Electron impact excitation of the Tl + ion: resonance and cascade transitions

Abstract

Results of experimental and theoretical investigations into the electron-impact excitation of the resonance → 6s 2 1 S 0 (132.2 nm), as well as the cascade 6s7s 1 S 0 → (309.2 nm) and 6p 2 1 D 2 → (150.8 nm) spectral transitions from the ground 6s 2 1 S 0 level in the thallium ion, are presented. Crossed beams of electrons and Tl+ ions in combination with a spectroscopic method were used in the experiment. Measured cross sections have a distinct resonance structure arising mainly from the electron decay of atomic 5d 96s 26p 2, 5d 106s7s(1 S)np (n ⩾ 7), 5d 106s6d(1 D)np (n ⩾ 6), 5d 106s6d(1 D)nd (n ⩾ 6) and ionic 5d 96s 2 np, 5d 96s 2 nd, 5d 96snp (n ⩾ 6) autoionizing levels, as well as radiative transitions from the higher 6s7s 1 S 0, 6s6d 1 D 2, and 6p 2 1 D 2, 1 S 0 ionic levels. Relativistic distorted wave (RDW) calculations are performed to obtain the effective cross sections for the above transitions. The absolute values of the cross sections are found to be 1.39 × 10−16 cm2 at 300 eV for the λ132.2 nm resonance line, and 0.11 × 10−16 cm2 and 0.56 × 10−16 cm2 at 100 eV for the λ309.2 nm and λ150.8 nm lines, respectively. The contribution of the cascade transitions being studied to the effective cross section of the λ132.2 nm resonance line at the energy of 100 eV is about 30%. A calculation using the semi-empirical Van Regemorter formula is also performed to obtain the effective cross section of the resonance λ132.2 nm line. The absolute value of the cross section at 300 eV is found to be nearly the same as that given by the RDW calculation.