. Among all processes of collision of electrons with atomic particles the study of the interaction between electrons and ions is extremely important and necessary for a better understanding of the basic collisional processes in plasma. In the processes of excitation, recombination, and ionization of ions by electron impact autoionization states play an important role. Their radiation decay makes an additional contribution to the photo-recombination process and leads to dielectronic recombination of an ion. Such contribution is most significant in the case of heavy many-electron ions such as the Tl+ ion. Excitation of the Tl+ ion by an electron impact at electron-ion collisions is practically not investigated up to date.
Methods. Experiments were performed using a spectroscopic method in conditions of electron and ion beams intersecting at an angle of 90°. The ribbon electron beam (cross section 1×8 mm2, energy Ee = 6–14 eV, current Ie = (2–10)×10–5 A) was formed by a low-energy three-electrode electron gun. The ion beam (cross section 2.5×2.5 mm2, energy Ei = 800 eV, current Ii = (5–7)×10–7 A) was obtained using a low-voltage arc discharge source. The radiation due to decay of the ion states excited during the collisions was detected perpendicular to the plane of the beams intersection. Spectral separation of the radiation in a vacuum ultraviolet region was carried out by 70° vacuum monochromator based on the Seya-Namioka scheme.
Results. Thorough measurements of the near-threshold region of the energy dependence of the effective cross section of the electron excitation of the resonance λ132.2 nm line (6s6p 11 → 6s2 1S0) of the Tl+ ion in the 6–14 eV energy range revealed resonant features both after and before the excitation threshold of the line. The widths of the before-threshold peculiarities is defined by the dispersion of the excited electrons energy. This is indicative of their resonant nature, i.e., they are due to the formation and decay of the atoionization states (AIS) of the Tl atom. Most likely, they are related to the formation and decay of the AIS of the 6s6pnp configuration (n ≥ 7) to the excited levels of the Tl atom. The nature of the after-threshold structure indicates that it is also the result of resonance processes and is due to the formation and decay of the AIS, most likely of the 5d96s26p2 configuration, in the electronic channel. The absolute values of the excitation cross sections of the dielectronic satellites were determined by comparison of their intensities with the intensity of the resonance λ132.2 nm line which effective excitation cross section was obtained by normalizing the experimental data on the theoretical calculation by the tight-binding method of two states of the excitation cross section of the 6s6p 11 level of the Tl+ ion at the 100 eV energy.
Conclusions. The energy dependence of the effective excitation cross section of the dielectronic satellites of the Tl+ ion resonance line showed that the absolute values of their excitation cross sections are ~ 10–16 cm2 and are of the same order of magnitude with that for the resonance line. It is shown that the main excitation mechanism of the resonance line satellites is dielectronic recombination which effectiveness essentially depends on the ratio of the probabilities of the AIS radiation and electron decay. In the case of the relativistic Tl+ ion a strong configuration mixing of levels leads to significant increase of the AIS radiation decay probability

Keywords: ion, electron, dielectronic recombination, thallium