Molecular effects in optical beam-foil spectroscopy usingH+,H2+,H3+,D+,D2+,D3+,HD2+, and HeH+projectiles

Relative beam-foil level excitations in hydrogen and helium have been studied as a function of the foil thickness using incoming beams of ${\mathrm{H}}^{+}$, $\mathrm{H}_2^{}{}_{}{}^{+}$, $\mathrm{H}_3^{}{}_{}{}^{+}$, ${\mathrm{D}}^{+}$, $\mathrm{D}_2^{}{}_{}{}^{+}$, $\mathrm{D}_3^{}{}_{}{}^{+}$, $\mathrm{HD}_2^{}{}_{}{}^{+}$, and He${\mathrm{H}}^{+}$, thus varying the average distance between the fragments at the exit of the foil. Large changes in level excitation are observed in many cases for molecular projectiles when using thin foils. For molecular-hydrogen projectiles the same enhancement of excitation is found for all hydrogen levels studied. However, a remarkable difference is found for the heteronuclear projectile He${\mathrm{H}}^{+}$. In that case, the uniform enhancement for different hydrogen levels vanishes. The results are discussed.