The n = 2-2 transitions in the FI LiI isoelectronic sequences of some fourth period elements

Abstract

The plasma produced by focusing the output of a high-powered, neodymium glass laser onto plane, metallic targets has been used as a source for studying the grazing incidence spectra of the elements in the periodic table from chromium to nickel. Over 400 new identifications are presented, the vast majority with transitions belonging to the n k n-1 k+12s 2p - 2s 2p transition arrays in the 0 I to Be I isoelectronic sequences. Of particular interest are identifications of inter combination lines in the N I to Be I sequences, which are observed for the first time in laboratory spectra of fourth period elements. The new data, taken together with other published results, allow a complete n k n-1 k+1 listing of the 2s 2p - 2s 2p transitions in the elements chromium to nickel, from which the appropriate term schemes have been determined. In addition to the n = 2 - 2 transitions, the wavelengths of some sodium-like transitions of the elements chromium to nickel are presented. The laser system used in this study has an output power of, typically, 50 GW in pulses of a few nanoseconds duration. Special features of this system are its high repetition rate and the reproducibility of its output and these features have been exploited in experiments to distinguish between the emission from different ionisation stages. A full description of the laser system is given. Consideration is, also, given to the various atomic structure calculations which have been used to give wavelength predictions of n = 2 - 2 transitions. The emphasis is placed on Hartree-Fock methods. The importance of the n = 2 - 2 transitions is evidenced by the variety of plasmas in which they occur and their uses both as a diagnostic of certain plasma parameters and for comparison with atomic structure calculations. Their occurrence in different plasmas is illustrated by the application of the term scheme analysis to tokamak and solar flare plasmas, both of which are of topical interest. Predictions of the allowed and forbidden, n = 2 - 2 transitions of metallic impurity elements which are expected to have sufficient intensity to be observed in tokamak discharges are listed and some new identifications in solar flare spectra are presented. Of particular note amongst these predictions and identifications are those of then forbidden transitions within the 2s 2p configurations, some of which fall in the near-UV and visible spectral regions, where high resolution spectroscopic techniques can be used. An example of a diagnostic use is the measurement of electron densities from line intensity ratios and this method is used to estimate the electron densities of some solar flare plasmas. In addition, the application of this method to high density plasmas, which is being investigated by other authors, is discussed. Finally, some of the observed wavelengths are compared with the results of atomic structure calculations.