Soft X-Ray Narrowband Continuum Emission from Laser Produced Plasmas

Abstract

Spectra from moderately ionised laser produced plasmas of the lanthanides are known to contain extensive line free continua throughout the VUV (vacuum ultraviolet) region of the spectrum1. In the case of samarium, the spectrum is line free from 4 to 200nm. These continua arise as a result of the complexity of the configurations found in the ions present in the plasmas. This complexity has its origins in the near degeneracy of the 5s, 5p and 4f electrons present in ion stages typically ranging from 6 to 12 times ionized. Due to the extremely high number of levels present in such ions, any transitions that do occur are too weak and too numerous to rise above the recombination radiation and bremsstrahlung from the plasma. The only feature of note is the intensity modulation of the continuum due to 4d → 4f transitions, seen at 8.8nm in cerium and 8.2nm in samarium. This emission is from more highly stripped ions where the 4d subshell is the highest occupied subshell, ranging from CeXIII to CeXXII in the case of cerium. The Unresolved Transition Array (UTA) model of Bauche-Arnoult et al.2,3 describes these narrow bands of quasi continuum, where even at the highest resolution it is impossible to resolve individual lines, using a statistical approach to the line and level densities. The optimisation of this emission feature is of importance in the search for a narrowband source of EUV (extreme ultraviolet) radiation. Such a source, when matched to a suitable multilayer reflecting optic, would have applications in fields ranging from biological imaging to EUV lithography. It important to minimise the level of off-band radiation that is incident on the mirrors, as this will cause heating and introduce distortions into any optical system. The approach of this work is to investigate the influence of target composition, with regard to the species and concentrations of rare earth ions, on the shape and intensity of the UTA.