Measurements Of Spectral Line Profiles Research Articles

Calibrating and stabilizing spectropolarimeters with charge shuffling and daytime sky measurements

Well-calibrated spectropolarimetry studies at resolutions of $R>$10,000 with signal-to-noise ratios (SNRs) better than 0.01\% across individual line profiles, are becoming common with larger aperture telescopes. Spectropolarimetric studies require high SNR observations and are often limited by instrument systematic errors. As an example, fiber-fed spectropolarimeters combined with advanced line-combination algorithms can reach statistical error limits of 0.001\% in measurements of spectral line profiles referenced to the continuum. Calibration of such observations is often required both for cross-talk and for continuum polarization. This is not straightforward since telescope cross-talk errors are rarely less than $\sim$1\%. In solar instruments like the Daniel K. Inouye Solar Telescope (DKIST), much more stringent calibration is required and the telescope optical design contains substantial intrinsic polarization artifacts. This paper describes some generally useful techniques we have applied to the HiVIS spectropolarimeter at the 3.7m AEOS telescope on Haleakala. HiVIS now yields accurate polarized spectral line profiles that are shot-noise limited to 0.01\% SNR levels at our full spectral resolution of 10,000 at spectral sampling of $\sim$100,000. We show line profiles with absolute spectropolarimetric calibration for cross-talk and continuum polarization in a system with polarization cross-talk levels of essentially 100\%. In these data the continuum polarization can be recovered to one percent accuracy because of synchronized charge-shuffling model now working with our CCD detector. These techniques can be applied to other spectropolarimeters on other telescopes for both night and day-time applications such as DKIST, TMT and ELT which have folded non-axially symmetric foci.

Precise measurements of spectral line profiles

We present a method of precise measurements of spectral line profiles that was elaborated and tested on the Main Stellar Spectrograph (MSS) of the 6-m BTA telescope. The method allows discarding corrections for flat field. To take account of inhomogeneity of the CCD chip sensitivity, two spectra are recorded, shifted against each other along the dispersion by one column. Recurrent ratios between the photo responses of the CCD pixels let us determine their relative sensitivity and correct the line profiles. To eliminate various instability effects, the spectra are registered with the use of a cyclic shift procedure along the dispersion, and a synchronous electronic image transfer across the spectrum dispersion (along the CCD matrix columns).

Oxygen 630.0- and 557.7-nm line source for thermospheric dynamics studies.

A novel design of a rf afterglow source of the oxygen forbidden lines at 630.0 and 557.7 nm for use in thermospheric dynamics studies is presented. With a repetitive discharge-afterglow excitation cycle, the source yields adequate afterglow intensities of the OI lines that are sufficiently free of background continuum for use in Fabry-Perot interferometer measurements of spectral line profiles. These lines provide accurate zero-velocity references in Fabry-Perot determinations of Doppler shifts in the OI thermospheric lines. The design considerations for the rf source are described, together with its preparation and filling by an ultrahigh-vacuum gas-handling system. Examples are given of the source's output spectrum, as measured by a grating spectrometer, and its spectral line profiles, as determined by a Fabry-Perot interferometer. Comparative interferometry between the OI afterglow source lines and nearby (within approximately 0.01-0.02-nm) lines from hollow-cathode sources illustrates the means of establishing secondary reference sources in cases in which the primary OI afterglow source is not available.

Optical Emission Spectroscopy on the Gaseous Electronics Conference RF Reference Cell.

A summary of the experimental observations of the optical emission from the Gaseous Electronics Conference (GEC) rf Reference Cell plasma will be discussed. Spatially and temporally resolved results are provided for various reference and non-reference conditions, including etching type plasmas. These measurements provide a detailed description of the temporal evolution of optical emission from different excited atomic states within different atomic and ionic species, as well as their radial and axial distributions. Some of the measurements have been placed on a absolute radiometric scale to provide comparisons to model calculations. Spectral line profile measurements are also presented to provide some insight into the multi-component particle velocities present in such plasmas.

Optical multichannel analyzer-based high-resolution, multipoint spectroscopic apparatus

A high-wavelength resolution (Δλ≤0.02 Å) spectroscopic apparatus has been developed and applied to the measurements of spectral line profiles emitted by thermonuclear plasmas yielding important plasma parameters, such as ion temperature and fluid velocity. The experimental apparatus consists of a spectrometer, an optical multichannel analyzer (OMA), and a lens. It allows precise wavelength measurements (Δλerror≤0.02 Å) when a computer and a ThI electrodeless discharge lamp (EDL) are used to calibrate the instrument. The apparatus yields single-shot space and time-resolved spectra for radiation in the 2000–8000-Å range.

Measurement of line profiles

The basic requirements for high precision spectral line profile measurements are reviewed, with the observatory at the University of Western Ontario serving to illustrate several of the points.

Collision cross sections for the noble-gas broadening of the Cs 6S-7P doublet using photon echoes

Collisional cross sections and broadening constants are presented for each of the fine-structure components of the second resonance transition in caesium (6S12/-7P32,1/2/, 4555 AA, 4593 AA) perturbed by the five noble gases He-Xe. The results were obtained at perturber pressures of less than 0.5 Torr using the photon echo technique. The cross sections correspond closely with those obtained from spectral line profile measurements, but there are some significant discrepancies.

Determination of Cs(np)-Xe interactions from spectral line profile measurements

The 6s-7p, 8p and 9p absorption transitions in caesium perturbed by xenon foreign gas at a temperature of approximately 500K and densities up to 1.2*1019 atoms/cm3 have been observed with a high-resolution computer-controlled spectrometer. A Fourier analysis technique to deconvolve the asymmetric line core from instrumental and Doppler effects yields width, shift and asymmetry for the 7p an 8p doublets independent of the instrument and directly comparable with line-broadening calculations. Normalised profiles of the wings of the 7p and 8p transitions, and of the entire 9p contour are also given from measurements at sufficiently high resolution that instrumental effects are negligible. The observed line core-parameters and line profiles are compared with Anderson-Talman lineshape calculations (1956). The authors also note the observation of multiple-perturber satellites as predicted by the Anderson-Talman theory based on the assumption of additivity of the interaction.

Spectral Line Profile Measurements from Calcium, Silver, and Aluminum Hollow Cathode Lamps

There has recently been a great deal of interest in the direct measurement of atomic spectral line profiles from sources which are commonly used in atomic absorption (AAS) and atomic fluorescence spectrometry (AFS), e.g., hollow cathode lamps (HCLs), electrodeless discharge lamps (EDLs), and vapor discharge lamps (VDLs). Knowledge of spectral line profiles is of great importance in theoretical AAS and AFS studies.

Experimental measurements of spectral line profiles

The general problem of the effect of the finite size of slits and apertures on the resolving power of high-resolution instruments is reviewed. In addition, the effect of Doppler and pressure broadening on the shape of lines produced by these instruments is discussed. There is some experimental evidence that lines in the quadrupole spectrum of H 2 are sharper than the normal Doppler width. This evidence is reviewed and discussed in the light of recent theory. Definitive experiments are proposed to determine the breadth and shape of quadrupole lines of H 2.