Populations of 2p and 3p terms in hydrogen excited by H+, H2+, and H3+ ions passing through thin carbon foils

Relative beam-foil populations of the 2p term in hydrogen have been measured as a function of the proton energy (0.015< or approx. =E< or approx. =1.10 MeV). For E> or approx. =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 (E<0.1 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 (0.1 or approx. =1.5 fs) is considered in this work. The variation of R = I/sup molec//I/sup atom/ (I/sup molec/ and I/sup atom/ are the Ly-α intensities per incident proton observed with molecular and atomic projectiles of the same velocity, respectively) with the projectile energy per nucleon (E/M) and the thickness (T) of the foil is well described by the following relation: R = R/sub infinity/[1-C exp(-kE/M)], where R/sub infinity/, C, and k are parameters depending only on T, and on the number of protons in the cluster. A qualitative explanation of the R(E/M,T) behavior is proposed