Interferometric Measurement of Resonance Transition Wavelengths

We have made the first interferomeric measurements of the wavelengths of the important ultraviolet diagnostic lines in the spectra C IV near 155nm and Si IV near 139nm with a vacuum ultraviolet Fourier transform spectrometer and high-current discharge sources. The wavelength uncertainties were reduced by one order of magnitude for the C IV lines and by two orders of magnitude for the Si IV lines. Our measurements also provide accurate wavelengths for resonance transitions in Al III, Al II, and Si II. The resonance transitions 2s S1/2 – 2p P1/2,3/2 near 155 nm in C IV and 3s S1/2 – 3p P1/2,3/2 near 139nm in Si IV are among the most important transitions for optical plasma diagnostics of hot plasmas in the interstellar medium, the intergalactic medium and stellar atmospheres. During the past decade, the increasing light gathering power of ground and space based telescopes has made it feasible to observe these lines with high resolution spectrometers. For example, the Goddard High Resolution Spectrograph (GHRS) and the Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telescope have modes of operation with resolving powers of up to 100000. For a reliable analysis of high resolution spectra from these instruments, transition wavelengths must be known with an accuracy of at least 1 part in 10. We have therefore undertaken new measurements of the C IV and Si IV resonance transition wavelengths. Our results confirm the observation of Smith et al. (1998) that the accuracy of available wavelength data for doublyand triply-ionized atoms is often very inadequate. The strongest motivation, at present, for a new measurement of the C IV and Si IV resonance lines came from the recent interest in determining National Institute of Standards and Technology (NIST), Gaithersburg, Maryland 20899, U.S.A. Harvard–Smithsonian Center for Astrophysics, Cambridge, Massachusetts 02138, U.S.A. bounds for temporal and/or spatial variations of the fine structure constant. This may be achieved through a measurement of the splitting of spectral lines in fine structure multiplets through spectroscopic observations of distant gas clouds seen against a background quasar. In a series of articles (Webb et al. 1999; Dzuba, Flambaum, & Webb 1999a,b), a method was suggested that considerably increases the sensitivity of optical measurements to variations of the fine structure constant. A recent analysis of quasar absorption spectra (Webb et al. 1999), based on accurate wavelengths for ultraviolet transitions in Mg II (Pickering, Thorne, & Webb 1998) and Fe II (Nave et al. 1991), found weak statistical evidence for a smaller fine structure constant in systems with cosmological redshifts z > 1. With this paper we intend to contribute accurate measurements of wavelengths for resonance transitions of several important ions in the vacuum ultraviolet to enable the analysis of quasar absorption spectra with much larger cosmological redshifts. Figure 1 is a block diagram of the experimental setup. The spectra were measured with the FT700 vacuum ultraviolet Fourier transform spectrometer (FTS) at the National Institute of Standards and Technology (NIST) (Griesmann et al. 1999). Photomultipliers with cesium-iodine photocathodes limited the optical bandpass of the spectrometer to wavelengths below ∼190nm which was es-