Nonequilibrium effects in the proton neutral fraction emerging from solids bombarded with MeV H0, H+, H2+, and H3+ beams

Abstract

We have investigated nonequilibrated neutral fractions in hydrogen beams transmitted through carbon foils bombarded with ${\mathrm{H}}^0$, ${\mathrm{H}}_2^{+}$, and ${\mathrm{H}}_3^{+}$ projectiles having energies from 0.5 to 2.4 MeV/amu. Measurements with neutral projectiles provide a direct determination of the charge-exchange cross sections for fast protons in solids. The fact that the solid cross sections have been found close to the corresponding gas cross sections is discussed and confronted with existing theories. For very short dwell times the role played by projectile electrons in neutral-atom production is demonstrated, and it is shown that the approach to equilibrium follows essentially the same law for ${\mathrm{H}}^0$, ${\mathrm{H}}_2^{+}$, and ${\mathrm{H}}_3^{+}$ projectiles. An overproduction of neutrals has also been observed with molecular ion projectiles for longer dwell times. An explanation of this effect is suggested in terms of a multistep process in which a target electron gets correlated with one proton before being captured by another proton of the cluster. Angular distributions of neutrals produced from incident molecular beams indicate the possible formation of a repulsive molecular state on emergence from the foil.