VUV emission and absorption spectroscopy with a multichannel near normal incidence spectrometer

Abstract

An experiment for vacuum UV emission and photoabsorption spectroscopy usmg a normal incidence multi-channel spectrometer is described. Two detection configurations were used in the experiments, one a Channel Electron Multiplier Anay with Photo Diode Array readout ( CEMA / PDA), the other a directly illuminated back thmned Charge Coupled Device ( CCD ). Emission spectra of plasmas of a number of low and high Z target matenals were recorded covering the majority of the vacuum UV spectral region ( 35 to 200 nm ). These experiments were designed mainly to study the nature of the emission ( line versus continuum) and it’s dependence on atomic number. The use of two detection configurations allowed for mter comparisons to be made on resolution, efficiency and dynamic range of the two multi-channel detector and spectrometer systems. Experiments were also performed on photoabsorption by helium gas. The spectra thus obtamed were used to help charactense the system’s application to absorption studies around the He principal series limit at - 50 43 nm. In addition, for the first time the instrument was used to study photoabsorption using the dual laser plasma technique ( DLP ). Results on the relative photo absorption cross sections of barium and barium ions using a samarium continuum back-light are presented which indicate that the instrument is well suited to absorption experiments m the range of 35 to 80 nm where second order of the contmuum is mainly absent and the grating is being used close to it’s blaze wavelength ( 80 nm ). Finally, the work concludes with details on the design, construction and preliminary testmg of a vacuum-optical beamlme which should both improve the aperture matchmg ability of the instrument and allow spatially resolved studies of the laser plasma light source to be made. The spectrometer with the additional spherical mirror was modelled by means of simple geometric optic calculations as well as with ray tracing software m order to estimate improvements in throughput