Under the experimental conditions of this early work, the maximum ion stage was estimated to be 15 or 16 times ionized and in the absence of self-absorption, calculations assuming collisional radiative (CR) equilibrium6 predicted that the spectra should be dominated by line emission. However, the variation of spectral profile with concentration, along with the observation of strong absorption lines, indicated that self-absorption needed to be taken into account and the reduction in continuum intensity could be traced to the lowering of the contribution from recombination radiation, due to the lower average charge in plasma where the major constituent was a low material since the intensity due to recombination scales as where is the ionic charge. Thus opacity was a limiting factor whose impact can be reduced by reducing the plasma density either through reducing the concentration of the element of interest in the target, using rather than solid state lasers or using prepulses and so interacting with lower density target plasmas. It was shown that the feature arose from transitions, which merge to form a UTA (unresolved transition array) consisting of tens of thousands of lines, while the peak position is sensitive to atomic number and moves to shorter wavelengths as increases; in Sn it lies near 13.5 nm.7,8 Configuration interaction in the final state is very important and leads to a spectral narrowing and, in the heavier lanthanides, better overlap between the emission from adjacent ion stages than would be obtained by consideration of or transitions alone. As a result, the UTA can be the brightest emission feature in the spectra of plasmas of these elements. The effects of CI are to concentrate the emission intensity in the high energy end of the array and cause the oscillator strength envelope of the entire array to narrow. The strength of this interaction depends on the magnitude of the , overlap integral, , and this factor also controls the degree of overlap in spectra from adjacent ion stages.