Radiometric calibration of a Fourier transform spectrometer in the vacuum ultraviolet using synchrotron radiation

Abstract

The interpretation of astrophysical spectra from both space−based and ground−based telescopes relies on accurate measurements of atomic wavelengths, energy levels and oscillator strengths. However below 160 nm laboratory oscillator strengths are sparse, even for the astrophysically−important spectrum of Fe II. One of the most widely−used techniques for measuring oscillator strengths of spectral lines longer than 160 nm is by combining measured branching fractions (i.e. the intensity ratios of all the lines with the same upper energy level) with the lifetime of the upper level of the transition. The poor performance of Fourier transform spectrometers in the vacuum ultraviolet, the lack of suitable radiometric standards below 160 nm, and the challenge of measuring lifetimes of high−lying energy levels have all contributed to the paucity of laboratory oscillator strengths below 160 nm.