CROSS-SECTIONS OF CHARGE-TRANSFER BETWEEN A GD ATOM AND ITS SINGLY CHARGED POSITIVE-ION IN METASTABLE STATES CLOSE TO THE GROUND-STATE

The charge-transfer cross sections between a gadolinium atom and its s ingly charged positive ion in metastable states have been measured wit h a crossed-beam apparatus. The primary ions are produced by photoioni zation with a tunable dye laser. The populations of primary ions in ex cited states have been measured by the laser-induced fluorescence meth od. By measuring the populations, the charge-transfer cross sections o f Gd+(0 cm(-1))+Gd, Gd+(262 cm(-1))+Gd, and Gd+(633 cm(-1))+Gd were ob tained successfully in the energy range from 30 to 1000 eV. It has bee n found that the cross sections of these reactions at 1000 eV are equa l to each other and are two times larger than that given by Sakabe's f ormula based on all the past experimental data for resonant charge tra nsfer [Phys. Rev. A 45, 2086 (1992)], and that the higher the state in which the primary ion is populated, the larger the cross section tend s to be. The impact-energy dependences are explained qualitatively by assuming the process to include both resonant and nonresonant charge t ransfer.