As part of an on-going laboratory spectroscopy program at the McMath-Pierce 1.0-m Fourier transform spectrometer (FTS), James E. Lawler (Wisconsin) and colleagues have been recording spectra of hollow cathode lamps. These data are analyzed to determine emission branching fractions for lines in the first and second spectra of many elements (Wickliffe and Lawler, J. Opt. Soc. Am. B 14, 737, 1997; Quinet et al., Mon. Not. R. astr. Soc. 307, 934, 1999).
The FTS is ideal for spectroradiometry on complex atoms and ions, including both transition metals (open d shell) and rare earths (open f shell). The FTS provides: (1) a limit of resolution as small as 0.01 cm-1, (2) wave number accuracy to 1 part in 108, (3) broad spectral coverage from the UV to IR, and (4) the capability of recording a million-point spectrum in 10 minutes. Another advantage of the FTS over a sequentially scanned grating monochromator is that its interferogram is a simultaneous measurement of all spectral lines. A sequentially scanned grating monochromator will, unlike the FTS, map any small drift in source intensity into a branching fraction error.
The combination of branching fractions from FTS spectra with radiative lifetimes from laser-induced fluorescence measurements has resulted in greatly improved atomic transition probabilities for the first and second spectra of many elements. Over the last 20 years, this combination of techniques from Fourier transform and laser spectroscopy has made the field of atomic spectroscopy more quantitative.
Accurate atomic transition probabilities are needed for quantitative spectroscopy in a variety of fields and are essential in astronomy. Sneden et al. (ApJ 533, L139, 2000) illustrate how accurate elemental abundance determinations are improving our understanding of heavy element nucleosynthesis and the relative importance of rapid (r-process) neutron capture verses slow (s-process) neutron capture.
Accurate data on atomic transition probabilities are also important to industry. Modern metal-halide high intensity discharge (HID) lamps use a mixture of metal salts in mercury arc lamps to improve their color and efficiency. Iodides of thulium (Tm), dysprosium (Dy), and homium (Ho) are widely used in HID lamps. Accurate atomic transition probabilities are essential in modeling and diagnosing these important lighting products.
See the "Publications" section of this issue for some recent publications pertaining to this topic.