[en] Absolute doubly differential cross sections for electron emission are presented for 0.5 MeV/u multi-charged ion impact on helium, neon, and argon targets. For the helium target, Bq+, Cq+ (q = 2--5) and Oq+, Fq+ (q = 3--6) projectiles were studied; for neon and argon, only Cq+ (q = 2--5) projectiles were used. Electron emission for 10 degrees ≤ Θ ≤ 60 degrees was studied. The measured cross sections were assumed to scale as the square of an effective projectile charge, Z_eff, which was determined as a function of emitted electron energy and angle. For distant collisions (low emitted electron energies), we find that Z_eff ∼ q for small q and Z_eff < q for the highest values of q investigated. For sufficiently close collisions (above the binary encounter peak), Z_eff > Z and increases as q decreases. This is true for all angles and targets investigated