A Study of the Beam-Foil Excitation Mechanism Using ⁴He⁺, ⁶Li⁺ and ⁷Li⁺ Projectiles

Beams of ⁴He⁺, ⁶Li⁺ and ⁷Li⁺ ions accelerated to energies between 10 keV and 75 keV have been sent through thin foils of beryllium, carbon, aluminium or gold, or into a helium gas target. The photon emission immediately after the foil has been measured in the wavelength range 200 nm to 600 nm. Excitation functions have been measured for 11 Li I^a spectral lines, 8 Li I^b lines and 10 Li II^a lines. It was found that the excitation functions within one level scheme have similar slopes, whereas differences exist between excitation functions of different level schemes. From the excitation functions relative population functions have been constructed and it is found that the populations of Li I^a levels are proportional to the probability of getting neutral Li, whereas the He I, He II, Li I^b and the Li II^a population functions differ from the charge state distribution curves. Comparisons between the population of corresponding levels in ⁶Li and ⁷Li indicate that the collision time is a more relevant parameter than the projectile energy. The statistical weights related to the orbital angular momentum and to the total spin of the electronic structure are found to influence the beam-foil interaction, whereas the statistical weight related to the nuclear spin is not apparent. The foil excitation is compared to single atomic collisions, and it is concluded that the beam-foil excitation is the result of a multi-collision atomic interaction. Sputtered excited atoms of beryllium and aluminium have been observed.