An apparatus is described for making direct measurements of the loss of forward velocity of argon ions in single encounters with helium atoms. Results are given in the form of collision cross sections for given percentage retardations, for ions of energies of 1000, 800, 600, 400, and 200 ev. The cross sections are zero for energy losses greater than 33.1 percent and increase both with decreasing percentage loss and decreasing beam speed. A correction to the readings necessary to obtain the results proves to be of unusual interest because the correction is itself the cross section for charge exchange between the argon ions and the helium atoms. Quantitative values of the cross sections for this unusual charge exchange are thus obtained. The values run roughly from 3.5 to 4.5 × ${10}^{-16\mathrm{}}$ ${\mathrm{cm}}^2$ per molecule in the energy range studied, compared with 28 × ${10}^{-16\mathrm{}}$ ${\mathrm{cm}}^2$ for ${\mathrm{A}}^{+}$ ions in A and 63 × ${10}^{-16\mathrm{}}$ ${\mathrm{cm}}^2$ for $\mathrm{N}_2^{}{}_{}{}^{+}$ ions in ${\mathrm{N}}_2$. All the results are in sufficiently precise form to invite detailed theoretical analysis.

Close

PrevNext

View figure in article