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The overlapping lasing lines in the collisional yttrium laser

The two dominant lasing lines in the Ne-like yttrium laser exhibit an apparently anomalous behaviour. The line at 157.14 AA demonstrates a marginally higher gain than that at 154.97 AA despite the fact that the latter is observed to be up to 100 times brighter. We describe a convincing explanation for this behaviour in terms of the 155 AA line being composed of two partially overlapping transitions-the existence of such an overlap (between a J=0-1 and a J=2-1 transition) has long been the suspected cause. We demonstrate that the intrinsic line shapes (in contrast to the gain narrowing of the spectral line shape) narrow along the path of the beam due to variations in electron density and ion temperature which arise as the beam refracts through the expanding medium. These variations are shown to be sufficient to partially decouple the lines at long plasma lengths. The resulting effect is that the beam undergoes large amplification initially, producing the observed intensity ratio between the 155 AA and 157 AA emission, but that the gain at 155 AA is reduced at the longer lengths examined experimentally.