Populations of2pand3pterms in hydrogen excited byH+,H2+, andH3+ions passing through thin carbon foils

Abstract

Relative beam-foil populations of the $2p$ term in hydrogen have been measured as a function of the proton energy ($0.015\ensuremath{\lesssim}E\ensuremath{\lesssim}1.10$ MeV). For $E\ensuremath{\gtrsim}0.1$ MeV, these populations are found to be proportional to the equilibrium neutral fractions of protons emerging from a carbon foil. At lower energies ($El0.1$ MeV), the behavior of the excitation function of the $2p$ term is compatible with a decrease of the ratio of the $2p$ population to the ground-state population with respect to that ratio at higher energies. The first precise measurements of the dependence on the projectile energy ($0.1lEl1.8$ MeV) of the population of the $2p$ term excited by the passage of $\mathrm{H}_{2}^{+}$ and $\mathrm{H}_{3}^{+}$ ions through carbon foils of various thicknesses (2.5-22 \ensuremath{\mu}g/${\mathrm{cm}}^{2}$) are reported. Only the long-dwell-time region ($t\ensuremath{\gtrsim}1.5$ fs) is considered in this work. The variation of $R=\frac{{I}^{\mathrm{molec}}}{{I}^{\mathrm{atom}}}$ (${I}^{\mathrm{molec}}$ and ${I}^{\mathrm{atom}}$ are the $\mathrm{L}\mathrm{y}\ensuremath{-}\ensuremath{\alpha}$ intensities per incident proton observed with molecular and atomic projectiles of the same velocity, respectively) with the projectile energy per nucleon ($\frac{E}{M}$) and the thickness ($T$) of the foil is well described by the following relation: $R={R}_{\ensuremath{\infty}}[1\ensuremath{-}C\mathrm{exp}(\frac{\ensuremath{-}kE}{M})]$, where ${R}_{\ensuremath{\infty}}$, $C$, and $k$ are parameters depending only on $T$, and on the number of protons in the cluster. A qualitative explanation of the $R(\frac{E}{M,T})$ behavior is proposed. Values of $R$ have also been measured for $\mathrm{L}\mathrm{y}\ensuremath{-}\ensuremath{\beta}$ radiation and are found to be significantly smaller than those obtained for $\mathrm{L}\mathrm{y}\ensuremath{-}\ensuremath{\alpha}$ radiation, for foils of thickness $T\ensuremath{\lesssim}6$ \ensuremath{\mu}g/${\mathrm{cm}}^{2}$.