The origin of line-free XUV continuum emission from laser-produced plasmas of the elements 62⩽Z⩽74

Abstract

The different plasma mechanisms responsible for the continuum emission observed in the 35-2000 AA wavelength region from laser-produced plasmas of targets with 62<or=Z<or=74 have been studied in detail. The continuum is found to be largely free-bound in origin, especially at shorter wavelengths. The nonappearance of line emission is due to the complexity of the electronic configurations in the ion stages present in the plasma which in nearly every case can contain open 4f subshells. Furthermore 4f, 5p and 4f, 5s level crossing lead to the population of a range of low-lying excited states which increase still further the number of anticipated transitions. As a result, the transitions merge to form narrow bands of quasi-continua which, in the soft X-ray region, enhance the underlying recombination continuum. The XUV emission is observed to be most intense in the 100 AA region where the radiance is estimated to be >1023 photons cm-2 sr-1 AA-1 s-1.