Optical gain on the 476.5 nm Argon-ion laser line in a gas-target excited by a heavy ion beam

Optical gain on the 476.5 nm Ar II 4p-4s ion laser transition has been observed in argon-gas excited by 2.5 ns pulses of 90 MeV S-32 ions with a repetition rate of 4883 Hz. The energy per pulse was 23-mu-J. The projectiles were stopped in the target at pressures between 5 and 20 kPa. Gain was determined from a measured transient increase of the intensity of a 476.5 nm probe laser beam sent along the ion beam axis and back reflected by an aluminum foil. The maximum gain observed was (0.4 +/- 0.1) x 10(-3) at a target-gas pressure of 5 kPa. Control experiments using krypton as target-gas were performed and yielded a null result. The optical gain observed in argon is consistent with the result from an analysis of spectroscopic studies of rare-gas targets excited by heavy ion beams.