We have made the first interferomeric measurements of the wavelengths of the important ultraviolet diagnostic lines in the spectra C iv near 155 nm and Si iv near 139 nm with a vacuum ultraviolet Fourier transform spectrometer and high-current discharge sources. The wavelength uncertainties were reduced by 1 order of magnitude for the C iv lines and by 2 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.
We report new cross section measurements, believed to be the first involving synchrotron radition together with a 'hot-wire' diode detector, in which we have observed the doubly excited series of C a I at a much improved signal to noise ratio using the radiation from the DORIS I I electron accelerator and the H 4 S Y L 4 B facilities. The data provide " e n information on the 'vanishing width' effect in Ca I and are compared with several calculations of the cross section reported in recent literature. The best representation of the detailed structure appears to be achieved by the most recent combination of MQDT with the R-matrix method. All the calculations ( R matrix and MBPT) underestimate the cross section in the 3d5p peak by 20-40%.
Recently, a technique for calibrating the modulation transfer function (MTF) of a broad variety of metrology instrumentation has been demonstrated. This technique is based on test samples structured as one-dimensional binary pseudo-random (BPR) sequences and two-dimensional BPR arrays (BPRAs). The inherent power spectral density of BPR gratings (sequences) and arrays has a deterministic white-noise-like character that allows direct determination of the MTF with uniform sensitivity over the entire spatial frequency range and field-of-view of an instrument. As such, the BPR samples satisfy the characteristics of a test standard: functionality, ease of specification and fabrication, reproducibility, and low sensitivity to manufacturing error. Here we discuss our recent developments directed to the optimization of the sample design, fabrication, application, and data processing procedures, suitable for thorough characterization of large aperture optical interferometers. Compared with the previous coded-aperture based design, the improved, 'highly randomized' BPRA pattern of the new test standard provides better accuracy and reliability of instrument MTF and aberration characterization, and enables operation optimization of large aperture optical interferometers. We describe the pattern generation algorithm and tests to verify the compliance to desired BPRA topography. The data acquisition and analysis procedures for different applications of the technique are also discussed.
When measuring the form errors of precision optics with an interferometer, calibration of the reference wavefront is of central importance. In recent years, ball averaging, or random ball testing, has emerged as a robust method for calibrating spherical reference wavefronts in converging beams. We describe a simple instrument, consisting of an air bearing and two electric motors, that can rotate the test ball around three axes as required for a ball averaging test. The performance of the instrument is demonstrated by using it to calibrate a concave transmission sphere. Further we discuss the effects of image sampling at random locations or on uniform grids, and the effect of correlated measurements. Finally, we describe a method to determine the number of measurements which are sufficient for a ball averaging calibration.
Branching fractions were measured for electric-dipole transitions from the 5p upper levels to the 5s levels in neutral krypton atoms. The measurements were made with a wall-stabilized electric arc and a 2-m monochromator for the spectral lines in the visible, and with a hollow cathode lamp and the NIST 2-m Fouriertransform spectrometer for the lines in the near infrared. A semiempirical calculation, based on accurately known lifetimes for six upper levels, was used to calculate lifetimes for which accurate measurements do not exist. This resulted in a complete set of lifetimes for all 5p levels. Branching fractions and lifetimes were used to calculate transition rates for the 5p-5s transitions. The relative uncertainties of the transition rates range from less than 1% for the strongest lines to about 10% for the weakest lines. Our data also reveal that most of the previous measurements appear to have been affected by opacity effects in the light sources.
We present new energy-level classifications for over 400 lines in the spectra of neutral and singly-ionized dysprosium. These lines were observed using Fourier transform spectrometry of high current hollow cathode lamps and cover the wavelength range 230 nm to 1.6 µm. They were classified by 43 new energy levels of Dy I and 24 new energy levels of Dy II. The standard uncertainty of the energy levels and the strongest lines is estimated at 0.005 cm-1.
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