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 Angstrom, demonstra tes a marginally higher gain than that at 154.97 Angstrom despite the fact that the latter is observed to be up to 100 times brighter. We de scribe a convincing explanation for this behaviour in terms of the 155 Angstrom line being composed of two partially overlapping transitions -the existence of such an overlap (between a J=0-1 and a J=2-1 transit ion) has long been the suspected cause. We demonstrate that the intrin sic line shapes (in contrast to the gain narrowing of the spectral lin e shape) narrow along the path of the beam due to variations in electr on density and ion temperature which arise as the beam refracts throug h the expanding medium. These variations are shown to be sufficient to partially decouple the lines at long plasma lengths. The resulting ef fect is that the beam undergoes large amplification initially, produci ng the observed intensity ratio between the 155 Angstrom and 157 Angst rom emission, but that the gain at 155 Angstrom is reduced at the long er lengths examined experimentally.