Absolute Detection and Collisional Destruction of 2.5-keV Metastable Hydrogen Atoms Produced by a Charge-Exchange Process in Cesium Vapor

Abstract

We have studied a near-resonant charge-exchange process between incident protons and a cesium vapor target. The cross section ${\sigma }_{+0\mathrm{}}$ for the production of all the neutral states of hydrogen decreases slightly with increasing energy. It varies from (10±3) × ${10}^{-15\mathrm{}}$ ${\mathrm{cm}}^2$ at 0.5 keV to (6.4±1.6) × ${10}^{-15\mathrm{}}$ ${\mathrm{cm}}^2$ at 2.5 keV. The cross section ${\sigma }_{+m}$ for the production of the $2S_{\frac{1}{2}}$ metastable state of hydrogen is (1.7±0.6) × ${10}^{-15\mathrm{}}$ ${\mathrm{cm}}^2$ at 2.4 keV. The percentage of metastable atoms in the outgoing neutral beam is found to be 0.27±0.08 for cesium target thickness less than ${10}^{13}$ atoms/${\mathrm{cm}}^2$. The outgoing fraction of $\mathrm{H}\left(2\mathrm{}{S}_{\frac{1}{2}}\right)$ atoms reaches a maximum equal to 0.13 for a cesium thickness of 1.2×${10}^{14}$ atoms/${\mathrm{cm}}^2$. The collisional quenching processes studied are the electron loss of $\mathrm{H}\left(2\mathrm{}{S}_{\frac{1}{2}}\right)$ on the noble gases ${\mathrm{H}}_2$, ${\mathrm{N}}_2$, and HI at 2.5 keV and the electron attachment of $\mathrm{H}\left(2\mathrm{}{S}_{\frac{1}{2}}\right)$ on ${\mathrm{N}}_2$ at the same energy. The electron-loss cross sections ${\sigma }_{m+}$, in units of ${10}^{-16\mathrm{}}$ ${\mathrm{cm}}^2$, and known with a 35% uncertainty, are 4.1 for He, 2.7 for Ne, 2.9 for Ar, 2.7 for Kr, 6 for Xe, 3.4 for ${\mathrm{H}}_2$, and 5 for ${\mathrm{N}}_2$. The cross section ${\sigma }_{m+}$ is always greater than ${\sigma }_{g+}$, the electron-loss cross section for the ground state of hydrogen. The maximum ratio 9.7 of these two cross sections is obtained with the halogen compound target HI, which is an interesting gas to selectively ionize $\mathrm{H}\left(2\mathrm{}{S}_{\frac{1}{2}}\right)$ in a beam containing the two species $\mathrm{H}\left(2\mathrm{}{S}_{\frac{1}{2}}\right)$ and $\mathrm{H}\left(1\mathrm{}{S}_{\frac{1}{2}}\right)$. A comparison of the data is made with theoretical predictions using an impulse approximation. Attachment cross sections of $\mathrm{H}\left(2\mathrm{}{S}_{\frac{1}{2}}\right)$ and $\mathrm{H}\left(1\mathrm{}{S}_{\frac{1}{2}}\right)$ have been measured on ${\mathrm{N}}_2$ at 2.5 keV. We obtain ${\sigma }_{m-}=(1.2\mathrm{}\pm 0.4)\times {10}^{-16\mathrm{}}$ ${\mathrm{cm}}^2$ and ${\sigma }_{g-}=(9.7\mathrm{}\pm 2)\times {10}^{-18\mathrm{}}$ ${\mathrm{cm}}^2$.