ABSTRACT
Fermat’s Principle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
The term “spectrometer” refers to any energy-resolving instrument; however, in this chapter we will concentrate on photon energy-resolving instruments or, to be more specific, wavelength-dispersive instruments. The basic theme of this chapter concerns techniques for the study of highly charged ions (HCIs), and one aspect of this should be their spectra. Spectra from HCIs
Ion
can cover very large ranges in photon energy, or wavelength, that is, from over 100 keV photons for H-like resonance lines in very-highly charged ions to less than 1 eV photons for hyperfine transitions. For example, the 1s hyperfine splitting gives a transition at 3858.2260 ± 0.30 Å (3.2 eV) in 203Tl80+ [1], whereas the 1s to 2p resonance transitions have energies more than 90 keV (simple Z scaling). Another example is provided by transitions in highly ionized iron. The resonance transitions in He-like iron, Fe XXV (from n = 2 to n = 1), occur in the x-ray region, at 1.85 Å, or 6.7 keV. These lines have been observed in solar flares and in tokamaks [2]. In Al-like Fe XIV, there is a forbidden M1 transition within the 3s23p2P ground term, a strong line in the solar corona, at 5303 Å. Already in 1945, Edlén [3] had identified 23 such corona lines, in highly charged Ar, Ca, Fe, and Ni, with wavelengths ranging from 3328 to 10,797 Å. The unit Å is not an official SI unit but is named after the Swedish physicist Ångström and is equal to 10−10 m, that is, 0.1 nm.
