Measurement of Cesium and Rubidium Charge-Transfer Cross Sections

Abstract

Charge-transfer cross sections for all cesium and rubidium ion-atom combinations were measured over energy ranges of 0.2 to 21 keV and 0.4 to 30 keV for the resonance and nonresonance combinations, respectively. A crossed-beam technique was used in which the atom beam was modulated by a mechanical chopper. Slow ions, produced by charge-transfer collisions, were extracted at the crossed-beam interaction region and detected using phase-sensitive lock-in techniques. Surface ionization was used to generate the ion beams and to detect the atom beams. The resonance cross sections are in general agreement with other experimental determinations and with theoretical predictions, except for the appearance of structure in our cross-section curves. The nonresonance cross sections show the expected adiabatic increase with velocity with the maximum near ${10}^{-14\mathrm{}}$ ${\mathrm{cm}}^2$ for ${\mathrm{Rb}}^{+}$-Cs at a velocity of 23×${10}^6$ cm/sec. The ${\mathrm{Cs}}^{+}$-Rb maximum is also expected to occur at this velocity, which is just beyond the measured range. For each nonresonance cross section, four subsidiary peaks were observed below the velocity at the maximum. The two curves are similar in shape, with all of the ${\mathrm{Cs}}^{+}$-Rb peaks occurring at the same velocities as the ${\mathrm{Rb}}^{+}$-Cs peaks. The low-velocity portion of our ${\mathrm{Rb}}^{+}$-Cs measurements is in good agreement with the data of Marino as regards the magnitude of the cross sections and the location of the first subsidiary peak. The cross-section curves of the four combinations are in very good agreement with the theoretical predictions of Rapp and Francis, particularly the ion velocity at the maximum for ${\mathrm{Rb}}^{+}$-Cs. The subsidiary peaks appear to be related to electron transitions between the first excited states of the incident and product atoms. Incident-atom excited states are attributed to polarization excitation by the incident ion during the initial phases of the collision, and it is concluded that the atomic polarizability greatly affects the magnitude and shape of the cross-section curves.