Formation ofH(n=2)atoms by the nearly resonant processH+in Cs. Multiple collision processes

Formation of H atoms in the $n=2\mathrm{}$ states for the nearly resonant charge-transfer process ${\mathrm{H}}^{+}$ + Cs (energy defect: 0.49 eV) is studied in the energy range 0.25-3.0 keV. The outgoing $\mathrm{H}\mathrm{}\left(2S\right)\mathrm{}$ atoms are quenched in a transverse electric field. The cross section for formation of $\mathrm{H}\mathrm{}\left(2S\right)\mathrm{}$ atoms exhibits typical nearly resonant characteristics. The principal maximum, (6.0 ± 0.25) × 1$0^{-15\mathrm{}}$ ${\mathrm{cm}}^2$ occurs at 0.5-keV ${\mathrm{H}}^{+}$ energy; a secondary maximum is observed at 1.35 keV. Structure is discussed in terms of the potential-energy curves of the quasimolecule ${\mathrm{}\left(\mathrm{CsH}\right)\mathrm{}}^{+}$. The cross section for formation of $\mathrm{H}\mathrm{}\left(2P\right)\mathrm{}$ atoms is obtained by an indirect method. The $\mathrm{H}\mathrm{}\left(2S\right)\mathrm{}$ fraction $f$ relative to the outgoing neutral beam for a thin Cs target is obtained; $f$ is close to the statistical value of 0.25 for ${\mathrm{H}}^{+}$ energies greater than 1 keV, and has a maximum value of 0.55 ± 0.075 at 0.6 keV. Multiple collisions are studied for Cs targets of variable thickness. The $\mathrm{H}\mathrm{}\left(2S\right)\mathrm{}$ fractional yield $F_m$ relative to the total outgoing beam has a maximum for a Cs target thickness of about ${10}^{14}$ atoms/${\mathrm{cm}}^2$. The largest value of $F_m$ is 0.30 at 0.5 keV. $F_m$ approaches zero as an equilibrium value. A four-component charge-exchange model is used to show that the primary destruction mechanism of $\mathrm{H}\mathrm{}\left(2S\right)\mathrm{}$ atoms is collisional deexcitation, with a cross section of about 5 × ${10}^{-15\mathrm{}}$ ${\mathrm{cm}}^2$. The influence of scattering and calibration of a secondary-emission neutral detector are discussed. Finally, the polarization $P$ of Lyman-$\alpha$ radiation from quenching of $\mathrm{H}\mathrm{}\left(2S\right)\mathrm{}$ atoms in a transverse electric field is measured as a function of applied electric field. $P$ is negative, varying from -0.31 ± 0.03 in a near-zero electric field to -0.50 ± 0.06 for a field of 475 V/cm. Data are in agreement with an adiabatic theory; in addition, oscillations about the theoretical curve are observed.