Observation of stimulated radiation as a result of transitions in He II pumped by a proton beam

When dense (p=0.5–2.5 atm) helium was pumped by a proton beam (j=2–12 A/cm²), stimulated emission was observed for the first time as a result of 4–3 (λ=468.5 nm) and 5–3 (λ=320 nm) transitions in helium-like He [font size=-3]II[/font] ions. Amplification was observed in a quasistationary supercooled plasma due to recombination pumping of the upper active levels.